On entering the cement show our friends saw on every side long rows of booths showing models of structures and articles that could be made of concrete. There were models of houses, subways, dams, bridges, dock works, retaining walls, sewers, bridges, pavements and even boats and furniture. In fact, so the men in the exhibition booths said, concrete can be used for practically every building purpose where strength, lasting qualities, and resistance to heat and cold are needed. "This is the concrete age," they declared. "Concrete is fireproof, waterproof, sanitary, and resists frost when properly used. Our timber supply is decreasing, the supply of iron ore for structural steel is limited, and stone is expensive; so concrete, reinforced with steel, and used by engineers who understand their business, will be the greatest building material of the future."

These are the things that the enthusiasts at all the concrete shows say, but they admit that there are certain kinds of construction in which concrete is not as effective as steel or granite. Also they say that the use of reinforced concrete requires the highest type of engineering skill, and a complete understanding of the technicalities of the subject.

One of the places where we know concrete best is in pavements and sidewalks, and several of the booths exhibited samples of such work. To show its strength the men in charge piled on weights, struck the slabs with hammers, or subjected them to any kind of hard usage suggested by the crowd. Then, too, there were sections of concrete buildings, and exhibitions of various systems of reinforced concrete construction. With these there were concrete chimneys, portable concrete garages, railroad ties, and what not.

"Oh, but look here," broke out the boy as he led his older friend about. "Here's a perfect model of a house."

"Yes," answered the man, "that is a model of the famous Edison poured cement, or 'one-piece' house, the latest invention of our great American inventor."

There the little building stood, perfect in every way, surrounded by a model concrete wall, a beautiful lawn, and approached by fine concrete walks and driveways.

"This model," explained the scientist, "represents what Thomas A. Edison is trying to get time to accomplish for workingmen and their families. Instead of being built piece by piece, the house is supposed to be made all at one time by pouring the concrete into a complete set of moulds. This house is so interesting that we shall look at it much closer a little later on."

"And here," said the boy; "what's this?"

He had paused before a perfect model of the Gatun locks of the Panama Canal, where the world's greatest work in concrete, or any other kind of masonry, is being carried on. The work is greater than the Pyramids of Egypt or the Great Wall of China. Though we will not bother ourselves much with figures, it will give an idea of the size of the job on the canal when we realize that it will require 8,000,000 cubic yards of concrete, and more then 900,000 tons of Portland cement.

In all there will be six great locks for the transportation of our ships from the Atlantic to the Pacific and back. Three of these locks are at Gatun on the Atlantic side of the canal, one at Pedro Miguel, and two at Miraflores. Each lock will be 1,000 feet long, 110 feet wide, and 45 feet deep—and practically all of this is done with concrete. So massive is most of the work that steel reinforcement is only necessary in certain parts of the project. The problem of sinking the great retaining walls to bedrock, and making them strong enough to hold in the face of the tremendous floods of the Chagres River, alone makes one of the most stupendous engineering works ever undertaken by man. Were it not for the use of concrete the cost of the work would be so great as to make it almost impossible of accomplishment.

The model of the Gatun locks showed the boy everything, just as it will be when the canal is opened for traffic in 1913. There was the wide Gatun lake, surrounded by the tropical forests, the great Gatun dam, and the series of locks in one solid mass of concrete. These locks when completed will be 3,800 feet long, and their tremendous height and thickness can be seen from the pictures of the work as it is actually being carried on. In the model there were perfect little ships on the lake and going through the locks. Besides the many present day uses of cement some of the concrete enthusiasts are suggesting that heavily reinforced concrete be used in place of steel in making bank vaults, as they declare that the material will resist the keen tools and the powerful explosives of bank robbers even more successfully than the hardest steel.

Then too, at the cement show, the boy saw, besides models of big works and examples of all kinds of concrete construction, exhibits of the various methods of placing steel bars and steel network in the cement to make it stronger, and the different machines used in mixing concrete and in making Portland cement, which is the binding element in concrete.

As concrete is a material that can be mixed by an amateur and used for a great many purposes, the booths where mixing and simple uses were demonstrated attracted a great deal of attention. For instance, in the last few years the farmers have found out that they can make watering troughs, drains, floors for stables, hen houses, and even fence posts, of concrete just as easily as they can of wood or iron. Moreover, the articles thus made will last practically forever. All that is needed is a supply of Portland cement, and a little careful study as to the best way of mixing it with the proper amounts of sand and gravel. The amateur has best results if he starts modestly and takes up the use of reinforced concrete after learning how to use the material in its simple form.

One of the most interesting uses of reinforced concrete for the amateur who has learned something of the craft is in making a good, seaworthy rowboat, or even a small motor boat. Poured boats are strong, graceful, and durable. If they are properly made there never is any danger of their leaking, and by a little extra pains it is possible to make them with air-tight compartments so that they are non-sinkable.

The usual method of making concrete boats is very simple. The kind of boat to be duplicated is borrowed and hung on the shore so that it swings free of the ground. Then a mould of clay is built all around it. A strong bank of sand is heaped around the clay, to hold it firm. Then the boat is worked a little each way so that a space of about an inch and a half is left all around between the outside of the boat and the clay. The space between the boat and the clay is the space into which the concrete is poured for sides and bottom after the reinforcing rods have been properly inserted. After the whole thing has stood a day or so the inside boat is taken out and the clay mould broken down, revealing a complete concrete hull.

Thus, we see that concrete can be used as a building material in practically any kind of construction, that it is easily handled since all that is necessary is to pour it into the moulds after the engineers have properly placed the reinforcement, and that it can be cast in practically any decorative design just as easily as plain. Add to this the fact that concrete is cheaper than stone or steel, and that it is practically indestructible when properly handled, and it is easy to see the reason for calling this the cement age, and concrete the building material of the future.

After the Panama Canal, the greatest engineering feat in which concrete figures as one of the chief materials used, is the Catskill aqueduct, by which water from four watersheds in the Catskill Mountains of New York State is to be piped to all five boroughs of New York City. The Ashokan reservoir, near Kensington, N. Y., was the first part of the work to be taken up, together with the Kensico storage reservoir twenty-five miles from New York, several smaller reservoirs, and the aqueducts to carry this water from the mountains to every home in greater New York. The dam and containing walls of the Ashokan reservoir are all made of reinforced concrete, and the size of the lake and the strength of the walls can be appreciated when one thinks that the 130,000,000,000 gallons of water it holds in check would cover all Manhattan Island with twenty-eight feet of water. A large part of the aqueduct proper, through which this great stream of water is carried from the mountains, under the Hudson River, and to the city where it runs more than a hundred feet below the street level, is made of reinforced concrete. For other examples of the use of this material in big engineering works a boy has only to look around him. There are the tunnels under the rivers around New York, the New York subways, the Philadelphia and Boston subways, the Detroit River tunnel, bridges, culverts, big piers and other dock works, miles of concrete snowsheds along the lines of the railroads that cross the continental divide of the Rocky Mountains, and in fact practically every big structural undertaking.

Almost anywhere we look these days we see a big machine crushing rock, mixing it with sand and mortar, and turning out concrete to be shovelled into a hole and perhaps used far below the surface by "sand hogs" working under compressed air, or hoisted to the towering walls of some great office building or factory that is being constructed of the artificial stone.

We are familiar with the falsework of a concrete building under construction. It is all, apparently, a maze of wooden beams that look like scaffoldings, and yet they seem to make the outlines of the building. This maze of woodwork, seemingly so lacking in plan or system, as a matter of fact is a triumph of engineering skill, for it is the mould for the building, and was all built by the most careful plans as to strains, stresses, floor loads, etc.

First, however, before building the mould for a residence, school, theatre, office building, or factory, the engineer decides what strength his foundations must have. The foundation for a small residence is an easy matter, but when it comes to a big factory, or an office building of a dozen stories or so, the most careful work must be done beforehand. In the old days, when it was desired to sink the foundations of a building down to bedrock, they used steel or wooden piles, but these will rust or rot, and the modern way is to use concrete piles. Either the great poles are moulded first and sunk like the ordinary wooden ones, or a pipe with a sharpened point is sunk and the concrete deposited in it by buckets designed for the purpose. Once these piles are driven, they are there for all time, if the work is done properly, and the engineer can be sure that his building is as good as if resting on bedrock.

From then on the erection of a reinforced concrete building is a most intricate matter, because while concrete in itself is a very simple substance, its use in buildings is a highly developed science. Of course there are many different methods of using concrete, and each one prescribes a different kind of steel network for the reinforcement. Then, too, some engineers cast parts of their buildings separately and put them in place after they have set, while others run the concrete for beams, floors, and walls into moulds, built right where those parts are to be in the finished structure. In laying the steel reinforcing rods, before the concrete is poured, the engineer sees that they make a perfect network so as to take care of all the strains, just as they will be put upon the building when it is completed. It is in the proper placing of reinforcement that the greatest engineering knowledge is needed in this kind of building.

As the wooden moulds for the first foundation beams and girders are completed and the reinforcement is placed, the concrete is poured in. The subcellar or cellar floor mould then is laid, the reinforcement placed and the concrete run in. Next the moulds for the cellar walls are built and perhaps the moulds for the beams and girders for the first floor. The reinforcing rods are placed in these moulds and the concrete run in, and so on, a story at a time, or a small section at a time, until the structure reaches the height called for in the plans, and it stands completed. As the building progresses and the concrete on the lower floor sets, the moulds can be taken down and used on higher stories. Concrete is even used for the roofs of buildings, as it can be moulded right in place or set up in slabs that can be later cemented together.

When properly used reinforced concrete is absolutely fireproof, so it is coming into extensive use in the construction of schools, theatres, warehouses, factories, and all other such buildings where a great height is not required. So far, none of the great skyscrapers has been built of reinforced concrete, although office buildings of sixteen stories have been erected with complete success.

There is still another method of using concrete as a building material. This is in the form of building blocks, and doubtless all who read this will recall seeing many beautiful residences built of blocks of stone that on closer inspection proved to be concrete. The blocks can be cast in any size or form and used in just the same way as structural stone.

Now, after having looked about the city and having seen the numerous ways that concrete is used as a building material, we come back to the very latest thing in the use of this man-made stone—the "one-piece" or poured house.

For a good view of it let us take a little jaunt out to West Orange, N. J., with the scientist and look into the library of Thomas A. Edison's laboratory, where we will see a perfect model of this marvel of invention. It is practically the same as the one at the cement show. Standing in the centre of the great room where Edison works is this perfect little cottage, about the size of a large doll's house. It represents not only Edison's latest invention, but also his favourite scheme. In years to come, when the boys who read this are grown men, it will probably be no novelty to build houses by pouring them all at once into a steel mould, but just at present it is one of the most startling developments in an age of epoch-making inventions. Every boy knows that Edison has never followed the ideas of others in working out his inventions, and the poured house is no exception to his rule. It will be interesting to take a little look back over a part of Edison's life and see how he came to enter the cement-making business, and how, when he had his process down to a fine point, he said to himself, "It is cheap and easy to build a house or an office building of concrete in sections, why not build it all in one piece?"

We shall see that no sooner had he asked himself this startling question than he began by making models, and satisfied himself that it was not only possible, but one of the cheapest and best methods of making small, simply arranged houses, such as could be bought or rented for a small sum.

Although Edison has within the last few years brought his idea to a state where it can be put to practical use, he himself is not trying to push it commercially, as he has his other great inventions like the phonograph, storage battery, and the motion-picture machine. In fact, he is content to let it be worked out by others just so long as it fulfills his idea of giving to workingmen good houses at a low price.

"Years ago, long before Edison had retired from active business affairs to give his whole attention to scientific research," said the scientist, as he and the boy walked about the laboratory, "he became interested in metallurgy, just as he was and always is interested in every other science where great difficulties must be overcome. In those days iron and steel were not used as extensively as they are now, but the scientists and leaders in the big industries saw that the day was coming when far, far greater quantities of iron ore would be needed to supply the great demand for steel to build skyscrapers, ships, machinery, and so on. Men were going farther and farther away in their search for iron ore, but Edison, with his never failing originality, said to himself that it was likely there was plenty of iron ore right around his laboratory in New Jersey if he only knew how to get at it.

"For one thing," continued the boy's friend, "Edison had seen on the ocean beaches great stretches of white sand with millions and millions of little black particles sprinkled through them. He knew that the specks were pure iron ore. You can prove this to yourself by simply holding a good magnet close to a pile of such sand, and watching the iron particles collect."

It was Edison's idea to concentrate the iron ore found in the earth, in just this way, for he had sent out a corps of surveyors who had reported vast quantities of low-grade ore in most of the Atlantic Coast States. Low-grade ore is that which contains only a small percentage of the metal desired, and hence it does not pay to smelt it, unless a very cheap process can be found. Edison thought he had a process cheap enough, for he simply intended to grind the mountains to sand and take out the particles of iron by running it through a hopper with a high-power magnet at the mouth.

The process sounds simple, but the machinery required was very complicated, to say nothing of being extremely heavy. Edison set up his mill in the mountains of New Jersey and started to blast down the cliffs of low-grade ore and run them through a series of gigantic crushers that ground them to a fine powder. The iron particles, called concentrates, after being extricated were pressed into briquets ready for delivery to the foundry.

After having spent close to $2,000,000 on the experiment, and satisfactorily proving its mechanical success, the discovery of vast quantities of high-grade ore in the Messaba range of Minnesota forced Edison to close his plant. "This would have been a crushing failure to most men," added the scientist, "but Edison's only comment was a whimsical smile. Indeed, even on his way home after closing his plant, Edison was planning new and more important activities, for with his experience at rock crushing he was satisfied he could enter the field as a maker of the building material called Portland cement."

At that time cement and concrete were even less used than were steel and iron, but Edison for many years had seen that in the future they would take the place of wood, stone, and brick.

"Well-made concrete, employing a high grade of Portland cement," said Edison on one occasion, "is the most lasting material known. Practical confirmation of this statement may be found abundantly in Italy at the present time, where many concrete structures exist, made of old Roman cement, constructed more than a thousand years ago, and are still in a good state of preservation.

"Concrete will last as long as granite and is far more resistant to fire than any known stone."

But Edison had something more than a successful business in mind when he returned from his rock-crushing plant, for he intended setting up cement-making machinery such as had never before been seen. With this end in view he began to read up on the subject, just as we have seen the Wright brothers read up on aviation. Incidentally, as an indication of the manner in which this wizard works, it may be said that all this time Edison was perfecting his new storage battery.

One big improvement upon the usual process in the manufacture of cement, planned by Edison, was that the grinding should be so fine that 65 per cent. of the ground clinker should pass through a 200-mesh screen instead of only 75 per cent. as is the usual rule. Thus, Edison made into cement 10 per cent. more material that other manufacturers sent back to be ground over again.

The success of Edison's Portland cement plant is not matter for our attention here, so we will pass over those busy years to the time of Edison's retirement to devote all his time to scientific research.

For many years he had watched the cities grow, had seen the great tenements become more crowded, and less comfortable each year. He had seen the children playing in the streets, and had compared their lives to the happy lives of the children whose parents could afford to live away from the great cities, where boys could have yards to play in. He decided that the boys of the city streets would have a far better time, that their mothers and fathers would have a far more cheerful life if they could live in comfortable little houses in the country with yards, and gardens, and plenty of room for every one.

Edison saw that what was needed was a building material cheap enough, and a method of using it cheap enough, so that dwellings could be put up at a cost that would place them within the means of workingmen and their families.

Concrete, he decided, was the material to solve the problem, and Edison set himself to the task of making houses poured complete into one mould so as to make the cost of labour as low as possible. The "one-piece" house was an assured thing from that time on. All that remained was for the "Wizard of Orange," as he is called, to work out the difficult details of a properly mixed cement and a practical system of moulds.

An incident that occurred at the time of the failure of his ore crushing plant in the New Jersey mountains was one of the things that brought the whole situation home to him. When the plant was closed and the buildings vacated, the fire insurance companies cancelled the policies, declaring that the moral risk was too great.

The inventor's reply was short and to the point. He made no protest against the cancellation of his policies, but simply said he would need no more policies, as he would erect fireproof buildings in which there would be no "moral risk."

This promise of Edison's, made at the time of his so-called failure and pondered during the years of his tremendous activities, was not redeemed until he had retired from the business of invention as a means of gaining riches. "I am not making these experiments for money," Edison has said many times. "This model represents the character of the house which I will construct of concrete. I believe it can be built by machinery in lots of 100 or more at one location for a price which will be so low that it can be purchased or rented by families whose total income is not more than $550 per annum. It is an attempt to solve the housing question by a practical application of science, and the latest advancement in cement and mechanical engineering."

Edison's plan, as we have seen before, was simply to make a set of moulds in the shape of the house he desired to build, run the concrete into them, let them stand until the material had settled, and then take down the retaining surfaces, exposing to view the finished house.

It was contrary to all the previous ideas in building, and was ridiculed by many famous architects. Nevertheless, tremendous obstacles are the stuff upon which Edison's genius feeds, and he only worked the harder to produce a concrete that would be liquid enough to fill all the intricate spaces and turns in the moulds and yet sufficiently thick to prevent the sand or gravel in the concrete from sinking to the bottom. Thus, it first had to run like thin mush and then set in walls and floors harder than any brick or stone. Another of the difficulties to be overcome was to discover a concrete that would give perfectly smooth walls.

Although this may sound very simple, it has not yet been completely worked out in this country, owing to the heavy demands on Edison's time. The perfected process, however, will be made known just as soon as the inventor can find time to complete certain small details that he wants to clear up before giving the system to the world. A French syndicate working along Edison's ideas for a poured house has made some progress and it is reported they have constructed two attractive dwellings with considerable success. One of these is at Santpoort, Holland, and the other near Paris.

Whether the houses are poured completely in one mould, or whether they are built a story at a time on different days, this newest form of house building is carried on along about the same lines.

"Let us just suppose," said the scientist, "that we are standing on a building site in some pretty suburb of a great city. We will also suppose that an Edison poured house is to be erected there. Plans are drawn beforehand for a small house of simple arrangement and a set of steel moulds in convenient sizes are turned out. These moulds all have connections so they can be set up and joined together in one piece. First, we see that a solid concrete cellar floor, called the 'footing', has been laid down just the size and shape of the house. A crowd of skilled workmen quickly set up the moulds on this footing and lock them together. The moulds make one complete shell of the house, from cellar to roof, just as it will appear when completed. Reinforcing rods are placed in the mould so that they will be left in the concrete walls, floors, etc., of the house after the steel shell is taken away.

"Nearby we see a few more skilled workmen mixing the concrete in great vats. When the mould and the material are ready we see the concrete taken to a tank on the roof and poured into troughs which carry the stuff to a number of different holes through which it flows into the mould. We hear it splash, splash, splash as it gradually fills every space in the shell, and finally after six hours or so it overflows at the roof. The main part of the work is now done and we can go away for a few days while the liquid in the shell sets, or turns to the hardest kind of stone.

"After about six days we return to see the moulds unlocked, taken down and the complete house standing ready with walls, floors, stairways, chimneys, bathtubs, stationary tubs in the cellar, electric-wire conduits, water, gas and heating pipes all complete. In making the moulds the spaces for bathtubs, wash-tubs, electric wiring and piping for gas, water, and heat, are just as carefully arranged as walls and floors. The only work necessary after the concrete has set is to put in the doors and windows, install the furnace and necessary fixtures for heating, lighting and plumbing and connect them up ready for use. No plaster is used in these houses, but the walls can be tinted or decorated just as the landlord or occupant desires."

The boy's friend went on to say that one might think that this was about as far as science could carry the use of concrete, but Edison said to himself: "If we can make houses, why can't we make furniture?" and he set about experimenting with poured furniture. He obtained some wonderful results with this newest use of concrete, and in his Orange laboratory he has several cabinets, chairs, and other articles of furniture that are every bit as attractive to look at as wooden furniture and that are practically indestructible.

"And my concrete furniture will be cheap, as well as strong," says Edison. "If I couldn't put it out cheaper than the oak that comes from Grand Rapids, I wouldn't go into the business. If a newlywed starts out with, say, $450 worth of furniture on the installment plan, I feel confident that we can give him more artistic and more durable furniture for $200. I'll also be able to put out a whole bedroom set for $5 or $6."

At present the weight of this concrete furniture is about one third greater than wooden furniture, but Edison is confident he can reduce this excess to one quarter. The concrete surface can, of course, be stained in imitation of any wood finish. The phonograph cabinet shown at the left of Edison in the picture opposite page 281 has been trimmed in white and gold. Its surface resembles enamelled wood. The cabinet at his right is the old style wooden type.

This concrete cabinet easily withstood the hard usage of shipment by freight for a long distance.

THE WORLD-WIDE USE OF CONCRETE

Of course, the poured concrete furniture is made in just the same way as the houses except that it is a much simpler process. It is a very easy matter to set up a steel mould for a chair, a cabinet, a dresser, or a bedstead, whereas a house, with its tubs, conduits, stairways, hallways, doorways, window frames, plumbing system, etc., is a most complex matter, requiring a set of moulds that could be put together properly by only a man who combined the highest abilities of an architect, a builder, an engineer, and a mechanic. Although concrete has been used for many years in making garden furniture, Edison's plan for making finished indoor pieces with it is entirely new.

But to return to the houses; Edison says it is just as easy to make poured dwellings in decorative designs as in plain ones. It is only necessary to have the moulds cast in the desired shape. It is his idea to have all the poured houses pretty as well as perfectly sanitary and substantial. He intends that there shall be many different kinds of moulds, and also that each set of moulds shall be so cast that it can be joined in different ways, in order to give the houses a variety of appearance. Thus, in a small town where a large number of poured houses were set up, there would be no two exactly alike if the owners preferred to have them different.

According to the plans Edison now has on foot, the first complete poured houses will have on the main floor two rooms, the living room and dining room, while on the second floor there will be four rooms, a bathroom and hallway. Of course as the main idea is to give perfectly sanitary and comfortable houses, there will be plenty of windows, for lots of fresh air and sunlight. Edison figures that he can build a house of poured concrete for $1,200 that would cost $30,000 if built of cut stone. Furthermore, he figures that the rent ought to be about $10 per month, as he will only license reputable concerns to use his patents, and his licenses will stipulate the approximate rent that can be charged.

Thus, the high cost of living about which we all hear so much at the family dinner table as well as everywhere else is being attacked by science and invention through a new channel, and Edison's latest invention can be expected soon to give good homes at low rents to thousands of families now paying exorbitant prices for dark stuffy city flats.

It was significant that at the celebration of Edison's sixty-fifth birthday, February 10, 1912, the great American inventor should sit at the head of the table surrounded by his family and associates facing a perfect model of one of his poured cement houses. The chair in which he sat, to all appearances was beautiful mahogany, but in reality was cast in a mould of Edison concrete at the Edison plant. At the place of each guest was a bronze paperweight, appropriately engraved, with Edison's favourite motto:

"All things come to him who hustles while he waits."

HISTORY OF CONCRETE

Although concrete is in truth the newest building material in our time, it is the oldest known to civilization because it was the stuff with which the eternal buildings of ancient Rome were constructed. Even before the Romans used concrete it was used by the Eygptians, more than 4,000 years ago. Every boy will remember from his history classes that the Egyptians, so far as we can learn, were the first people in the history of the world to reach a high state of civilization. Every boy also will remember that the only way we know this is through the evidence of ruins of tombs and buildings. Many of these buildings were made of a material very much like concrete that must have been made in some such manner as concrete is made nowadays.

About 2,000 years later, long after the Egyptian civilization had died, the men of Carthage discovered concrete for themselves and built a marvellous aqueduct 70 miles long, through which water was brought to their city. It was carried across a great valley over about 1,000 arches, many of which are still standing in good condition.

To the Romans, however, we are indebted for some of the best examples of ancient concrete work. They used this material in their wonderful city for buildings, bridges, sewers, aqueducts, water mains, and in fact in a great many of the ways that we have seen it is used to-day. The great Coliseum and the Pantheon at Rome are relics of the skill of the ancient architects in the use of concrete.

Although many historians think that the secret of making cementious building material was lost from the fall of Rome until the middle of the eighteenth century, there are ruins of ancient castles which stood in mediÆval times in Europe which indicate at least some use of concrete.

The real discoverer of natural cement in our modern times though, was John Smeaton, who will be remembered by the readers of "The Boy's Second Book of Inventions" as the man who built the first rock lighthouse at Eddystone, England, in 1756. In his great work he discovered a kind of limestone with which he could make a cement that would set, or harden, under water. His discovery was hailed as the recovery of the secret of the ancient Romans of making hydraulic cement. It was so called because it would harden under water.

In 1796, Joseph Parker, another Englishman, made what he called Roman cement. Several others followed, and in 1818 natural cement was first made in the United States by Canvass White near Fayetteville, N. Y. The material was made from natural rock and was used in the construction of the Erie Canal.

All of these early cements are called natural cements by engineers nowadays, because they were made from natural rock. It was only necessary to find a clayey limestone which contained a certain percentage of iron oxide and two other minerals known as silica and alumina. The limestone was crushed to a convenient size and was burned in a kiln. The heat turned the stuff into cinders which, when ground to a fine powder and mixed with water, would make a cement that would harden under either air or water very quickly, and last for practically all time. Just for the sake of those who have studied chemistry we will say that in this process the heat drives off the carbon dioxide in the limestone, and the lime, combining with the silica alumina and iron oxide, forms a mass containing mineral properties called silicates, aluminates, and ferrites of lime. These properties mixed with the water make natural cement. In the United States, natural cement was called Rosendale cement, because it was first made commercially in a town of New York State by that name.

The supply of natural cement, however, is limited, because the proper kind of limestone is only found in a few places. Consequently, when an artificial mortar called Portland cement was invented in 1824, the world took a step forward that could not be measured in those days.

Most authorities give the credit for the invention to Joseph Aspdin, a bricklayer of Leeds, England. He took out a patent on the material and in 1825 set up a large factory. In 1828 Portland cement was used in the Thames tunnel, making the first time that the material figured in any big engineering work. In those days even the most enthusiastic supporters of cement little dreamed that in this modern age it would be the material that would make possible such tremendous victories over the obstacles of nature as the Panama Canal, the tunnels under the rivers that surround New York and the great dams that hold back the waters all over the country.

Aspdin, however, is not given the credit for the invention of Portland cement by all authorities, as some claim that Isaac Johnson, also an Englishman, who early in 1912 died at the age of 104, was really the first man to invent a practical, commercial, artificial cement.

The advantage in Portland cement is that it can be made of a number of different kinds of earth, to be found in many different parts of the world, and makes a far stronger rock. It sets more slowly than natural or hydraulic cement, but is more satisfactory for use in reinforced concrete work. In the Lehigh Valley, where about two thirds of the Portland cement used in the United States is made, the raw material is a rock, called cement rock, and limestone. In New York State they make Portland cement of limestone and clay; in the Middle West they make it of marl and clay, while in other Western States they make it of chalk and clay. In Europe slag is sometimes used. The artificial product contains lime oxide, silica, alumina, iron oxide, and other minerals in varying quantities, but the necessary ones are silica, alumina, and lime. In making Portland cement the raw material is ground into a fine powder and poured into one end of a long cylindrical kiln which looks like a smokestack lying on its side. Powdered coal is shot into the kiln, where it is kept burning, at a heat of about 2,500 to 3,000 degrees Fahrenheit. After the raw material has been burned thoroughly and is taken from the kiln it looks like little cinders or clinkers about the size of marbles. The cement clinker is then cooled and ground to a powder, after which it is stored away for a little while to season.

The first Portland cement ever made in the United States was turned out by David O. Saylor, of Coplay, Pa., in 1875, but the development of the new industry was very slow, as builders and engineers seemed to be blind to the great possibilities of the material that built Imperial Rome. In 1890, nearly twenty years after the process was introduced in America, only 335,500 barrels of Portland cement were manufactured in this country. The country woke up to the situation a few years later, and in 1905 there were manufactured in the United States 35,246,812 barrels of Portland cement. In 1911 the industry turned out the stupendous total of 77,877,236 barrels.

This was because the age of concrete had dawned on the world and man had learned in those years that by mixing gravel and sand with cement he could make a material cheaper, more easily handled, and far more lasting than wood, brick or some stone.

As Edison once said to some of his associates: "I think the age of concrete has started, and I believe I can prove that the most beautiful houses that our architects can conceive can be cast in one operation in iron forms at a cost, which, by comparison with present methods, will be surprising. Then even the poorest man among us will be enabled to own a home of his own—a home that will last for centuries with no cost for insurance or repairs, and be as exchangeable for other property as a United States bond."

The technical definition of concrete is as follows: "Concrete is a species of artificial stone formed by mixing cement mortar with broken stone or gravel. Cement is the active element called the matrix and the sand and stone forms the body of the mixture called the aggregate."

The ingredients are mixed in different proportions for different work. A common proportion is 1 part cement, 2 parts sand, and 5 parts broken stone or gravel. Cement users speak of this as a "1: 2: 5 mixture." Sometimes the gravel is left out and a mixture of 1 part cement to 3 or 4 parts sand is made. The cement binds the mass together and sand fills up any little vacant spaces about the gravel, making what is called a dense mixture.

THE SILENT KNIGHT MOTOR

Two views of the latest automobile engine. At the top can be seen the sliding sleeves, the inlets and outlets which do away with valves.