The next step after, and sometimes even before, you have thought out your great idea is to make a drawing of the invention it represents.

Nearly every one can do a little free hand drawing and this is a good way to make rough sketches to aid the mind in further developing thought.

But if you can make a simple working drawing of your device, that is a picture in which all of the parts are drawn in proportion, or to scale as it is called, the whole thing will stand out clearly before you and you can see where it is wrong and make the needed changes on paper before you try to build a model.

Tools for Making Simple Drawings.—To make simple working drawings, or mechanical drawings as they are called, all the tools you need are a good, straight 12-inch rule, as shown in Fig. 13, compasses as shown in Fig. 14, a medium hard lead pencil, a rubber eraser and some smooth white paper.

How to Make Simple Working Drawings.—At A in Fig. 15 is shown a drawing in perspective, that is as it would look to the eye, of a rectangular box, while B is a top view, C is a side view and D is an end view of the same box; of course the bottom and the other end and side cannot be seen but you can imagine pretty well that they are there if you try to.

To show the top, bottom, sides and ends of a box, or other device, you don’t need to draw out the whole thing in perspective but you can make a flat, or plan view of each part as shown at B, C and D in Fig. 15, that is an outline drawing shown as though you were looking squarely at it in the center and with the measurements marked upon it.

If now you will make a set of these working drawings of, say, a box and draw each part to scale, that is measured off in proportion, as shown in B, C and D, and saw out of a board the top, bottom, sides and ends and nail them together you will have a box like that shown in perspective at A.

Plan views are easy to draw because they are formed of horizontal and vertical lines, and wheels are shown as true circles. After making your plan views, though, the safest way is to make a perspective drawing to the same scale for when you are looking at a square object as it really is it always appears larger than the plan views would indicate. But this is ahead of the story.

Now suppose you wanted to show how the box would look if it was sawed lengthwise through the middle. You simply make a cross-section view of it as shown at E and any one who knows how to read drawings will understand it. To show the hook on the front of the box more clearly it can be drawn separately as at F and this is called a detail drawing.

Exactly in the same way any device, apparatus or machine can be shown by top, side and end views and by cross-section and detail drawings.

Just to see how something a little more complicated would work out on paper, let’s take the cylinder and steam chest of a steam engine. First draw a side view of these parts as shown in Fig. 16.

As the steam chest is a rectangle and every side of it is flat it can be shaded by drawing fine parallel lines spaced equally apart. The cylinder, pipes and rods are round, or rather cylindrical, and to get this effect these parts should be shaded with parallel lines drawn close together beginning at the top and bottom and making them ever farther apart as you get toward the middle and this will give it a rounded appearance.

Next draw the end view of the cylinder and steam-chest. Since the cylinder has been given a diameter of 3¼ inches in the side view, of course it must have the same diameter in the end view as shown in Fig. 17.

By looking again at Fig. 16 you will see that the steam chest is 4½ inches long and that it is 2½ inches high but it is in the end view Fig. 17, that the width of it is shown. The end of the steam chest is shaded with straight parallel evenly spaced lines and the cylinder head is shaded with concentric circles, that is with circles equally spaced apart and having the same center.

In this and many other cases a side view and an end view give all the outside dimensions needed but sometimes a top view must also be made, and this is shown in Fig. 18.

While all of these views show the outside of the steam chest and the cylinder they give no hint as to how the inside is made. Suppose you had invented the steam engine of course you would know how the inside should be made and so you make a cross-sectional drawing of the parts as shown at Fig. 19, and then the construction and even the operation of the engine looms up as though you had turned a searchlight on it.

That is all of it will be clear except perhaps the slide valve and this is where a detailed drawing comes in to show a small part, or a part that is hard to understand by looking at the side, end and top views. The slide valve, see Fig. 20, is drawn in detail and the picture is made large and bold. The slide valve is made of a cast piece of metal hollowed out. It and the completed steam chest and cylinder are both drawn in perspective, that is just as the eye would see them if they were actually made of metal. The latter is shown in Fig. 21.

A Simple Way to Draw in Perspective.—Did I hear you ask how you can make a drawing in perspective? List and I will tell you the simplest way—a way so that you do it the first time you try.

Buy a quire of isometric (pronounced i-so-met´ric) cross-section paper 6 by 9 inches, at a cost of 15 cents, of any dealer in drawing materials. This paper is lined in faint colored ink in three directions, as shown in Fig. 22, and which represent length, breadth and thickness.

Now isometric comes from iso which means equal and metric which means measure, so isometric means equal measure and the three lines used in isometric perspective are at equal distances from each other. The lines which cross the vertical lines on isometric cross-section paper are 30 degrees from the base, or horizontal line and the vertical line is, of course, 90 degrees from the horizontal as shown in Fig. 23. Having everything at hand suppose you try to draw a square frame. Begin by making the first upright and you will see by looking at Fig. 23 that all you have to do is to draw three vertical lines and join the top and bottom by marking over the 30 degree lines. This done draw three more uprights in the same way and when you have these on paper it is easy to put beams on top or struts between them as shown at Fig. 24.

As all the lines are of equal measure you can mark on the exact dimensions as shown in many of the isometric perspective drawings in this book. For a drawing of some device, or of a whole machine, to give to some mechanic to make for you the better way is to hand him a perspective drawing together with the top, side and end views, rather than the latter views alone, and then he will not need to figure out how they are put together.

To show to a better advantage how isometric perspective works out look at Fig. 25 and you will see how the bearings of a crankshaft of a four cylinder gas engine stand out in a vertical line, up and down and in a horizontal line right and left as though they were real and made in three dimensions.

How to Make Isometric Paper.—To make isometric perspective drawings you can get along without the cross-section paper described above though this is the easiest and most accurate way to get results.

But you can make these drawings on any kind of paper if you know how to use a protractor and measure of 30 degrees. To do it right you should have some drawing tools and if you are an inventor you should have them anyway.

Drawing Tools You Need.—For making drawings of any kind you should by all means have a drawing-board as shown at A in Fig. 26. As a drawing board must be perfectly square and made so that it cannot warp it is better to buy one of a dealer in drawing materials.

A good board is built up of thoroughly seasoned strips of white pine glued together and fitted with end ledges; a small board say 12 by 17 inches on the sides can be bought for 50 cents or a little more and it will serve you well. A 12 inch triangular boxwood architect’s scale is shown at B in Fig. 26 and is much handier to use than a common rule.

A beginner’s set of drawing instruments consisting of compasses, with pen and pencil points, a ruling pen and a box of leads all in a nice pocket case, as shown at C, Fig. 26, can be bought for $1.25 and these compasses are easier to handle than the one shown in Fig. 14.

But the chief instrument you need is a protractor, as shown at D, Fig. 26. This is a semicircle of brass, or of German silver, 3¾ or 4½ inches in diameter and costs 10 cents or 40 cents, according to the size and metal it is made of.

A protractor, as you may or may not know, is used to lay off angles and to measure angles in degrees. The curved part or scale of the protractor is divided into 180 degrees since there are 360 degrees in a circle. The figures start at both corners with 0 so that an angle of any number of degrees right or left can be marked off. Now the lines formed by marking off angles of 30 degrees are the only ones you will have to make for isometric perspective. To do this fasten a sheet of paper to your drawing board with thumb tacks at each corner and draw a straight line across the paper near the bottom. Put your protractor on the edge of the paper and the pencil line exactly as shown in Fig. 27; lay your rule so that its edge crosses the straight part of the protractor at the middle, marked A in the drawing and also on the line of the scale of the protractor marked 30 degrees and then draw a line on the paper along the edge of your rule.

This done place the protractor on the opposite and left hand edge of the paper and the horizontal line and lay your rule with its edge crossing the middle of the straight part of the protractor as before and on the 30 degree line of the scale and so that when you draw the line it will cross the other 30 degree line as shown in Fig. 27.

If now you draw another line at 90 degrees, that is vertically, between the two crossed lines, also as shown in Fig. 27, each of the three lines will be exactly the same distance apart in degrees. You can go ahead now and draw lines ? inch apart parallel with each of the three lines and you will have a sheet of isometric cross section paper of your own making.

How to Draw Isometric Ellipses.—An Easy, Rough Way.—There is just one more little thing you should know about making isometric perspectives and that is how to draw disks, wheels and anything else that is circular in form so that they will look right and be right.

In isometric perspective everything that is round in reality is drawn in the shape of an ellipse, that is a closed curve that is longer than it is wide as shown at A in Fig. 28; there are different shaped ellipses but there is only one used for isometric drawing and this is always in the ratio of 1¼ to 2; that is if an ellipse is 2 inches long it will be 1¼ inches wide; an ellipse 4 inches long will be 2½ inches wide and so on.

An easy, though rough way to draw an isometric ellipse is to make a line as long as the diameter of the disk or wheel you intend to represent; draw another line which is the width of the ellipse through the center and at right angles across it, see A again and then draw the curved line around the end of them free hand.

How these ellipses are made to appear as if they were set either in a vertical or a horizontal position and at right angles to each other is shown at B in Fig. 28. The axis, that is the spindle, or shaft on which the disk, or wheel, is mounted, must always follow the 30 degree line running at right angles to the edge of the board or whatever it is supposed to be fastened to or goes through; and the thickness of the disk or wheel is always shown on the same sides as the thickness of the board or other part on which it is mounted, all of which is brought out clearly at B in Fig. 28.

How to Draw an Isometric Ellipse.—A Harder but More Accurate Way.—Begin by drawing a straight line as long as you want the longest axis of your ellipse to be, as shown at A B, Fig. 29. Divide this line into four equal parts. Now take your compasses and with the needle at the center of the line O draw a circle having the line as its diameter.

Next start at A with your dividers and divide the whole circle into ten equal parts and then take your rule and draw a line from the point C on the circle through the point G on the diameter and produce, or extend it to the bottom of the circle; draw a line from D through G and extend it to the top of the circle; draw a line from E through H and extend it to the bottom of the circle when it will intersect the line C G at the point J; and finally draw a line from F through H to the top of the circle which will intersect the line D G at I.

Take your compasses and using G as a center draw the arc K A L; then using H as a center draw the opposite arc M B N; using the point J as a center, draw the arc K M so that its ends will meet the upper ones of the end arcs perfectly; using the point I as a center draw the fourth and last arc L N when the ellipse is completed.

When making isometric ellipses much care must be taken to make all the points and draw all the lines with the greatest accuracy as the slightest error will distort the whole thing.

How to Shade Drawings.—Besides the few hints for shading perspective drawings which I have given above there are certain ways to shade cross-sections and elevations to show whether it is made of metal, glass, wood, liquid, cork, carbon, insulation or other materials. There are also different kinds of shading to show fine and coarse fabrics and the various colors.

The patent office has prepared a chart showing the shading that should be used to represent the different materials and colors and these are reproduced in Fig. 30. The letters of the alphabet both upper and lower case, as the capitals and little letters are called, which are used by mechanical draftsmen are also shown in Fig. 30. As these letters and figures are clear, easy to make and are preferred by the patent office they are good ones for you to use.

How to Make Electrical Symbols.—In making drawings, either for yourself or for the patent office, of electrical apparatus to show how it is connected up you do not need to draw out a plan view or a perspective of each part but you can make what are called symbols.

Symbols are simply a few lines or signs that stand for or represent a certain piece of apparatus; as an illustration suppose you want to show a dry cell, all you need to do is to make a couple of parallel lines, one shorter and heavier than the other like this:

and if you want to show a battery you make as many pairs of parallel lines as there are cells in this fashion:

And just so with every separate piece of electrical apparatus, and all of them are shown at A and B in Fig. 31.

How to Read Electrical Diagrams.—From the plates of symbols given at A and B in Fig. 31, you will see that the symbol for a battery is a pair of parallel lines as shown above, that the symbol for a motor is made in this fashion:

and that a switch is made like this:

now if you want to show a battery, a motor and a switch wired together all you have to do is to join the symbols with lines as shown at C in Fig. 31 and you will have what is called a diagram.

You can read a diagram, that is understand how it is connected up, in an instant for you can see at a glance how the wires run. Because the wiring is shown so simply and clearly diagrams of this kind are usually called wiring diagrams.