No sooner was the third of these oppositions past than he wrote another book on the subject, with the title “Mars and its Canals”; and this in no sense a supplement to the earlier one, but an entirely new and independent presentation of the subject, covering the old ground and much more. He was enabled to do this because the copyright of the earlier work belonged to him. The later one was published by The Macmillan Company in December 1906, and dedicated to Schiaparelli. Like the earlier book, he wrote it by no means for astronomers alone, but for the interested public; and in the preface he tells why he did so: “To set forth science in a popular, that is in a generally understandable, form is as obligatory as to present it in a more technical manner. If men are to benefit by it, it must be expressed to their comprehension. To do this should be feasible for him who is master of his subject, and is both the best test of, and the best training to that post.... Nor is it so hard to make any well-grasped matter comprehensible to a man of good general intelligence as is commonly supposed. The whole object of science is to synthesize, and so simplify; and did we but know the uttermost of a subject we could make it singularly clear.” At the same time there was nothing in these writings of the nature of what is commonly called popularizing science. He expounded his subject in a strictly scientific way, but avoided unfamiliar technical terms if possible, and sought to raise his readers or audience to his level of thought, not to descend to theirs. Such statements for the public were very often preceded by technical ones in the Bulletins of the Observatory or elsewhere, and yet it cannot be doubted that the former tended to alienate some scientific scholars who were slow to admit his discoveries, and did not sympathize with his method of presenting them, or perhaps with the attractive style of the man of letters as well as of exact thought.

Still there are pitfalls in taking the public into one’s confidence; as he found in December 1900, when a telegram sent by the usual channels to the astronomical world, that the night before a projection had been observed on Mars that lasted seventy minutes, was taken by the press to mean an attempt by Martians to signal to the Earth, and as such was proclaimed all over America and Europe. The cause of the excitement, as he explained a year later to the American Philosophical Society in Philadelphia, was the reflection from a cloud on the horizon of the planet.

“Mars and its Canals” is frankly a demonstration that the planet is habitable, and that from what takes place there it must in fact be inhabited by highly intelligent beings. For that purpose the book is divided into four parts, entitled: Natural Features; Non-Natural (that is, artificial) Features; The Canals in Action; and Explanation. His general thesis, which he was to expound more fully later (and which although not essential to his argument for life on Mars he connected therewith) was that all planets go through the same process of development—varying, however, with their size which determines their power to retain the gases of their atmosphere—and that one element therein is the gradual leakage of water through cracks into its interior as the planet cools. He cites geologists to prove that the oceans formerly covered much more of the surface of the Earth than they do now; argues that the desert belts around it are of comparatively recent geologic origin, as shown by the petrified forest of Arizona; and points out the similarity in color, as seen from the San Francisco Peaks, of the forested hills and the painted desert there, to that of the blue-green and reddish-ochre spaces of Mars as presented by the telescope. He notes also that to get water in our deserts plants and animals have sought the higher altitudes, and are able to exist and multiply in an air less dense and a climate cooler with a shorter warm season than in their natural habitat, adjusting themselves to these conditions.

This idea of the lack of water on Mars he derives from observation of its surface and the changes thereon; for the supply of water is in great part locked up in the snow or ice of the polar caps during the Martian winters of the two hemispheres and distributed over its surface as summer comes on. Therefore he naturally begins his account of the natural features of the planet by a description of these polar snow caps, their formation and melting. In doing so he cannot resist a sarcastic reference to the endless enthusiasm, useless expenditure of money and labor, and the scientific futility of arctic exploration.

“Polar expeditions exert an extreme attraction on certain minds, perhaps because they combine the maximum of hardship with the minimum of headway. Inconclusiveness certainly enables them to be constantly renewed, without loss either of purpose or prestige. The fact that the pole has never been trod by man constitutes the lodestone to such undertakings; and that it continues to defy him only whets his endeavor the more. Except for the demonstration of the polar drift-current conceived of and then verified by Nansen, very little has been added by them to our knowledge of the globe. Nor is there specific reason to suppose that what they might add would be particularly vital. Nothing out of the way is suspected of the pole beyond the simple fact of being so positioned. Yet for their patent inconclusion they continue to be sent in sublime superiority to failure.

“Martian polar expeditions, as undertaken by the astronomers, are the antipodes of these pleasingly perilous excursions in three important regards, which if less appealing to the gallery commend themselves to the philosopher. They involve comparatively little hardship; they have accomplished what they set out to do; and the knowledge they have gleaned has proved fundamental to an understanding of the present physical condition of the planet.”

Then follows the story of the melting of the polar snows, the darkening of the blue-green areas by the growth of vegetation due to the flow of water; and a summary, at the close of [Part I] (Natural Features), of the reasons for believing that from its atmosphere, temperature, and the actual, though scanty, supply of water, Mars is capable of supporting life. In fact the presence of vegetation proves that life of that kind does exist, in spite of the fact that five-eighths of the surface is desert; and if plants can live animals might also. But, unlike vegetation, they could not be readily seen, and save in the case of intelligent operation on a large scale, their presence could not be detected. This is the significance of the canals, to which much of the observation of the last three oppositions was directed.

Close to the limit of vision, and only to be seen at moments when the atmosphere is steady, the fainter canals are very hard to observe. Percival describes the experience in this way:

“When a fairly acute eyed observer sets himself to scan the telescopic disk of the planet in steady air, he will, after noting the dazzling contour of the white polar cap and the sharp outlines of the blue-green areas, of a sudden be made aware of a vision as of a thread stretched somewhere from the blue-green across the orange areas of the disk. Gone as quickly as it came, he will instinctively doubt his own eyesight, and credit to illusion what can so unaccountably disappear. Gaze as hard as he will, no power of his can recall it, when, with the same startling abruptness, the thing stands before his eyes again. Convinced, after three or four such showings, that the vision is real, he will still be left wondering what and where it was. For so short and sudden are its apparitions that the locating of it is dubiously hard. It is gone each time before he has got its bearings.

“By persistent watch, however, for the best instants of definition, backed by the knowledge of what he is to see, he will find its coming more frequent, more certain and more detailed. At last some particularly propitious moment will disclose its relation to well known points and its position be assured. First one such thread and then another will make its presence evident; and then he will note that each always appears in place. Repetition in situ will convince him that these strange visitants are as real as the main markings, and are as permanent as they.”

Strangely enough fine lines, from the continuity of the impression they make upon the eye, can be recognized when of a thickness that would be invisible in the case of a mere dot. To determine how narrow a line on Mars would be perceptible, experiments were made with a wire of a certain size, noting the limit of distance at which it could be seen; and then, from the magnifying power of the telescope, it was found that a Martian canal would be visible down to about a mile wide. From this the conclusion was drawn that the canals probably ran from two or three up to fifteen or twenty miles in width, the minimum being much less than had been thought at earlier oppositions. The distance apart of the two branches of double canals he estimated at about seventy-five to one hundred and eighty miles, save in one case where, if a true instance of doubling, it is over four hundred. Of the oases, whereof one hundred and eighty-six had been observed, much the larger part were from seventy-five to one hundred miles in diameter.

The later oppositions enabled him also to complete the topography of the planet, showing that the canals were a vast system, running from the borders of both polar caps, through the dark areas of natural vegetation where they connected, at obviously convenient points, with a still more complex network in the ochre, or desert, regions, and thus across the equator into the corresponding system in the other hemisphere. By this network the greater part of the canals could receive water alternately from the melting of the north and south polar caps, or twice yearly, the Martian year, however, being almost twice as long as our own. But to perfect his proof that this actually takes place he had to show that the canals, that is the streaks of vegetation bordering waterways, sprang into life—thereby becoming visible or darker—in succession as the water spread from the poles to the tropics; and this he did with his usual thoroughness at the opposition of 1903.

Since there was then no mechanical means of measuring the variations in visibility of the canals,—and under the atmospheric conditions at any place in the world perhaps there never will be,—the record had to be made by the eye, that is in drawings by the observer as he saw the canals; and these, as he said, must be numerous, consecutive and extended in time. The consecutive could not be perfectly carried out because “as Mars takes about forty minutes longer to turn than the Earth, such confronting (of the observer) occurs later and later each night by about forty minutes, until finally it does not occur at all while Mars is suitably above the horizon; then the feature passes from sight to remain hidden till the difference of the rotations brings it round into view again. There are thus times when a given region is visible, times when it is not, and these succeed each other in from five to six weeks, and are called presentations. For about a fortnight at each presentation a region is centrally enough placed to be well seen; for the rest of the period either ill-placed or on the other side of the planet.” But with changes as gradual and continuous as those of the darkening of the canals this did not prove a serious drawback to the continuity of the record.

There was another element in the problem. The drawing being the estimate of the observer on the comparative darkness of the markings from time to time it was of the greatest importance to avoid any variation in personal estimates, and therefore Percival made all the drawings himself. From April 6 to May 26 he drew the planet every twenty-four hours, and although “the rest of the time did not equal this perfection, no great gap occurred, and one hundred and forty-three nights were utilized in all.... But even this does not give an idea of the mass of the data. For by the method employed about 100 drawings were used in the case of each canal, and as 109 canals were examined this gave 10,900 separate determinations upon which the ultimate result depended.”

For each canal he plotted the curve of its diminishing or increasing visibility as the season advanced, and this curve he called the cartouche of the canal. Now combining the cartouches of all the canals in each zone of latitude, he found that those in the several zones began to become more distinct—that is the vegetation began to come to life—in a regular and approximately uniform succession, taking from the northern arctic down to the equator and past it to the southern sub-tropic about eighty Martian days. From north latitude 72° to the equator, a distance of 2,650 miles, took fifty-two of these days, at a speed of fifty-one miles a day, or 2.1 miles an hour. Now all this is precisely the opposite of what happens on the Earth, where vegetation in the spring starts in the part of the temperate zone nearest to the equator, and as the season advances travels toward the pole; the reason for the difference being, he says, that what is needed on Earth to make the sap run is the warmth of the sun, what is needed on Mars is water that comes from the melting of the polar snows. He points out also that the water cannot flow through the canals by nature, because on the surface of a planet in equilibrium gravity would not draw it in any direction toward or away from the equator. “No natural force propels it, and the inference is forthright and inevitable that it is artificially helped to its end. There seems to be no escape from this deduction.” In short, since water certainly cannot flow by gravity both ways in the same canal, the inhabitants of Mars have not only dug the canals, but pump the water through them.

OBSERVING AND DRAWING THE CANALS OF MARS