In the intervals of personal observation Percival was often giving lectures or writing on astronomical subjects for the publications of the Observatory, and for scientific societies and periodicals. The substance of most of these found their way into his books, which are summations or expositions of his conclusions. In December 1902, for example, he gave six lectures on “The Solar System” at the Massachusetts Institute of Technology, of which he was a non-resident professor, and they were published by Houghton, Mifflin & Company. Then in the autumn of 1906 he gave a course of eight lectures at the Lowell Institute in Boston on “Mars as the Abode of Life.” These were so crowded that they had to be repeated, were then printed as six papers in the Century Magazine, and finally re-published by The Macmillan Company under the same title. Two years later, in the winter of 1909, he gave at the Massachusetts Institute of Technology, another course of six lectures on “Cosmic Physics: The Evolution of Worlds,” which were brought out in December by the same publisher with the latter half of the title. Although their names are so diverse, and far more is told of Mars in the book whose title contains its name, they all deal essentially with the same subject, the evolution of the planets and the development and end of life upon them. In the Preface to “Mars as the Abode of Life,”—for a preface, although printed at the beginning, is always written after the book is finished, and is the author’s last word to the reader, giving his latest thought as the work is being launched,—he tells us:[17] “Though dealing specifically with Mars, the theme of the lectures was that of planetary evolution in general, and this book is thus a presentation of something which Professor Lowell has long had in mind and of which his studies of Mars form but a part, the research into the genesis and development of what we call a world; not the mere aggregating of matter, but what that aggregation inevitably brings forth. The subject which links the Nebular Hypothesis to the Darwinian Theory, bridging the evolutionary gap between the two, he has called planetology, thus designating the history of the planet’s individual career. It is in this light that Mars is here regarded: how it came to be what it is and how it came to differ from the Earth in the process.”

At each opposition, in fact at every opposition during Percival’s life and long thereafter, Mars was observed at Flagstaff and more detail was discovered confirming what had been found before. He tells of a slight change in the estimated tilt in its axis; the fact that the temperature is warmer than was earlier supposed;[18] and he had found how to discover the gases by spectroscopic analysis applied according to an ingenious device of his own known as “Velocity Shift” and much used thereafter.[19] He tells also of an ingenious and elaborate experiment with wires, and with lines on a wooden disk, which showed that such lines can be perceived at a greater distance and therefore of smaller size than had been supposed, so that the canals might have less width than had been assumed. It is, however, needless, in describing his planetary theory, to do more than allude to his evidence of Martian habitation drawn from the canals, with which the reader is already familiar. Curiously enough, however, it is interesting to note that on September 9, 1909, about the time when “The Evolution of Worlds” was going to press, a strange phenomenon appeared in Mars. Two striking canals were seen where none had ever been seen before, and the most conspicuous on that part of the disk. Moreover, they were photographed. After examining all the maps of canals made at Flagstaff and elsewhere, Percival discussed them in the Observatory Bulletin No. 45, and concluded that they must not only be new to us, but new to Mars since its previous corresponding season of two of our years before: “something extra ordinem naturae.” We may here leave Mars for the time, and turn to the more extensive study of the evolution of the planetary system.

The desire to rise from a particular case to a more general law was characteristic of his attitude of mind, constructive and insatiable, and appears throughout these volumes. It may have been influenced by his great master Benjamin Peirce, who ever treated any mathematical formula as a special instance of a more comprehensive one. In such a subject as the evolution of the planets, especially of life on them, it involved dipping into many sciences, beyond the physical laws of matter; and he says in the same preface: “As in all theses, the cogency of the conclusion hangs upon the validity of each step in the argument. It is vital that each of these should be based on all that we know of natural laws and the general principles underlying them.” This did not mean that all his premises would be universally accepted, but that he found out all he could about them, convincing himself of their accuracy and of the validity of the conclusions he draws therefrom. That is all any man of science can do in a subject larger than his own special, and therefore limited, field.

But from the time of his resumption of research and the direction of the observatory in 1901, he was constantly enlarging his own field by the study of astrophysical subjects, and the methods for their determination. With this object he was initiating and encouraging planetary photography. He was constantly writing Dr. V. M. Slipher about procuring and using spectrographic apparatus and about the results obtained by him therefrom. By this process the rotations of planets were determined; and the spectra of the major ones—often reproduced in astronomical works—have been a puzzle to astrophysicists until their interpretation in very recent years. He was interested also in nebulae, especially in spiral ones, taking part in Dr. Slipher’s pioneering spectrographic work at the observatory, which showed that they were vast aggregations of stars of different spectral types, moving with great speed, and far beyond the limits of our universe. For over fifteen years the observatory was almost alone in this field of research, as well as in that of globular clusters. It is in fact, the discovery of the rapid motion of the spiral nebulae away from the solar system that has given rise to the conception of an expanding universe.

But these discoveries were still largely in the future, and to return to his books on the planetary system it may be noted that in the two larger and more popular ones the general planetary theory is expounded in the text, while the demonstrations of the more complex statements made, and the mathematical calculations involved, are relegated to a mass of notes at the end of the volume.

The first of his books on the solar system is the small volume bearing that title; but since all three of the books here described are several expositions of the same subject it may be well to treat his views on each topic in connection with the work in which he deals with it most fully. Indeed, “The Solar System” is not a general treatise, but rather a discussion of some striking points, and it is these which one thinks of in connection therewith.

In considering the origin of the planets he had become much interested in the meteors, shooting stars, meteoric streams and comets, all or almost all of which he regarded as parts of the solar system, revolving about the Sun in elliptic orbits, often so eccentric as to appear parabolas.[20] The old idea that comets came from outer space and therefore travelled in hyperbolas can, he points out, be true of few, if any, of them. “Very few, three or four perhaps, hint at hyperbolas. Not one is such beyond question.” Many of them are associated with the meteoric streams with which everyone is familiar at certain seasons of the year. Indeed seventy-six of these associations were then known, and comets sometimes break up into such streams.

Now if the comets are travelling in orbits around the Sun they must be throughout their course within its control, and not within that of some other star; and therefore he computes how far the Sun’s control extends. Taking for this purpose our nearest star, a Centauri, a double with a total mass twice that of the Sun, at a distance of 275,000 astronomical units, in other words that number of times our distance from the Sun, he finds that the point at which its attraction and that of the Sun become equal is 114,000 of these units. This he calls the extent of the Sun’s domain, certainly an area large enough for any, or almost any, comet known.[21]

He then turns to some of the planets,—Mercury to show the effect of tidal action in slowing the rotation of a planet or satellite, and causing it to turn the same face always to its master.[22] This involved a highly interesting comparison of Newton’s theory of the tides, long generally accepted, but not taking enough account of the planet’s rotation, and that of Sir George Darwin based upon the effect of such rotation. The general conceptions are even more different than the results, and the later theory is less concerned with the tides in oceans, which probably affect only our Earth, than with those of a planet in a fluid or viscous condition, which may still continue to some extent after the surface has become partly solidified. He therefore studies the tide raising force, and the tendency to retardation of rotation, by the Sun on the planets, and by these on their satellites while still in a fluid state, tabulating some very striking results.

What he says about Mars is more fully dealt with in his other writings; and the same is true of Saturn’s rings, except for the reference to the calculation by Edward Roche of the limit of possible approach by a fluid satellite to its planet without being disrupted, and for the fact that this limit in Saturn’s case falls just beyond the outer edge of the rings. In discussing Saturn’s satellites he brings out a curious analogy between the order of distribution of these attendants of the three best known major planets and the order of the planets themselves about the Sun. In each case the largest of the bodies so revolving is nearly in the centre of the line, as in the case of Jupiter among the planets; the second largest the next, or not far, beyond, as in the case of Saturn; while there is another maximum farther in, for as the Earth is larger than any planet on either side until Jupiter is reached, so a like order is found in the satellites of Jupiter, Saturn and Uranus. In other words, the size in each case rises with increasing distance, falls off, then rises again to the largest and thence declines. This he believed cannot be an accidental coincidence, but the result of a law of development as yet unexplained.

To the ordinary reader the most novel thing he says about Jupiter relates to its family of comets, for no less than thirty-two of these bodies have their aphelia, or greatest distance from the Sun, near its orbit. Moreover, their ascending nodes—that is the place where their paths if inclined to the plane of the ecliptic pass through it—are close to its orbit. At some time, therefore, in the vast ages of the past they must have passed close to the planet, and if so have had their orbits greatly changed by its attraction. He considers the various effects Jupiter may have upon a comet, and shows—contrary to the opinion of Professor H. A. Newton—that any such body moving by the attraction of the Sun would be going too fast for Jupiter to capture completely. Then he takes up other effects of deflection. The comet’s speed may be accelerated and its direction changed even so much as to drive it out of the solar system; it may be retarded so that its path is contracted and the aphelion drawn nearer to the planet’s orbit. After calculating the possible conditions and analyzing the actual orbits of Jupiter’s family, he comes to the provisional conclusion that these comets have been drawn from the neighborhood. “It is certain,” he says, “that Jupiter has swept his neighborhood.... If we consider the comet aphelia of short-period comets, we shall notice that they are clustered about the path of Jupiter and the path of Saturn, thinning out to a neutral ground between, where there are none. Two-thirds of the way from Jupiter’s orbit to Saturn’s, space is clear of them, the centre of the gap falling at 8.4 astronomical units from the sun....

“Jupiter is not the only planet that has a comet family. All the large planets have the like. Saturn has a family of two, Uranus also of two, Neptune of six; and the spaces between these planets are clear of comet aphelia; the gaps prove the action.

“Nor does the action, apparently, stop there. Plotting the aphelia of all the comets that have been observed, we find, as we go out from the Sun, clusters of them at first, representing, respectively, Jupiter’s, Saturn’s, Uranus’, and Neptune’s family;[23] but the clusters do not stop with Neptune. Beyond that planet is a gap, and then at 49 and 50 astronomical units we find two more aphelia, and then nothing again till we reach 75 units out.

“This can hardly be accident; and if not chance, it means a planet out there as yet unseen by man, but certain sometime to be detected and added to the others. Thus not only are comets a part of our system now recognized, but they act as finger-posts to planets not yet known.”

We shall hear more of this last suggestion hereafter.

In both “Mars as the Abode of Life” and “The Evolution of Worlds,” he accepts the proposition that our present solar system began with a collision with some dark body from interstellar space, as had been suggested by Chamberlin and Moulton a few years before. He points out that stars which have finished contracting, grown cold and ceased to be luminous, must exist, and although we cannot see them directly we know about some of them,—such as the dark companion of Algol, revolving around it and cutting off two-thirds of its light every three days. Many dark wanderers there must be, and the novae, as he says, are sometimes, at least, due to a collision with such a body,—not necessarily an actual impact, but an approach so near that the star is sprung asunder by the tidal effect. In such a case the opposite sides of the victim would be driven away from it, and if it was rotating would form spirals. Now we know that the apparently empty spaces in our solar system still contain a vast number of little meteoric particles, which as judged from their velocity do not fall from outer space, but are members of our system travelling in their own orbits around the sun. As he puts it, “Could we rise a hundred miles above the Earth’s surface we should be highly sorry we came, for we should incontinently be killed by flying brickbats. Instead of masses of a sunlike size we should have to do with bits of matter on the average smaller than ourselves[24] but hardly on that account innocuous, as they would strike us with fifteen hundred times the speed of an express train.” That these meteorites are moving in the same direction as the Earth he shows by an ingenious calculation of the proportion that in such a case would be seen at sunrise and sunset, which accords with the observed facts. Moreover, their chemical composition shows that they were once parts of a great hot body from which they have been expelled.

The meteorites that are seen because they become hot and luminous in traversing our atmosphere, and occasionally fall upon the Earth, are the remnants of vastly larger numbers formerly circling about the sun, but which, by collision and attraction, were, as he describes, gathered into great masses, thus forming the planets. The force of gravity gradually compacted these fragments closer and closer together, thereby generating heat which if the body were homogeneous would be in proportion to the square of its mass. The larger the planet therefore the more heat it would generate, and owing to the fact that mass is in proportion to the cube and its radiating surface to the square of the diameter the slower it would radiate, and thus lose, its heat, so that the larger ones would be hotter and remain hot longer than the smaller ones.

Some of the planets may once have been white-hot, and luminous of themselves, some were certainly red-hot, some only darkly warm; all growing cooler after the amount radiated exceeded the amount generated. Now by the difference in the heat generated and retained by the larger and smaller bodies he explains the diverse appearance of those whose surfaces we know, the Earth, Mars and the Moon. As the surface cools it forms a crust, but if the interior still remains molten it will continue to contract, the crust will be too large for it and crinkle, like the skin of a dried apple; and this will be more true of a large than a small body. “In like manner is volcanic action relatively increased, and volcanoes arise, violent and widespread, in proportion; since these are vents by which the molten matter under pressure within finds exit abroad.” By a calculation, which agrees with the formula of Laplace, he finds that the effective internal heat of the Earth might be 10,000 degrees Fahrenheit, enough to account for all the phenomena; and for Mars only 2,000, which is below the melting point of iron, and would not cause volcanic action. Now the observations of Mars at Flagstaff show that there can be no mountains on it more than two or three thousand feet high, and that the surface is singularly flat.

But here he met a difficulty; for the Moon ought to be flatter still if it had evolved in the ordinary way, whereas it has enormous volcanic cones, craters 17,000 feet high, some exceeding 100 miles in diameter, and a range of mountains rising to nearly 30,000 feet. An explanation he finds in the analysis of the action of the tides in the Earth-Moon system by Sir George Darwin, who showed that when traced backward it “lands us at a time when the Moon might have formed a part of the Earth’s mass, the two rotating together as a single pear-shaped body in about five hours.... For in that event the internal heat which the Moon carried away with it must have been that of the parent body—the amount the Earth-Moon had been able to amass. Thus the Moon was endowed from the start of its separate existence with an amount of heat the falling together of its own mass could never have generated. Thus its great craters and huge volcanic cones stand explained. It did not originate as a separate body, but had its birth in a rib of Earth.”[25]

The Flagstaff site having been selected for the purpose of planetary observation yielded facts less easily detected elsewhere. Mercury, for instance, is so near the Sun that it could be observed in the dark only a short time after sunset and before sunrise, an obstacle that gave rise to errors of fact. Schiaparelli led the way to better results by observing this planet in broad daylight. Up to that time it had been supposed to rotate on its axis in about twenty-four hours, and therefore to have a day and night like those of the Earth, but daylight observation showed him markings constant on its illuminated face, and therefore that it turns nearly the same side to the Sun. Before knowing his conclusions, and therefore independently, the study of Mercury was taken up at Flagstaff in 1896, and the result was a complete corroboration of his work. It showed that, as in the case of the Moon with the Earth, tidal action on the still partially fluid mass had slowed its rotation until it has little with regard to the central body around which it revolves. He discovered also other facts about Mercury, which Schiaparelli had not, that its size, mass and density had not been accurately measured.

A similar discovery about the period of rotation had been made in the case of Venus. For more than two centuries astronomers had felt sure that this period was just under twenty-four hours, figured, indeed, to the minute. But again it was Schiaparelli who doubted, and once more by observing the planet at noon; when he noted that the markings on the disk did not change from day to day, and concluded that the same side was always pointed at the Sun. At Flagstaff in 1896 his observations were verified and the inference later confirmed by the spectroscope, which was, indeed, first brought to the Observatory for that purpose. Thus Venus, which from its distance from the Sun, its size and density, is most like the Earth, turns out to be in a totally different condition, one face baked by unending glare, the other chilled in interstellar night, and as he puts it: “To Venus the Sun stands substantially stock-still in the sky,— ... No day, no seasons, practically no year, diversifies existence or records the flight of time. Monotony eternalized,—such is Venus’ lot.”[26]

On the movements and physical condition of the Earth it was needless to dwell, and he passed to the asteroids. He describes how they began to be discovered at the beginning of the last century by searching for a planet that would fill a gap in Bode’s law. This, a formula of arithmetical progression for the distances of the planets from the Sun, has proved not to be a law at all, especially since the discovery of Neptune which is much nearer than the formula required; but for nearly a century it had a strong influence on astronomic thought, and the gap in the series between Mars and Jupiter was searched for the missing link. Two were found, then two more, about the middle of the last century another, and then many, smaller and smaller, until by the time Percival wrote six hundred were known, and their number seems limitless. Only the four first found, he remarks, exceed a hundred miles in diameter, the greater part being hardly over ten or twenty. But here he points out a notable fact, that they are not evenly distributed throughout this space; and although massed in a series growing thicker toward its centre there are many gaps, even close to the centre, where few or no asteroids are found. Now it is the large size and attraction of Jupiter by which Percival explains the presence of asteroids with gaps in their ranks, instead of a planet, in the space between it and Mars; but we shall hear much more of this subject when we come to his work on Saturn’s rings and the order in the distribution of the planets.

Jupiter, he tells us, having a mass 318 times that of the Earth, and a volume 1400 times as large, is much less dense, not much more than water, in short still fluid; and as it has a tremendous spin, rotating in less than ten hours, it is more oblate than the Earth; that is, the diameter at its equator is larger in proportion to that from pole to pole. The observations at Flagstaff brought out some interesting facts: first, that the dark belts of cloud that surround it are red, looking as if the planet within were still molten;[27] second, that the bright central belt lies exactly upon its equator, without regard to, and hence independent of, its tilt toward the Sun, and that the belts of cloud on each side appear at the planet’s morning just as they left it in the evening. All which shows that Jupiter’s cloud formation is not due to the Sun, but to its own internal heat, an interpretation of the phenomena that has a direct bearing on his explanation of the Earth’s carboniferous age.

Saturn is still less dense, even more oblate; but its most extraordinary feature is of course the rings. Assumed by the early astronomers to be solid and continuous, they were later shown to have concentric intervals, and to be composed of discrete particles. They have usually been supposed flat, but when the position of the planet was such that they were seen on edge knots or beads appeared upon them; and in 1907 these were studied critically at Flagstaff, when it was found that the shadows of the rings on the planet were not uniform, but had dark cores; these thicker places lying on the outer margin of each ring where it came to one of the intervals. These phenomena he explained in the same way as the distribution of the intervals among the asteroids.[28]

About Uranus and Neptune he tells us in this book little that was not known, and save for their orbits, masses and satellites not much was known of their condition. But later, in 1911, the spectroscope at Flagstaff determined the rotation period of Uranus, afterwards precisely duplicated at the Lick; and later still the spectral bands in the vast atmosphere of the giant planets were identified as due to methane, or marsh, gas.[29]