Percival Lowell, of the Flagstaff Observatory, Arizona, is the strongest advocate of the positive side, and as he is also the best observer of Mars and has at hand a larger and better mass of actually observed facts than any other astronomer, he is in a position to assail any argument that is claimed to be based on facts.

The only safe way, therefore, to study the problem is to take Lowell's own writings, to grant his observed facts, and then to analyze his deductions and examine his arguments. It is only in this manner that the battle may be fought on even ground, for when it comes to reasoning, a recluse in his cell is on a par with the best observer.

Lowell has collected all his observed facts and expressed his ideas in two popular books, "Mars and Its Canals" and "Mars as the Abode of Life," published respectively three years and one year ago. When quoting these works I shall call them I. and II., respectively, followed by the page number. But before proceeding to review these books it will be necessary briefly to recall the principal facts concerning our sister planet as they are given in astronomical textbooks.

Mars, as we know, is the planet whose orbit is next outside the earth's. It requires 687 days, or one year and ten a half months of our reckoning, to complete its circuit about the sun. Its distance from this luminary is about fifty percent. greater than ours, so that it receives only about half as much light and heat per square mile as the earth does.

The planet itself is a globe like the earth, and is about as much flattened at the poles. It is certainly a solid, as the permanence of its surface markings proves. Its diameter is about 4,200 miles, a little more than half the earth's. Its volume is only one-seventh as much, and its surface area about twenty-eight per cent., that is to say, 100 pounds on earth would weigh only 38 pounds on Mars. It turns on its axis in 24 hours, 37 minutes, 22.67 seconds, so that its day and night are only a little longer than ours. Its equator is inclined to its orbit at very nearly the same angle that the earth's is, so that its seasons are identical with ours, except that they are about twice as long.

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Mars has a very rare and transparent atmosphere. The barometer which registers a terrestrial pressure of about thirty inches at sea level would sink down to less than four inches on Mars. Human beings, therefore, like ourselves could evidently not exist there. This is admitted by all. This rare atnsosphere is seldom obscured by clouds, and these clouds are rather dust than water vapor. On account of this wonderful transparency of its atmosphere, combined with the fact that when Mars is nearest the earth it turns a fully

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must therefore be a negligible factor in Martian surface meteorological phenomena. Both density and temperature can be but little affected by such cause." (Ι. 63.)

Owing to the rarity of the air and the general absence of clouds, "insolation on Mars is more of a factor than with us" (I. 79). Hence during the long days of summer, which itself is about twice as long as ours, heat may accumulate to a considerable extent in spite of the rarity of the atmosphere. This is a point well taken by Lowell. There may, therefore, be sufficient heat for the support of life, which the equally long and extremely cold winter would only cause to hibernate without destroying. (II., 187.) "The Martian climate is one of extremes.

In summer and during the day it must be decidedly hot, certainly well above any possible freezing. The maximum temperature, therefore, cannot be low. The minimum, of course is. Organic life is not in the least debarred from finding itself there." (I. 380.)

And in reference to observations that he has himself made on the top of the San Francisco mountains near his observatory at Flagstaff, Arizona, Lowell says (II. 96) that "the fact of a few warm weeks made life possible, outweighing the impossibility of all the other long, cold, forbidding months."

He claims that "the mean temperature of the surface air of Mars should be about 48 degrees F.; that of the earth is only 60 degrees F." (II. 86.) Here, however, he is at variance with Poynting (Monthly Weather Review, November, 1904) and the generality of astronomers, who say that the four terrestrial planets, Mercury, Venus, Earth and Mars, part with the sun's heat by radiation into space as fast as they receive it, and that therefore their surfaces must be at a constant mean temperature. This temperature is for the earth about 62 degrees F., and for Mars 36 degrees below

zero.

With such a low average temperature it is hard to see how life could endure. However, Lowell remarks (II., 103) that "man can endure 70 degrees below zero F. if the air is still, but perish at 40 degrees below under the least wind. Even a breeze, therefore, is equivalent to a fall of 30 degrees F. in the temperature." The rarity of the Martian atmosphere precludes the possibility of violent winds, and in so far favors the existence of life.

Nor is this rarity of the Martian atmosphere such an essential bar to life as is generally supposed. "Another point the presence of the animals on the San Francisco Mountains serves to bring out their indifference to thinness of the air." (II. 96.) Lowell says that the species of deer, bear and other animals are the same at 10,000 feet elevation, where the barometer is only 18 inches, as they are at sea level, with 30 inches. In the same way meadow larks at 8,000

He claims that the polar caps cannot be carbonic acid, as some have maintained, because although this when frozen is as white as snow, it changes from the solid to the gaseous state without passing through the liquid state. Now the polar caps of Mars are always fringed with a deep blue line, which cannot be anything else but water. (I., 39.)

This fine blue line fringing the caps seems to be Lowell's only real argument for proving the presence of water on Mars. It is a rather slender thread to support such a weighty conclusion. It is needless to say that he is alone in this contention.

And as water vapor is there, Lowell infers that therefore the less volatile gases—nitrogen, oxygen and carbonic acid-must be there also (II., 104). As all these gases, which are the constituents of our own atmosphere, found their presence on Mars ultimately on the fine blue line which fringes the polar caps, and which Lowell is the only one to have seen, the argument is rather weak and unconvincing. The direct spectroscopic proof of the presence of oxygen in the atmosphere of Mars, which Lowell claims recently to have obtained, still awaits confirmation.

We come now to the most interesting part of Arean topographythe famous canal system. Here we must trust Lowell almost implicitly, since no other person has ever been able, even with superior optical power, to see as much as he has depicted. The keenness of his eyesight, as well as his assiduity, are truly marvelous. According to Lowell the canals cover the whole surface of the planet (II., 194). They are extremely fine lines, almost entirely beyond the visibility of most obervers. They run in arcs of great circles, that is, to use an unmathematical term, they are perfectly straight. Their smallest width is about one mile (I., 181), and as to length, 2,000 miles is common; many exceed 2,500; one is 3,540 miles long (I., 183). This is one-third of a circumference (I., 183), or, rather, only one-fourth (II., 150), and as long comparatively as if it extended on earth from London to Denver, or from Boston to Behring Strait. They are to be found in all latitudes, longitudes and directions (I., 190). They are, however, visible only during the summer; they "hibernate" (I., chap. xxv.) and disappear during the winter (II., 197).

But the most wonderful feature of these canals is their gemination or doubling, two fine lines being perfectly parallel and equidistant throughout their whole length. Lowell estimates an average double canal to be about 2,250 miles long, each one being 20 miles wide and 130 miles from its fellow (I., 206). Out of the 437 canals (II., 151) on Mars only 51 have been seen double (I., 208; II., 159). He says that this gemination is seasonal (I., 212), and that doubles are an