Page:The American Cyclopædia (1879) Volume XI.djvu/204

192 MARS Madler, Sir W. Herschel, and Huygens in 1672, (1 .lured a rotation period of 24h. 37m. 22'62s. Lastly, the present writer, by combining ob- s. -nations made in 1873 with Hooke's obser- vations in 1G66, deduced the period 24h. 37m. 22'73d. As this differed more from Kaiser than could be explained if neither Huygens's observations nor Hooke's had been misunder- stood, Kaiser went afresh over his work, and obtaining his former result expressed the opin- ion that Hooke's observation was untrustwor- thy. But the present writer, having carefully examined Kaiser's work, found that Kaiser had apparently counted the years 1700 and 1800 as leap years ; at any rate, he had somehow count- ed two days too many in the interval of about 200 years. This excess of one day per century really corresponded to a defect of 37m. 23s., since a Martial rotation too many had been of course introduced along with the extra day, and a Martial rotation requires 24h. 37m. 23s. instead of 24h., so that the additional terres- trial day fell short by 37m. 23s. of what was really required to provide (so to speak) for the added Martial day. Now if we divide 37m. 23s. by the number representing all the Mar- tial rotations in a century, or roughly if we divide 2,250 seconds by 35,000, we obtain about 0-065 of a second ; and adding this to Kaiser's value of the rotation period, we obtain 24h. 37m. 22'685s. This is near enough to the wri- ter's value (24h. 37m. 22'735s.) to show that both Huygens's picture and Hooke's can be re- lied upon, the difference resulting merely from such errors in drawing as might be expected. We may therefore assign a part of the differ- ence to error in Hooke's picture, and the rest to error in Huygens's. After a critical exam- ination of the two pictures for this purpose, the present writer finds that the most proba- ble value of the rotation period is 24h. 37m. 22'715s., the true value almost certainly lying between 24h. 37m. 22'7s. and 24h. 37m. 22 : 73s. This is the only case in which the rotation of a planet has been determined, or probably can be determined, with so great an approach to ex- actness. The marks on Mercury and Venus are too uncertain to be trusted, and the planets Jupiter and Saturn do not in all probability show their real surface to us. The determina- tion of a planet's rotation period is not a mere matter of curiosity ; for when the period has been determined with considerable accuracy, a planet may be regarded thenceforth as a sort of celestial chronometer, by which changes in the rate of other motions may be inferred if not gauged. The surface of Mars has been carefully studied by many skilful observers. Hooke, Cassini, Huygens, and Fontana were the first to recognize any of those markings which are now known to belong to the real surface. They noticed that the polar parts of the planet appeared to be occupied by white matter ; the idea does not seem to have presented itself that this matter might be like the snow and ice which are found in the polar regions of our earth. On the contrary, when Maraldi in 1720 studied these white regions, and found that one of them had diminished in size, he predicted 'its entire disappearance. Tt was not until Her- schel had carefully examined them for a con- siderable time, and found their variations to correspond to the progress of summer and win- ter in the northern and southern hemispheres of the planet, that the resemblance between the white spots and our arctic and antarctic snows was recognized, and that Herschel, ever on the watch for analogies of the kind, express- ed the opinion that these spots are the snows of Mars. The other parts of the planet present two chief colors, a faint ruddy tint, apparently representing the continents of the planet, and a still fainter indigo-green tint, which from an early period has been regarded as indicating the presence of seas and oceans upon that dis- tant world. It was for a long time impossible, however, as Dr. Whewell pointed out, to be certain that this interpretation of the white spots and of the greenish markings was cor- rect, or that water existed in any form on the surface of Mars. But recently the spectrosco- pic analysis of the light of Mars has shown be- yond question that at times the vapor of water exists in the planet's atmosphere, since the same bands are seen which appear in the solar spectrum when the sun is low down and shining through the denser and more moisture-laden parts of our atmosphere. Nevertheless, it is not easy to understand how the condition of Mars as to temperature can so nearly resemble the earth's, as we should have to believe if we considered only the relative extent of the snowy polar regions in the two planets. Mars is so much further from the sun that the solar radi- ation is reduced, as compared to that to which our earth is exposed, in the ratio of about 1 to 2 when Mars is in perihelion, and of about 1 to 3 when he is in aphelion. Moreover, be- ing a much smaller planet, we should expect his atmospheric envelope to be much less dense, since if reduced as his volume it would be re- duced as the cube of his linear dimensions; whereas it would extend over a surface reduced only as the square of those dimensions. On this assumption there would be less air per square mile of the planet in the proportion of about 1 1 to 20 ; and as gravity at the surface of Mars is less than gravity on the earth in the ratio 387 to 1,000, the atmospheric pressure would be less as about 4,257 to 20,000, or would be little more than one fifth that of the earth's sea level. Of course, we have no certain assurance that the assumption here made is even approxi- mately correct. But since, to make the climate of Mars as warm as our earth's, the atmosphere should be much denser than ours, whereas the assumption which must be regarded as the most probable would make the atmospheric density barely one fifth of ours, it seems diffi- cult to regard the climate of Mars as probably like that of our earth. There are reasons, therefore, for viewing as at least worthy of