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Rh planet varied so much that no reliance could be placed on the result as a means of ascertaining whether our day remains the same from age to age.

Herschel considered the planet Mars a more favourable field for experiment than Jupiter. On Mars he saw spots of a different nature: "Their constant and determined shape, as well as remarkable colour, show them to be permanent and fastened to the body of the planet. These will give the revolution of his equator to a great certainty, and by a great number of revolutions, to a very great exactness also." A circumstance, with which Herschel was not acquainted, materially helped him in his observations on Mars. The atmosphere on that planet is not nearly so dense as the earth's, and similar trade-wind belts to those on Jupiter do not seem to exist. By these means he concluded that the length of a day on Mars is a little longer than our day, or 24 hrs. 39 min. 5 sec. The value of an accurate measure of the length of day in other planets he conceived to be this: "Future astronomers may be enabled to make some estimate of the general equability of the rotatory motions of the planets. For if in length of time they should perceive some small retardation in the diurnal motion of a planet, occasioned by some resistance of a very subtle medium in which the heavenly bodies perhaps move, or, on the other hand, if there should be found an acceleration from some cause or other, they might then ascribe the alteration either to the diurnal motion of the earth, or to the gyration of the other planet, according as circumstances, or observed phenomena,