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40 life; that its climates, its seasons, and the length of its days totally differ from ours; that without dense clouds (which the Moon has not) there can be no rain; perhaps no rivers, no lakes". His belief in the habitability of our satellite arose from the view that its inhabitants "are fitted to their conditions as well as we on this globe are to ours".

Of the planets, Mercury alone was neglected by Herschel; he studied it only when in transit across the solar disc. His observations on Venus—"an object," he said," that has long engaged my particular attention"—were commenced in April, 1777, and were continued for sixteen years. Herschel had four objects in view—to measure the rotation period of the planet, to ascertain the presence or absence of an atmosphere, to determine accurately the planet's diameter, and to give "attention to the construction of the planet with regard to permanent appearances". He satisfied himself that Venus did rotate, but the diurnal motion, he said, "on account of the density of the atmosphere of this planet, has still eluded my constant attention, as far as concerns its period and direction". The spots which Herschel discovered on the planet's surface were faint and ill-defined. His observations on Venus were in direct contradiction to those of Schröter. The German astronomer had not only estimated the planet's rotation period at 23½ hours, but had announced the existence of mountains on Venus, whose height exceeded five or six times the perpendicular elevation of Chimborazo. "As to the mountains on Venus," said Herschel, "I may venture to say that no eye which is not considerably better than mine, or assisted by much better instruments, will ever get a sight of them." Herschel's negative conclusions have been on the whole confirmed by later astronomers.

Herschel may be safely called the founder of Martian astronomy. In 1777 he commenced observations on Mars, and he early satisfied himself that "the constant