360 OF THE PHÆNOMENA AND AFFECTIONS

lunar mountains are much higher than any wehave upon the earth.

Most astronomers are of opinion that themoon has no atmosphere, because she is neverobscured by clouds or vapours, and because thefixed stars, at the time of an occultation, dis-appear instantaneously, without any gradual di-minution of their light. But if we consider theeffects of days and nights, which are near thirtytimes as long as with us, we may readily grantthat the phænomena of vapours and meteorsmust be very different. And besides, the va-pourous or obscure part of our atmosphere isonly about the 1980th part of the earth’s dia-meter, as is evident from the height of theclouds, which is seldom above three or fourmiles j and, therefore, as the moon’s apparentdiameter is only about thirty-one minutes anda half, or 1890 seconds, the obscure part of heratmosphere, when viewed from the earth, mustsubtend an angle of less than one second; whichis so small a space, that observations must be ex-tremely accurate to determine whether the sup-posed obscuration takes place or not.

In looking at the moon through a telescope,we constantly observe the same face, and fromthis it is evident that she turns only once roundupon her axis in the time of every periodicalrevolution ; so that the inhabitants of the moonhave but one day and night in the course of a

month.