on the Atmosphere of the Moon. 351 
tially interrupt, or, at times, wholly prevent this crepuscular 
light, either at one or the other cusp, and sometimes at both. 
I cannot hence but consider the discovery I here announce as 
a very fortunate one, both as it appears to me decisive, and as 
it may induce future observers to direct their attention to this 
phsenomenon . 
Admitting the validity of this new observation, which, I 
think, cannot well be called in question, I proceed now to de- 
duce from it the following inferences. 
1. It confirms, to a degree of evidence, all the selenotopo- 
graphic observations I have been so successful as to make, on 
the various and alternate changes of particular parts of the 
lunar atmosphere. If the inferior and more dense part of this 
atmosphere be, in fact, of sufficient density to reflect a twi- 
light over a zone of the dark hemisphere 2 0 34', or 10^ geogr. 
miles in breadth, which shall in intensity exceed the light re- 
flected upon its dark hemisphere by the almost wholly illu- 
minated disk of our earth ; and if, by an incidental computa- 
tion, this dense part be found to measure 1356 feet in per- 
pendicular height, it may, according to the strictest analogy, 
be asserted, that the upper, and gradually more rarified strata, 
must, at least, reach above the highest mountains in the moon. 
And this will appear the more evident, if we reflect that, not- 
withstanding the inferior degree of gravitation on the surface 
of the moon, which Newton has estimated at somew'hat less 
than ^ of that on our earth, the lower part of its atmosphere 
is, nevertheless, of so considerable a density. This considerable 
density will, therefore, fully account for the diminution of 
light observed at the cusps, and on the high ridges Leibnitz 
and Doerfel, w'hen illuminated in the dark hemisphere ; as 
Z z 2 
