240 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/92. 



hence but consider the discovery I here announce, as a very fortunate one, both 

 as it appears to be decisive, and as it may induce future observers to direct their 

 attention to this phenomenon. Admitting the vaHdity of this new observation, 

 which I think cannot well be called in question, I proceed now to deduce from 

 it the following inferences. 



1. It confirms, to a degree of evidence, all the selenotopographic 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 twilight over a zone 

 of the dark hemisphere 2° 34', or 104- geogr. miles in breadth, which shall in 

 intensity exceed the light reflected on its dark hemisphere by the almost wholly 

 illuminated disc of our earth ; and if by an incidental computation, this dense 

 part be found to measure 1336 feet in perpendicular height, it may, according 

 to the strictest analogy, be asserted, that the upper, and gradually more rarefied 

 strata, must at least reach above the highest mountains in the moon. And this 

 will appear the more evident if we reflect that, notwithstanding the inferior 

 degree of gravitation on the surface of the moon, which Newton has estimated 

 at somewhat less than -^ of that on our earth, the lower part of its atmosphere 

 is nevertheless of so considerable a density. This considerable density will there- 

 fore fully account for the diminution of light observed at the cusps, and on the 

 high ridges Leibnitz and Doerfel, when illuminated in the dark hemisphere ; as 

 also for the several obscurations and returning serenity, the eruptions, and other 

 changes I have frequently observed in the lunar atmosphere. This observation 

 also implies, 



1. That the atmosphere of the moon is, notwithstanding this considerable 

 density, much rarer than that of our earth. And this indeed is sufficiently con- 

 firmed by all our other lunar observations. I think I may assert, with the 

 greatest confidence, that the clearer part of our twilight, when the sun is 4° below 

 our horizon, and when we can conveniently read and write by the light we 

 receive from it, surpasses considerably in intensity the light which the almost 

 wholly illuminated disc of our earth reflects on the dark hemisphere of the moon 

 1\ days before and after the new moon. But should we even admit an equal 

 degree of intensity, it will however appear from computation, that our inferior 

 atmosphere, which reflects as strong a light over 4° as that of the moon does 

 over 1° 34'of their respective circumferences, must be at least 8 times higher than 

 that of the moon. 



3. The striking diminution of light I noticed, in my 12 years observations on 

 Venus, likewise indicates that the atmosphere of that planet, which is in many 

 respects similar to ours, is much denser than that of the moon ; and this will 

 be still further corroborated, if we compare together the several measurements 



