348 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 66. 



over a denser medium. 1. Because this elevation is gradual, rising higher to- 

 wards the moon, according to the degree of the density of its atmosphere. 3. 

 Because this elevation of the sun against the cusps seemed to be a little higher 

 than a mountain wliich jetted out the most on aside view. 4. Because the 

 penumbra, which was observed at the ingress and egress, and which certainly 

 is much less defined than towards the middle, is very probably occasioned by the 

 mixture of the rays, which are refracted in passing through the lunar atmosphere. 

 This appears to be more likely than that it should be caused by the inflexion of 

 light, from the globe of the moon; because such inflected light sliding on the 

 globe ought rather to lessen its dark, limb; whereas, on the contrary, the 

 penumbra, which was sufficiently thick, served to enlarge that limb. He thinks 

 he has seen enough to conclude, that the moon's atmosphere extends to at least 

 double the height of the highest mountains in it; and that it has some dry 

 cavities, which sink below the disk. The most surprizing circumstance, was that of 

 the mountains seen in profile, and perfectly welldefined. The3 summits of N°. 1 , 2, 

 and 3, were very distinct, and especially the hollows between them, surprizing on 

 account of their depth, for they made cavities within the circle of the disk. I am 

 no longer astonished at the luminous points, which are seen starting from the 

 crescent; but I wonder that no notice should hitherto have been taken of these 

 mountains seen sideways in the eclipses of the sun : a 4-foot telescope well fixed is 

 necessary to observe them distinctly. Large refracting telescopes cannot serve 

 for that purpose, as it is difficult to prevent entirely their shaking. 

 Uy He is inclined to believe that there is no water in the moon, for the two 

 cavities which penetrated within the disk, continued concave to the bottom, 

 whereas if there had been there any water, the bottom ought to have been 

 convex. The mountains in the moon must be very high and hollow, and that 

 is the reason of their white circle. If ever Tycho could be seen edge-ways, 

 it would make a beautiful appearance. 



2. Observation of the Eclipse of the Sun the 5th Aug. 1766, near the Cour- 

 gainat Ca/aw.— Beginning 5^ 39™ 9% end 7^ 19"" 13^ 



The sun did not set in the sea till about 14 minutes after the end of the 

 eclipse; that is, at least 10 minutes later than the almanack makes it to do at 

 Paris. His lower limb touched the sea at 7^ 23"' in thick vapours, which made 

 Mr. M. prefer the setting of this lower limb rather than the centre. It is easy to 

 conclude the difference which should result from the different situations of Paris 

 and Calais. No inequalities nor mountains were discernible, nor could any be 

 seen on that part of the moon. The elevation of the light of the sun's cusps 

 was but small at the middle of this eclipse (about as much as in the last eclipse) 

 enough, however, to indicate an atmosphere, as in the former observation; but 

 the air was charged with strong vapours, which hindered him from seeing it so 

 distinctly as he then did. 



