18 C. PIAZZI SMYTH ON 



shire, I witnessed a remarkable proof that " cumulus " clouds can assume, and 

 keep up for a short time, an excessive brilliancy of reflection. 



The case was this ; at 6 p.m. when walking in the Park there, and looking 

 S. East, I noted and sketched the half-Moon and a great thunder-cumulus' 5 ' 

 cloud close to it thus, 



The Moon was exceedingly pale as compared with the great mass of the 

 cloud on which the Sun was shining out of a clear sky in the West. The 

 cloud was indeed estimated at 7 to 10 times as bright as the Moon ! This 

 extreme brightness of the cloud however only lasted about half an hour; when 

 its brighter part went down to something like 3 times that of the Moon. 



But as Sir John Herschel well remarked in his " Cape Observations," and 

 when contrasting tclescopically the brightness of the Moon setting behind the 

 summit-cliff of Table Mountain, with that cliff then illuminated by the brilliant 

 rising Sun of South Africa, — their then brightnesses were equal, just as their 

 illuminations by the Sun, at the same distance therefrom in space, were equal 

 also. But the solid surface of the Moon must be a nearly constant of reflect- 

 ing power ; therefore when it was so vastly transcended in brightness by the 

 Buxton cloud in daylight, — the said cloud must have been an anomaly surpass- 

 ing all ordinary terrestrial surface materials in reflective power, just as did the 

 Edinburgh night clouds when distinctly luminous from their reflection of gas- 

 lights far below them. 



What enabled clouds then, on these two or three occasions, to reflect so 

 very strongly whatever light, whether Solar, or artificial, that struck upon 

 them ? The only reason I can suggest powerful enough for the occasion, but 

 one that seems to fulfil all the conditions, is, — that the molecules of the clouds 

 were then in a hollow, vesicular condition, rather than drops or spherules of 

 water. The former state is evidently most compatible with their floating in 



* The thunder-cumulus, when weather is approaching a condition for electric discharge differs 

 from the ordinary cumulus cloud, by being more tightly made up as it were, and with smaller or sort 

 of cauliflower figures. 



