10 SEVENTH REPORT—1837. 
calculation ; but it is hoped that the results will enable the eminent 
individuals who take the lead in such inquiries, to connect Glasgow 
and Dundee with the other ports around Britain, at which good obser- 
vations have been made. 
It is a singular circumstance, that the time of high water at Port 
Glasgow and Greenock generally precedes the meridian passage of the 
moon instead of following it. This was evident from observations 
made before the meeting of the Association ; but since that period the 
author has been furnished with observations made at Greenock with great 
care, under the superintendence of Mr. James Thomson, civil engineer ; 
and in the former diagram the zig-zag line on the left represents the 
intervals between the meridian passage of the moon and high water, 
from new moon September 29th, till full moon October 13th, 1837. 
When the curve is on the right of the vertical line a B, the times of 
high water are after the moon’s southing ; but when on the left, they 
precede the moon’s southing, and to an extent sometimes of nearly 2 
hours. 
On an Optical Phenomenon observed at Mont Blane. 
By M. De ta Rive. 
When the sun has set at Geneva, it is observed that Mont Blanc 
remains illuminated by its direct rays for a much longer time than the 
surrounding mountains. This phenomenon is owing to the great 
height of Mont Blane. But, after it has ceased to be illuminated, the 
summit of Mont Blanc sometimes reappears at the end of ten or fifteen 
minutes, less intensely enlightened than at first, but nevertheless in a 
manner very decided, and often very brilliant. This phenomenon 
takes place especially when the atmosphere is very pure—highly 
charged with aqueous vapour in an invisible state—and consequently 
very transparent. The author has satisfied himself (by the exact ob- 
servation of the time which elapses between the two successive illumi- 
nations of the mountain, combined with the calculation of the sun’s 
progress) that the phenomenon is due to the rays of the sun which 
traverse the atmosphere at a distance from the earth less than the 
height of Mont Blanc, but greater than half that height, and which 
arrive at rarer regions of the atmosphere, under an incidence so great 
that they are reflected instead of refracted. This interior reflection is 
facilitated by the humidity of that part of the atmosphere which the 
rays traverse until they reach the point of incidence. The reflected 
rays falling on the snowy summit of Mont Blanc, produce this second 
illumination ; and the humidity (by augmenting the transparency of 
the air) renders the illumination more brilliant. 
