J. LeConte on the Descent of Glaciers. 337 
The total descent @) 0 yf the point B, by the elongation and 
contraction; being obvtously equal go’ the sexcess ‘of its ‘descent 
above its ascent, is, wit determined by fee equation 
a d=4Lel +5, tan a ~4Le I=; tan a 
: tan a 4 
sd-Le. ona’) 
His attention having been first drawn to the influence of varia- 
tions in temperature to cause the descent of a sheet of metal rest- 
ing on an inclined plane, by observing that a portion of the lead 
which covers the south side of the choir of the Bristol Cathedral, ° 
which had not been properly fastened, had descended bodily 
eighteen inches into the gutter; Mr. Moseley proceeds to show, 
by the application of the foregoing formula (2), that the cause 
“assigned is adequate to account for the fact observed. Thus far 
Mr. Moseley’s reasoning is perfectly satisfactory aud in accord- 
ance with what might be expected from one of the clearest and 
best writers on mechanical philosophy: but when he attempts to 
apply these facts and principles to the solution of the mechanical 
problem of the motion of glaciers, the physical aspect of his rea- 
soning is singularly exceptionable. It is to this portion of the pa- 
per, that the attention of the reader is bectage o- directed. 
After referring to the recent experiments of MM. Schumacher, 
Pohrt and Moritz, which show that the linear expansion of ice 
for a given increment of temperature, is greater than that of any 
other solid body hitherto examined; Mr. Moseley remarks, “Ice, 
therefore, has nearly twice the expansibility of lead, so thata 
sheet of ice would, under co circumstances, have descended 
a plane similarly inclined, twice the distance that the sheet of 
lead, referred to in the rboedioe article, descended. Glaciers are, 
on an increased scale, sheets of ice placed upon the slopes 
_—. and subjected to atmospheric variations of tempera- 
re throughout their masses, by variations in the quantity and 
ve stipe of the water, which, flowing from the surface, every- 
where percolates them. That the must, from this canse, descen 
into the valleys, is a aia certain.” (p. 64.) To show the ade- 
quacy of this cause, he applies the formula (2) to the Glacier de 
Léchaud, the Glacier du Géant, and the Mer de Glace. From the 
data in relation to length and inclination, furnished by the meas- 
urements of Prof. J. D. Forbes and others, assuming the mean 
daily variation of temperature to be 5°°321 Centigrade,* and the 
linear expansion of ice for an interval of 100° Centigrade to be 
aa 
* 
eT a a. ge ee a 
= Ace ee 
* This is the mean daily range of temperature, according to the observations of 
De ote in the month of July, at the Col du Géant ; and, of course, has refer- 
ence to the air and not to the mass of the glacier. 
icon Sertss, Vol. XX, No, 60.—Nov., 1855. 43 
