38 
heat passes abruptly into a state of intense cold when any part of the glacier 
falls into shadow by an alteration of the position of the sun, or even by the 
passing over it of a cloud. 
There is no substance the dilatation or contraction of which, by changes 
of temperature, is more thoroughly and accurately known than that of ice. 
Experiments were made upon it by three independent observers, at the 
observatory of Pultowa, in the winters of 1845 and 1846, between the tem- 
peratures of 0°9 R. and 22*82 R., from which it resulted that ice is by far 
the most dilatable of all known solid substances, being nearly twice as 
dilatable as lead.* These experiments have been described by Baron W. 
Struve, in the transactions of the Academy of St. Petersburgh. f 
The ice of a Glacier behaves itself in its descent exactly as the lead did in my 
experiment. The Mer de Glace moves faster by day than by night. § Its mean 
daily motion is twice as great during the six summer as during the six winter 
months. The connection between its rate of motion and the external tempera- 
ture is most remarkable. It has been carefully observed, and the results as 
recorded by Prof. Forbes|| leate no doubt of the fact, that no change of external 
mean temperature is unaccompanied by a corresponding change of glacier motion. 
From this it follows that the two are either dependent on same common 
cause, or that the one set of changes stands in the relation of a cause to the 
other. That both sets of phenomena— the changes of the sun's heat, and 
the changes of glacier motion — should be due to some common independent 
cause, seems impossible. We are forced therefore on the conclusion that 
one is caused by the other. And as the changes in the glacier motion 
cannot cause the changes of solar heat, it must be the changes of solar 
heat which cause the changes of glacier motion. Nor is this to be considered 
a startling or improbable conclusion. Heat is but another form of 
mechanical power. This power is constantly streaming into the glacier; 
for ice is dia-thermanous. It is readily penetrable by radiant heat- 
This has been shewn by Tyndall; who having sent a beam of heat 
through a block of Wenham Lake ice, saw its course starred by the 
dilatations of the ice. I have, moreover, myself obtained an ice lens by 
causing ice to be turned in a lathe to a spherical surface by means of a 
templet of iron. Through this lens the rays of the sun streamed in 
abundance, and were concentrated in its focus with such intensity as to burn 
the hand and instantly set fire to a match. There can be no doubt therefore 
* The coefficient of the dilatation of ice is -00002856 for 1° F. 
+ "Forbes' Travels in the Alps," p. 97. 
% Forbes' Occasional Papers, p. 12. 
|| Forbes' Travels in the Alps, p. 148. 
