39 
that the rays of the sun, which in those Alpine regions are of such 
remarkable intensity, find their way into the depths of the glacier. Tbey 
are a power, and there is no such thing as the loss of power. The 
mechanical "work" which is their equivalent, and into which they are 
converted when received into the substance of a solid body, accumulates 
and stores itself up in the ice under the form of what we call elastic force 
or tendency to dilate ; until it becomes sufficient to produce actual dilatation 
of the ice in the direction in which the resistance is weakest ; and by its 
withdrawal to produce contraction. From this expansion and contraction 
follows of necessity the descent of the glacier. How much heat entering 
the surface of a glacier is necessary to this result has been made the subject 
of calculation. Supposing the depth of the ice to be the same as that at the 
Tacul, its motion at different depths that which Tyndall found it to be 
there, and its surface motion that which he measured lower down the Mer 
de Glace at LesPonts, and supposing the resistance to shearing of ice to be 751bs. 
per square inch ; then the mechanical work, which acting within the mass is 
necessary to put the glacier in motion, as it actually moves is Glf units of 
work per square inch of the surface of the glacier per day. Now this quantity 
of work would be supplied by .0635 heat -units entering the ice per square inch 
of surface per day and diffusing itself through it, each heat-unit being the heat 
necessary to raise 1 lb. water by 1°F. Far more than this heat probably 
falls on the surface of a glacier. 
It has been argued in opposition to this theory that the temperature of the 
ice of glaciers is, by the observations of Agassiz.J but very little below 32°, 
and that if radiant heat found its way into it, it would not expand, but 
melt it. To this there is the obvious answer that radiant heat does find its 
way into ice as a matter of common observation, and that it does not melt 
I The observations of Agassiz on the temperature of the ice of the Aar 
glacier in 1841, 1842, were made in borings from 15 to 200 feet deep. 
Thermometers placed in these borings never fell below 0.3 Cent., although 
the external temperature descended at night to 5° or 6 Q Cent. Commonly 
they shewed exactly zero. (Bulletin de Geneve, torn. 44, p. 349. J Nothing 
can, however, be concluded from these experiments because the thermometers 
were not frozen into the ice of the glacier, or the mouths of the borings so 
effectually stopped as to prevent the access of air, or the percolation of 
water from the disintegrated ice near the surface. The maintenance of a 
constant state of humidity in the air of the boring not being thus provided 
against, the included thermometer could not but remain at zero, however 
low might be the temperature of the surrounding ice. For the water of the 
contained air freezing on the sides of the boring (like hoar frost) would raise 
the temperature around the bulb, by the latent heat set free in freezing, to 
zero, Cent. And the humidity of the air being continually renewed this 
process would always go on. 
