37 
because the same thrust of dilatation of the element at X would not be able 
to push so great a length of the bar up the plane as it would down it. 
In this state of the temperature of the plate, let a point Xi be taken 
such that, if it were divided there, the strain necessary to pull the part Xi Ai 
down the plane would just equal that necessary to pull Xi Bi up it. Let 
the temperature of the element at Xi be so diminished as by its contraction 
just to produce this strain, and let the temperatures of all the elements from 
Xi to Ai in succession be similarly reduced. Each will contract more than 
the one before it, because a less resistance will be offered to its contraction ; 
and the displacement Ai A2 of Ai down the plane will equal the sum of these 
separate contractions. In the same way the displacement Bi B2 of Bi up the 
plane will equal the sum of the separate contractions of the elements of Xi Bi . 
The point Xi will be further from Ai than Bi because the same strain of con- 
traction of an element at Xi would pull a greater length of the bar down 
the plane than up it. 
It is by the dilatation of the greater length of the plate XB favoured by its 
weight that the extremity B is displaced down the plane when the temperature 
is raised ; whilst it is by the contraction of the less length Xi B against its 
weight that it is displaced up the plane when the temperature is lowered. 
The extremity B is therefore more displaced down the plane by a given 
raising of the temperature than it is displaced up it by a corresponding 
lowering. On the whole, therefore, the extremity B is made to descend the 
plane by a given alternation of temperature. It is by the dilatation of the 
less length X A that the extremity A is displaced up the plane, and by the 
contraction of the greater length Xi Ai that it is displaced down the plane. 
It is therefore less displaced up by dilatation than it is down by contraction, 
and on the whole it descends by a given alternation of temperature. Both 
the extremities A and B of the plate are therefore made to descend when it 
is subjected to a given elevation and then to a corresponding depression of 
its temperature ; that is, the whole plate is made to descend. 
Now, a theory of the descent of glaciers, which I have ventured to 
propose myself, is that they descend, as the lead in this experiment does, by 
reason of the passage into them and the withdrawal of the sun's rays, and 
that the dilatation and contraction of the ice so produced is the proximate 
cause of their descent, as it is of that of the lead. All Alpine travellers, from 
De Saussure to Forbes and Tyndall, have borne testimony to the intensity of 
the solar radiation on the surfaces of glaciers. ' 4 1 scarcely ever, " says Forbes, * 
"remember to have found the sun more piercing than at the Jardin." This 
"Forbes' Travels in the Alps," p. 97. 
