512 THE LIFE OF JAMES D. FORBES. [CHAP. 



when he commenced his work, as it is that of many 

 views on the subject which are even DOW persistently 

 maintained, and stoutly defended with the choicest 

 weapons of the mathematician o the physicist. 



It is difficult in thought to place ourselves in the 

 position of Forbes at the time when his discoveries were 

 made, but the more successfully we attempt to do so, the 

 more are we led to admire the cautious boldness and 

 the indistractable penetration of the young explorer. 



This is not a scientific but a historical work, so that 

 here is not the place to enter fully into the physical ex- 

 planation of glacier motion, though it may be well to 

 show briefly that in all probability J. Thomson's beautiful 

 discovery of the dependence of the freezing-point on 

 pressure, which has already explained much, will ulti- 

 mately be found capable of explaining almost all that is 

 at present known about the formation, structure, and 

 motion of glaciers. This was not Forbes' opinion, so that 

 we will first give a letter which he wrote a few years 

 ago, with the full intention that it should some day 

 be published, containing a brief but complete statement 

 of his final views as to the physical explanation of the 

 facts of observation whose ensemble was concisely 

 enunciated as the Viscous THEORY. In one of his 

 letter-books, in which are kept in copying ink duplicates 

 of almost every note he ever wrote, it has the following 

 heading : 



' On some Debated Points connected with the Properties 

 of Ice and the Motion of Glaciers. 



' MY DEAR PROF. TAIT, January I2th, 1865. 



' ... It occurs to me that I may as well note 

 down the ideas I expressed to you the other day as to 

 the properties of ice in relation to its plasticity. Though 

 untoward circumstances have made me very shy of 

 publishing anything which can possibly lead to con- 

 troversy on this vexed subject, I am glad to put on 

 record the opinions which I have for many years enter- 

 tained. 



