6 Professor Forbes' Account of his recent 



bly vertical so long as it stands at all ; for the velocity of the 

 surface is sensibly the same as that at 10 or 20, or probably 

 even 100 feet deep in most glaciers. It is only near the bot- 

 tom or bed that the velocity is materially affected, as I have 

 found also, that, in respect to breadth, it is in the immediate 

 neighbourhood of the sides that the velocity diminishes rapid- 

 ly, and that, for half its breadth in the centre, the velocity does 

 not vary by more than from ^'g to ^-^ of its amount. It is 

 farther worthy of notice, that whenever a glacier is of no great 

 thickness, and, at the same time, highly inclined, that is, in 

 circumstances calculated to produce a great difference between 

 the motions of points of the glacier in a vertical line, there 

 the fissures are not transverse but radiated, as in almost all 

 glaciers of the second order, and, therefore, the fissures are 

 not liable to distortion. 



I might put it rather as a direct result of observation than 

 as a hypothesis, that the motion of a glacier resembles that of 

 a viscid fluid, not being uniform in all parts of the trans- 

 verse section, but the motion of the parts in contact with the 

 walls being determined mainly by the motion of the centre ; 

 but it yet remains to be shewn what is the cause of the pres- 

 sure which conveys the motion, whether it is the mere weight 

 of the semifluid mass, or the dilatation of the head of the glacier 

 pushing onwards. The answer to this question involves the 

 fate of the rival theories of De Saussure and De Charpentier. 

 I still entertain the same difiiculties with respect to both, 

 which I have stated in an article in the Edinburgh Review ; 

 but these difficulties amount, I think, to a proof of insufficien- 

 cy, if taken in connection with the observations which I have 

 made this summer. On the one hand, if it were possible that 

 the glacier could slide by the mere action of gravity in a trough 

 inclined only 3, or 4, or 5 degrees, it is probable that one of two 

 things would happen ; either it would slide altogether with an 

 accelerated velocity into the valley beneath, or else it would move 

 by fits and starts, heing stayedbyobstacles until these were over- 

 come by the melting of the ice beneath, or by the accumulated 

 weight of snow above and behind. Now, neither of these 

 things happen ; the glacier moves on day and night, or from 

 day to day, with a continuous regulated motion, which, 



