ICE AND GLACIERS. 125 



determined by himself and afterwards by Agassiz, and 

 they found that each year it had moved downwards. 

 Fourteen years later, in the year 1841, it was 4,884 feet 

 lower, so that every year it had on the average moved 

 through 349 feet. Agassiz afterwards found that his 

 own hut, which he had erected on the same glacier, had 

 moved to a somewhat smaller extent. For these observa- 

 tions a long time was necessary. But if the motion 

 of the glacier be observed by means of accurate measuring 

 instruments, such as theodolites, it is not necessary to 

 wait for years to observe that ice moves — a single day 

 is sufficient. 



Such observations have in recent times been made 

 by several observers, especially by Forbes and by Tyn- 

 dall. They show that in summer the middle of the 

 Mer de Glace moves through twenty inches a da,y, while 

 towards the lower terminal cascade the motion amounts 

 to as much as thirty-five inches in a day. In winter the 

 velocity is only about half as great. At the edges and 

 in the lower layers of the glacier, as in a flow of water, 

 it is considerably smaller than in the centre of the sur- 

 face. 



The upper sources of the Mer de Grlace also have 

 a slower motion, the Glacier du Geant thirteen inches 

 a day, and the Glacier du Lechaud nine inches and a 

 half. In different glaciers the velocity is in general 

 very various, according to the size, the inclination, the 

 amount of snow-fall, and other circumstances. 



Such an enormous mass of ice thus gradually and 

 gently moves on, imperceptibly to the casual observer, 

 about an inch an hour — the ice of the Col du Geant 

 will take 120 years before it reaches the lower end 

 of the Mer de Glace — but it moves forward with un- 

 controllable force, before which any obstacles that man 

 could oppose to it yield like straws, and the traces 



