THE ICE AGE IN NORTH AMERICA. 



denly gives out an enormous amount of heat ; on the con- 

 trary, when ice melts, a corresponding amount of heat is 

 absorbed in accomplishing the result. To melt a cubic foot 

 of ice requires as much heat as to raise a cubic foot of water 

 80° C. or 1M Fahr. 



For our knowledge of the nature of the movements tak- 

 ing place in glaciers, we are largely indebted to the investi- 

 gations of Louis Agassiz and Professor Forbes between the 

 years 1840 and 1842, and later to more detailed investigations 

 of Professor Tyndall and other physicists. The mode of 

 measurement with all these investigators was essentially the 

 same. Stakes were driven across a glacier in a line at right 

 angles to the direction of the movement ; and, by means of a 

 theodolite, accurate notations were taken, from hour to hour 

 and day to day, of any changes in the relative position of 

 the points where the stakes were driven. The uniform re- 

 sult of these observations was that the 

 line of stakes began immediately to curve 

 slowly down near the middle, showing 

 that the motion on the surface was greater 

 near the middle than on the sides. This 

 curve continued to increase as long as 

 the stakes remained standing. 



Professor Tyndall's observations show 

 also that the most rapid line of motion 

 on the surface of a glacier is not exactly 

 in the middle ; but that, wherever there 

 is a bend in the glacial current, the more 

 rapid movement is uniformly on the con- 

 vex side of the channel, so that the curve 

 of the line of most rapid motion is more tortuous than that 

 of the main channel. This conforms to the facts concerning 

 the movement of water in a crooked river-bed, and illustrates 

 again the analogy between the movement of ice and that of 

 water. 



The most rapid motion observed by Tyndall, in the sum- 

 mer time, in the center of one of the largest of the Alpine 



a Z c <£ e f 3 



V 



;,3 



Fig. 1.— The letters a, b, 

 c, d. e, f, g, represent 

 stakes driven across the 

 surface of a glacier, at 

 right angles to its line 

 of motion ; a', b', c', 

 d', e', /', q\ represent 

 these positions at a 

 subsequent stage. 



