﻿234 Royal Society. 



To determine how great a force, in addition to its weight, would 

 be necessary to cause the descent of a glacier of uniform section and 

 slope such as has been supposed in the calculations, let u represent, 

 in inch-lbs., the work of that force in twenty-four hours. Then 

 assuming the unit of shear (ju) in glacier ice to be 75 lbs., it follows, 

 by the principle of virtual velocities, that 



m=94134000 + 1012560-2668400 

 = 92478160 inch-lbs. = 77065 13 foot-lbs.* 



This computation has reference to half only of the width of the^ 

 glacier, and to 23'25 inches of its length. The work, in excess . ot 

 its weight, required to make a mile of the imaginary glacier, 466 

 yards broad and 140 feet deep, descend, as it actually aoes descend 

 r>er twenty-four hours, is represented by the horse-power of an engine, 

 which, working constantly day and night, would yield this work, or by 



2 x 7706513 X 5280 X 12 = 883 . 78 h> 

 l^RTx 24 X 60 X 33000" 



The surface of the mass of ice, on which the work u is required 

 to be done, in aid of its weight, to make it descend as it actually 

 does is 124771-5 square inches. The work required to be aone 

 on each square inch of surface, supposing it to be equally distributed 



over it, is therefore, in foot-lbs., J^tW^^ 1 ' 7 ^' 



These 61*76 foot-lbs. of work are equivalent to '0635 heat-units, 

 or to the heat necessary to raise '0635 lb. of water by one degree of 

 Fahrenheit. This amount of heat passing into the mass of the gla- 

 cier per square inch of surface per day, and reconverted into mecha- 

 nical work Mere, would be sufficient, together with its weight, to 

 bring the glacier down. 



The following considerations may serve to disabuse some persons 

 of the idea of an unlimited reservoir of force residing somewhere in 

 the prolongation of a glacier backward, and in its higher slopes, from 

 which reservoir the pressure is supposed to come which crushes the 

 glacier over the obstacles in its way. < 



Let a strip of ice one square inch in section, and one mile in 

 length, in the middle of the surface of the imaginary glacier, be con- 

 ceived to be separated from the rest throughout its whole length, ex- 

 cept for the space of one inch, so that throughout its whole length, 

 except for that one inch, its descent is not retarded either by shear 

 or by friction. Let, moreover, this inch be conceived to be at the 

 very' end of the glacier, so that there is no glacier beyond it. Now 

 it may easily be calculated that this strip of ice, one inch square and 

 one mile long, lying on a slope of 4° 52', without any resistance to 



^ohrme as soft puttv, and its consistency about the same, it would descend by its 

 weight only, without the aid of any other force. It would not, however, be pos- 

 sible to walk on such ice. .... 



* Thus the work to be done in aid of the weight is thirty-four times the work 



of the weight. 



