1869.] of the Descent of Glaciers by their Weiyht only. 207 



thrown in, a few at a time, and driven home by sharp blows of a mallet on 

 a wooden cylinder. By this means a solid cylinder of ice was constructed, 

 accurately fitting the hole. Weights were then suspended from the rope, 

 passing over the pulley until the cylinder of ice was sheared across. As 

 by the melting of the ice, during the experiment, the diameter of the 

 cylinder was slightly diminished, it was carefully measured with a pair of 

 callipers. 



1st experiment. — Radius of cylinder '65625 in., sheared with 98 lbs. 



2nd experiment. — Radius of cylinder '7031 2 in., sheared with 119 lbs. 



By the first experiment the shear per square inch, or unit of shear, was 

 72*43.3 lbs. ; by the second experiment it was 76*619 lbs. The main unit 

 of shear of ice, from these two experiments, is therefore 75 lbs. 



Now it appears by the preceding calculations, that to descend by its own 

 weight, at the rate at which Prof. Tyndall observed the ice of the Mer de 

 Glace to be descending at the Tacul, the unit of shearing force of the ice 

 could not have been more than 1*3193 lb.* 



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 (//) in glacier ice to be 75 lbs., it follows, by the principle of virtual 

 velocities, that 



2^=94134000+ 1012560-2668400 

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



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 of its weight, re- 

 quired to make a mile of the imaginary glacier, 466 yards broad and 140 

 feet deep, descend, as it actually does descend per twenty-four hours, is 

 represented by the horse-power of an engine, which, working constantly day 

 and night, would yield this work, or by 



2x 7706513x5280x 12 QOO _ , 

 23*2x24x60x3:5000 =883,/8 h ' P- 

 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 

 124 77 1 '5 square inches. The work required to be done on each square 

 inch of surface, supposing it to be equally distributed over it, is therefore, 



in foot-lbs., 7706513 =61*76. 

 124771*5 



* By an experiment on the shearing of putty, similar to that which was made on the 

 shearing of ice, its unit of shear was found to vary from 1 lb. to 3 lbs., according to its 

 degree of hardness. If ice were of the same weight per unit of volume as soft putty, 

 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 possible to walk on such ice. 



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

 weight. 



