208 On the Descent of Glaciers by their Weight only. [Jan. 



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 glacier per square inch 

 of surface per day, and reconverted into mechanical work there, 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 prolon- 

 gation of a glacier backward, and in its higher slopes, from which reservoir 

 the pressure is supposed to come which crushes the glacier over the obsta- 

 cles 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 conceived to be sepa- 

 rated from the rest throughout its whole length, except for the space of one 

 inch, so that throughout its whole length, except for that one inch, its de- 

 scent 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 its descent, except at its end, must press against its end, by 

 reason of its weight, with a force of 194*42 lbs. But the cubical inch of 

 solid ice at its extremity opposes, by the shear of its three surfaces, whose 

 attachment to the adjacent ice is unbroken, a resistance of 3 X 75 lbs., or 

 225 lbs. That resistance stops therefore the descent of this strip of ice, 

 one mile long, having no other resistance than this opposed to its descent, 

 by reason of its detachment from the rest*. It is clear, then, that it could 

 not have descended by its weight only when it adhered to the rest, and 

 when its descent was opposed by the shear of its whole length; and the 

 same may be proved of any number of miles of strip in prolongation of 

 this. Also, with obvious modifications, it may be shown, in the same way, 

 to be true of any other similar strip of ice in the glacier, whether on the 

 surface or not, and therefore of the whole glacier. 



It results from this investigation that the weight of a glacier is insuf- 

 ficient to account for its descent ; that it is necessary to conceive, in addi- 

 tion to its weight, the operation of some other and much greater force, 

 which must also be such as would produce those internal molecular dis- 

 placements and those strains which are observed actually to take place in 

 glacier ice, and must therefore be present to every part of the glacier as its 

 weight is, but more than thirty-four times as great. 



* If, however, the glacier were inclined at 35° 10', instead of 4° 52', and a strip were 

 detached from its surface, as described above, it would equal the shear of one cubic inch 

 at its lower end, if it were 300 yards long, and if the glacier were vertical, when it was 

 172-8 yards long. 



