210 



Mr. W. R, Browne. 



ice, are those of Moseley ("Phil. Mag.," January, 1870). He found 

 that with pressures from 100 to 110 lbs. per square inch, cylinders of 

 ice sheared slowly across the two planes in contact, sliding over each 

 other without losing continuity. The distance sheared through was 

 about f inch in half an hour. A load of 119 lbs. per square inch was 

 sufficient to shear through a cylinder of 1-J inches in diameter in two 

 to three minutes. From these experiments it would appear that the 

 lowest shearing stress which will cause ice to flow is about 100 lbs. 

 per square inch ; but sufficient time was not allowed in the experi- 

 ments to make this a matter of certainty. 



There is another way in which the shearing resistance of ice may 

 be tested. In the case of a block of ice of vertical sides, gravity of 

 course produces a shearing resistance along all planes passing through 

 the base. Let h be the height of such a block in feet, and consider 

 the shearing force due to gravity on any square foot of a plane making 

 an angle with the vertical. This shearing force is given by — 



vjh X h tan 



"2 Xcos ^ wh 



=~7y sin cos 0. 



h sec z 



This expression is a maximum when =45°, and its value is then — 



wh 

 4 ' 



What is the greatest height at which a vertical cliff of ice will 

 stand ? I am not able to state this precisely, but it is very consider- 

 able. Mr. Whymper mentions crevasses in South America 300 feet 

 deep. Cliffs of fully that height have been seen standing out of water 

 in the case of icebergs, and as so small a part of an iceberg projects 

 above water, these cliffs probably extend below to a considerable 

 depth. Taking, however, only 300 feet for the value of h, or for the 

 maximum height of an ice cliff, this would give about 30 lbs. per 

 square inch as the lowest shearing force upon a plane of ice which 

 would cause it to assume the condition of flow. 



Let us now suppose a glacier of thickness a, lying upon a slope 

 whose inclination to the horizontal is /3 : then the force per square 

 foot, tending to shear the ice at its junction with the slope, is clearly 

 aw sin (3. 



Supposing sin (3 to equal J, and that the shearing resistance is 

 30 lbs. per square inch, we get. a = about 290. Hence we may say 

 that a glacier lying on a slope of 1 in 4 will not move at all under its 

 own weight, unless it be at least 300 feet thick, and that if it be more 

 than this, the upper 300 feet will move as one solid mass, the part 

 below alone representing the conditions of flow. 



It is needless to say that there are hundreds of glaciers which are 



