94 Rev. C. Trotter. Physical Properties of Ice [Jan. 29, 



which can be produced in a descending glacier by gravity ; and that 

 therefore the sheariog which the measurements of Forbes and others 

 have shown to be an essential part of the motion of a glacier cannot 

 be produced by gravity alone. 



Mr. W. Mathews ("Alpine Journal," vol. iv, pp. 411 — 427), Mr. J. 

 Ball (" Phil. Mag.," vol. xl, pp. 1 — 10), and others replied to Canon 

 Moseley, but none of the answers were altogether complete, though 

 both the above-mentioned authors clearly pointed out the weak point 

 in Moseley 's argument. 



Moseley's objection is no doubt decisive if the shearing strength of 

 ice as deduced from his experiments represents even approximately 

 the resistance to shearing under the actual circumstances of glacier 

 motion. But, as Mr. Mathews pointed out {loc. cih, p. 426), the 

 time during which the shearing force acted is an important element 

 in the experiments. Moseley found (" Phil, Mag.," vol. xxxix, p. 8) 

 that when the shearing took place in three minutes the shearing 

 strength was about 118 lbs. per square inch, while when the operation 

 lasted thirty minutes the result was about 112 lbs. per square inch. 

 In one experiment the ice sheared in thirfcy-six minutes under a force of 

 about 98 lbs. per square inch. Mr. Mathews pertinently remarks, 

 " I am curious to know what weight would have sheared the ice if a 

 day had been allowed for its operation." I have attempted to answer 

 a question of this kind in the experiments recorded in the present 

 paper. 



In the section of the paper which immediately follows I have 

 given a short notice of the most important recent experiments upon a 

 small scale which bear upon the question of the viscosity of ice. In 

 Section III I have described my own experiments on the subject. In 

 Section IV I have dealt with the objection to the viscosity of ice 

 drawn from its supposed inextensibility. In Y I have discussed a 

 novel argument of Mr. Browne's in support of a high value of the 

 shearing strength of ice. In YI I have discussed the question of 

 the probable temperature of different parts of the glacier, and of 

 the significance of the " Bergschrund ; " while YII contains a few 

 concluding remarks upon the general drift of the paper. 



II. Previous Experiments hearing upon the Viscosity of Ice* 



Since the date of Canon Moseley's papers a good many experiments 

 have been published which tend to show that ice will change its form 



* By viscosity, I understand the property in virtue of which bodies change their 

 form under the continued influence of forces which would be insufficient to cause an 

 immediate change of form of similar amount. Probably nearly all, if not all, sub- 

 stances have more or less viscosity ; but by a viscous solid may be understood a 

 substance in which a large change of form is produced by adequate forces in a time 

 which is neither exceedingly small nor enormously great. 



