﻿370 Canon Moseley on the Uniform Motion 



Now the unit of shear in ice is at the least 75 * lbs. per square 

 inch. It is therefore mechanically impossible that the Mer de 

 Glace should descend by its weight only. Some other force in 

 addition to its weight must act upon it to cause it to descend. 

 This force must, moreover, be such as would produce those 

 molecular displacements and strains which are observed actually 

 to take place in glacier-ice, and must therefore be, as its weight 

 is, present to every part of the glacier, but more than thirty-four 

 times as great. 



LI I. On the Uniform Motion of an Imperfect Fluid. 

 By Henry Moseley, F.R.S. §-c.f 



BY a perfect fluid I understand a fluid to the displacement 

 of whose parts among one another no resistance is opposed 

 either of shearing or friction ; and by an imperfect fluid one to 

 the displacement of whose parts among one another there is op- 

 posed the resistance of shearing but not of friction. All fluids 

 are perhaps in this sense imperfect fluids. 



Let A a b B represent half a transverse section of an im- 



perfect fluid descending in a straight channel, having a uni- 

 form slope and a rectangular section everywhere of the same di- 

 mensions. Conceive the plane surface D dc C to be replaced by 

 a curved surface, and let AD^cCB (bounded in front by this 



* The unit of shear in soft putty is from 1 lb. to 3 lbs. per square inch. 

 The Mer de Glace would therefore descend by its weight only if it were of 

 the same degree of hardness as that substance and of the same specific gra- 

 vity. Although the resistance to shearing in ice is great enough to stop 

 effectually the descent of a glacier by its weight, yet it is probably less than 

 the resistance to shearing of any other solid. The unit of shear in wrought 

 iron is 50,000 lbs. per square inch ; in cast iron 27,000 lbs.; in oak 2300 

 lbs. ; in ash and elm 1400 lbs. ; in larch 970 to 1700 lbs. j in spruce 600 

 lbs. ; and in red pine from 500 to 800 lbs. 



t Communicated by the Author. 



