Distribution of Flow in a Strained Elastic Solid. 503 



The method of using the still is as follows i — The tap E 

 is opened, F is closed ; a water-pump then exhausts the whole 

 system, and the mercury to he acted on rises from the cis- 

 tern A. The cistern being large and shallow, only a slight 

 change takes place in the height of the mercury in the bulb, 

 when the level of the mercury in the cistern changes. While 

 the pump is exhausting the ring of gas-jets is lit, and in 

 about ten minutes, in the case of the still in our laboratory, 

 the mercury fills the tube D, any metal which comes over 

 being caught in the bulb G. The tap E is then closed and 

 F opened ; the still then continues to work by virtue of the 

 vacuum formed by its own mercury. It has been found 

 necessary to place a gas-regulator on the pipe which supplies 

 the jets, as the change of pressure in the gas-mains is consi- 

 derable. An automatic arrangement, depending for its action 

 upon the height of the mercury in the cistern, shuts off the 

 gas when the surface of the mercury falls below a certain 

 point. In using stills of this class the mercury before distil- 

 lation should be carefully freed from moisture, as a minute 

 quantity of water will often cause a fracture in the heated tube 

 or bulb. 



LVIII. On the Distribution of Flow in a Strained Elastic Solid. 

 By Chaeles A. Carus-Wilson, B.A., A.M.I.C.E., De- 

 monstrator in the Mechanical Laboratory at the Royal Indian 

 Engineering College, Coopers Dill*. 



IF a metal bar of uniform section throughout its length be 

 subjected to uniform longitudinal stress, the elements of 

 the bar will become distorted ; if the strain should exceed the 

 limit of elasticity the distortion will be partly permanent. 



In consequence of the straining of the elements, every ele- 

 ment will experience a displacement relative to three fixed 

 axes in the bar ; say ox parallel to the length of the bar, oy 

 parallel to one side, and o z parallel to a second side, both o z 

 and oy being at right angles to ox\ such displacement is 

 generally spoken of as " flow." 



It is clear that if the metal be homogeneous, the stress 

 uniform, and the section at right angles to the axis every- 

 where the same, an element will experience a displacement at 

 right angles to the planes xoz, yoz proportional to its distance 

 from these planes ; since such displacement depends on the 

 straining of the elements between it and these planes. Hence 

 a bar with its sides originally parallel to ox will remain so 



* Communicated by the Physical Society : read May 2, 1890. 



2Q2 



