234 Mr. C. Chree on some Applications of 



and the weight of one ton as unit of force. If a portion of 

 the bar increase in length from 100 to 100"01 inches, and the 

 increase be uniformly distributed over the portion lengthened, 

 the longitudinal strain is (IOO'01-IOO) x 10~ 2 , or -0001. 



The writers who have had most influence on the present 

 scientific usage of English terms dealing with physical 

 properties are unquestionably Professor Clerk Maxwell and 

 Sir William Thomson. The former gives the following 

 definitions in his ' Theory of Heat''* : — " A body which when 

 subjected to a stress experiences no strain would, if it existed, 

 be called a Perfectly Rigid Body. There are no such 

 bodies. . . " 



" A body which when subjected to a given stress at a 

 given temperature experiences a strain of definite amount, 

 which does not increase when the stress is prolonged, and 

 which disappears completely when the stress is removed, is 

 called a Perfectly Elastic Body." 



"If the form of the body is found to be permanently 

 altered when the stress exceeds a certain value, the body is 

 said to be soft or plastic, and the state of the body when 

 alteration is just going to take place is called the Limit of 

 Perfect Elasticity." 



" If the stress, when it is maintained constant, causes a 

 strain . . . which increases continually with the time, the 

 substance is said to be viscous." 



A viscous material may be either solid or fluid. It is 

 regarded by Maxwell as fluid when any stress, hoivever small, 

 produces a constantly increasing strain. Maxwell draws a 

 distinction between elasticity of bulk and elasticity of shape — 

 the latter being peculiar to solids- — which is more fully treated 

 of by Sir W. Thomson. A body possesses perfect elasticity 

 of bulk when on the removal of the stress it returns to its 

 original volume, even though the form of its surface be per- 

 manently altered. Both writers regard it as certain that solid 

 bodies will retain perfect elasticity of bulk under compressive 

 stresses which far exceed the limit of elasticity of shape. 

 The following statement embodies the views of Sir W. 

 Thomsonj: — "If we reckon by the amount of pressure, 

 there is probably no limit to the elasticity of bulk in the 

 direction of the increase of pressure for any solid or fluid ; 

 but whether continued augmentation produces continued 

 diminution of bulk towards zero without limit, or whether for 

 any or every solid or fluid there is a limit towards which it 



* 5th edition, chapter xxi. 



t Mathematical and Physical Papers, vol. iii. pp. 7-8. 



