LAW OF ELASTICITY. 21 



5. Those bodies which alter their form under the 

 influence of external forces, and resume their original 

 form on the cessation of these external forces, are called 

 elastic. The greater these alterations are, the greater 

 is the elasticity of the body. The external force pro- 

 ducing the alterations may be either tension, extending / ; 

 the body in one particular direction ; or it may be pres- 

 sure, compressing the body into a smaller space ; or, 

 again, it may be tension combined with pressure, bend- 

 ing the body. We are only concerned with the force 

 of tension, which acting on the body in a longitudinal 

 direction extends it ; that is to say, we are about to 

 study the elasticity of muscle tension. Physicists 

 have experimented on elastic tension in bodies of the 

 most diverse kinds. But bodies of regular shape, rods 

 or threads, the length of which considerably exceeds 

 the thickness, are best adapted for such experiments. 



On firmly fastening a body of this kind, for instance 

 a steel wire or a glass thread, to a beam in the ceiling, 

 and, after accurately measuring its length, attaching 

 weights to the lower end, it will be found that the ex- 

 tension caused by these weights is greater in the first 

 place in proportion as the weights causing the extension 

 are greater, and in the second place in proportion as the 

 body which is extended is longer. And, on the con- 

 trary, with any given weight and length, the extension 

 will be found to be less in proportion as the body is 

 thicker, or, in other words, the larger is its cross-section. 

 This latter circumstance may be easily understood by 

 assuming that the rod or thread consists 'of a number 

 of smaller rodlets or tiny threads which lie evenly side by 

 side. If, for instance, we select for this experiment a 

 steel rod, the cross-section of which measures exactly 



