76 



recovers itself again. If the force is considerable, change in the length of the 

 wire may be easily observed ; with a less force it will not be so apparent but still 

 measurable, and, finally logic requires us to believe that if the force applied and 

 withdrawn is infinitely small, there will still be a change in the length of the wire 

 acted upon. 



That which is true of a wire is equally true of all masses of metal of what- 

 ever proportion. A cube of steel may resist an enormous load tending to crush 

 it, and yet the application of a slight force effects a decrease in its height. The 

 change in form under light loads is certainly small, but actual, nevertheless. 

 That which is true of steel is, in a general sense, equally true of wrought and 

 cast irons, and, in fact, metals of every sort. 



The machine designer, therefore, can not, as some suppose, make the several 

 parts of his machine so strong that they will remain fixed in form, but he must 

 choose rather so to distribute the metal with reference to the stresses to be trans- 

 mitted, that the change in form which is sure to occur, will not interfere with the 

 action of the proposed machine. 



Some years ago the writer became interested in tests involving a measure- 

 ment of the strain, that is the change in form, of various parts of steam engines, 

 parts supposedly invariable, while the engines were being worked under load. 

 The apparatus employed consisted of a fine micrometer screw mounted upon a 

 frame work wholly apart from the engine and having no connection with it, but 

 so arranged that the screw could be brought in contact with the part to be exam- 

 ined. In making observations, one terminal wire from a telephone receiver was 

 attached to the part of the engine which was under examination, and the other 

 terminal from the telephone to the micrometer; the observer placed the telephone 

 to his ear, and slowly screwed the micrometer in towards the desired point on 

 the engine. If the part of the engine in question was in vibration it first touched 

 the point of the advancing screw for an instant for each oscillation, the contact 

 being made manifest to the observer by a sharp click in the telephone as the cir- 

 cuit was made and broken again. This fixed one boundary by which the ampli- 

 tude of the vibration was to be determined. Next the micrometer was advanced 

 towards the engine until the screw did not break contact with the machines, a con- 

 dition which was denoted by a cessation of sound in the telephone, while for all 

 intermediate points the clicking in the receiver kept time with the_ revolutions of 

 the engine. It was assumed that this last position of the micrometer marked the 

 other boundary of the vibration. The difference of the two readings of the 

 micrometer gave the amplitude of the vibrations, or the extent of the motion in 

 the part examined. A large number of readings from several parts of two engines 



