Vibration of Heated Metals, 1 43 



those partial and sudden expansions which give the tremor- 

 causing impulses. 



When the bar lies with a flat surface upon the block, the 

 movement of course takes place in a vertical position only ; but 

 when the bevelled face of the bar is presented to the block, and 

 the bar rests on the longitudinal ridge, then, along with the ver- 

 tical, the transverse vibration also occurs, and communicates the 

 rocking movement. 



This rocking may be induced by two causes ; either by some 

 slight inequality in the weight of the portions of the bar on the 

 two sides of the ridge, or some difference in the condition of the 

 surface of that part of the block which the ridge of the bar 

 touches. 



1st, If the first mentioned inequality exist, as soon as the 

 bar receives an upward heave, the greater weight of the one 

 side will cause it to incline to that side, and as soon as that 

 heave ceases, and the contraction succeeds, the bar approaches 

 its original position, but will not remain in it, for the inclination 

 given to the preponderating side will, on its return, of course, 

 cause the bar to incline to the opposite side. 



The impulses which the bar receives in this position from the 

 renewed expansion and elongation, will not only renew the up- 

 ward heave, but also give it a shove to the preponderating side, 

 and thus increase the lateral movement, which, like the vertical, 

 though altogether insensible at first, by incessant frequent repe- 

 tition accumulates and increases till the rocking becomes conspi- 

 cuous, and is rendered much more so by the transverse rod. 



2d, If there exist any difference in point of asperity, in the 

 condition of the surface of that part of the block upon which 

 the ridge of the bar rests, it must necessarily follow, that the im- 

 pulse given to the bar on that side which is the most rough, will 

 be greater than on the other, and consequently the upward 



