S4 



COMPENSATION PENDULtTM. 



it. It has cost me much labour, time, and expense; in« 

 deed, it has occupied almost the whole of my attention for 

 the last nine months. 



If any objections should be made to it, I will endeavour 

 to answer them, and make any further experiments re- 

 quired. 



I am Sir, 



Your obedient servant, 



HENRY >VMRD. 



nescriii-tion of 

 it. 



Its action. 



Smeaton's ex- 

 pansion table 



used. 



PI. II, fScf. 7, h h i i, are two fiat rods of iron or steet, 

 about half an inch wide, and an eiMbth of aa inch thick. 

 k k is a rod of zinc interposed between then>, and is nearly 

 a quarter of aji inch thick. The corners of the iron rods 

 are bevilled off, that they may meet with less resistance 

 from the air; and it likewise gives them a much lighter ap- 

 peal ance. These three rods are kept together by means of 

 three or four screws ////, which pass through oldnng holes 

 in the bars h h k k, and screw into the rod i i. The rod h h 

 is connected to the rod k k by the screw yn, which I call the 

 adjusting screw. This screw turns in the rod h h, passes 

 thiOLigh the zinc rod A; A;, and screws into the iron rod / f. 

 The rod i i has a shoulder at its upper end turned at right 

 angles, and^ bears on the top of the zinc rod k k, and is sup- 

 ported by it. It is necessary to have several holes for the 

 screw m, in order to adjust the compensation. See fig. 8. 



Now it is evident, that if any degree of heat or cold be 

 applied to this compound rod, the one of zinc expands and 

 contracts as much as the tv;o iron ones together ; the dis- 

 tance from the point of suspension to the centre of oscilla- 

 tion therefore must remain the same. 



In proportioning the length of the rods, I made use of 

 Mr. Smeaton's table of expansion of metals, in the 48th 



vol. of the Philosophical Transactions : where he sh 



by 



experiments made with a pyrometer, that the expansion of 

 iron is to that of unhauiniered zinc, with the same dcTee 

 of heat, as 151 to 353, and to that of zinc hammered half 

 an inch per foot, as 151 to 373. This great expanding pror 



lerty. 



