392 



the expansions of the balance-spring of chronometers consequent on 

 variations of temperature, the authors came to the conclusion that 

 there exist certain physical defects in the substances employed for its 

 construction, beyond the most perfect mechanical form that can be 

 given to it, which interfere with the regularity of its agency : so that 

 however exquisite may be its workmanship, and however complete its 

 power of maintaining a perfect figure when in different degrees of 

 tension, yet the imperfect distribution of its component parts may give 

 rise to great incorrectness in its performance. Hence the balance- 

 spring not only should be made of a substance most highly elastic, 

 but its elasticity should not be given to it by any mechanical or che- 

 mical process : as a body in motion, it should be the lightest possible ; 

 and, as far as the case admits of, it should be free from atmospheric 

 influence. Glass suggested itself as the only material possessing, in 

 the greatest degree, all these desirable properties. Its fragility, al- 

 though apparently a great objection to its employment, was found, on 

 trial, to constitute no obstacle whatever ; for it was found to possess 

 a greater elastic force than steel itself, and thus to admit of greater 

 amplitude in the arc of vibration. 



It was first proposed to ascertain how far a glass balance-spring 

 would sustain low temperatures ; and it was found by experiment that 

 it resisted completely the effects of a cold as great as that of -f- 12° of 

 Fahrenheit's thermometer ; thus satisfactorily removing any objection 

 which might be brought against its use from its supposed fragility in 

 these low temperatures. The next object of solicitude was to deter- 

 mine whether it would withstand the shock arising from the discharge 

 of cannon in the vicinity; and its power of resisting concussions of 

 this nature was fully established by experiments made with this view 

 on board H.M.S. Excellent at Portsmouth. 



On comparing the performance of glass balance-springs with me- 

 tallic ones, when the temperatures were raised from 32° to 1 00°, it 

 was found that while the loss in twenty-four hours in the gold spring 

 was 8 m 4 s , that of steel 6 m 25 s , and that of palladium 2 m 31 s , that of 

 a glass spring was only 40 s . These differences the authors ascribe 

 principally to the different degrees in which the substances had their 

 elasticity reduced by an increase of temperature. As glass was thus 

 found to suffer a much smaller loss of elasticity by this cause than 

 metals, they proceeded to construct a glass balance suited to the cor- 

 rection of the small error still occasioned by this cause, employing a 

 glass disc for this purpose. The compensation being completed, they 

 next tested the isochronism of the glass spring, and it proved to be as 

 perfect as any metallic spring. Chronometers thus constructed are 

 now in course of trial at the Royal Observatory. In common with all 

 other instruments of the same kind they have shown a disposition to 

 progressive acceleration, the cause of which is but little known, but 

 which appears to be influenced by the action of the air. 



The Society then adjourned over the Whitsun week, to meet again 

 on the second of June next. 



