Chronometers. 



335 



experiments by trying a gold balance spring ; also one of palladium ; 

 and their several results are shewn in the following table : the num- 

 ber of vibrations were 18,000 when the chronometer shewed mean 

 time. 



This table shews that metal balance springs vary in iheir results, when 

 under different degrees of heat, in the order in which they stand in a 

 table of expansions ; and with this conclusion we might have remain- 

 ed satisfied, had there not been such a wide difference between the 

 results with the glass and the gold springs. This at once led us to 

 consider that such a difference; could not arise entirely from an in- 

 crease of length in the springs caused by direct expansion, but prin- 

 cipally from a loss of elasticity occasioned by change of temperature. 



Our attention was next directed to separate these two causes of 

 error, and if possible to account for the anomaly between the glass, 

 steel, and gold balance springs. We may naturally conclude, if these 

 alterations in the rates of the chronometers arose entirely from an in- 

 crease on the length of the balance spring, due to the change of tem- 

 perature, that by shortening the spring by the same quantity by which 

 it had been augmented by the increase of temperature, a very near 

 correction would have been effected, (making an allowance for a very 

 small change in the inertia of the balance.) This was by no means 

 the case ; the following experiment will shew that it arose principally 

 from a variation in the elastic force of the springs. 



We first measured the length of the steel spring, and, on referring 

 to a table of expansions, we found that being 11.04 inch in length it 

 would increase its length .0065, in a temperature from 32° to 100°. 

 This quantity being so small, we therefore preferred .01 inch, being 

 in excess of that given by the table. Having shortened the spring 

 by this quantity, which we could determine by our gauge, and keep- 

 ing the glass disk applied, we obtained the difference occasioned 

 thereby in the rate of going at 32° and 100°, then reasoning from 

 the table of expansion, that as the spring had lengthened .0065 in- 

 ches, it was fair to conclude that we had reversed the order of going, 

 and that the chronometer would then go at the same rate when at 

 100° that it had gone before 32°. Instead of this, we had only ad- 



