334 Chronometers. 



As chronometers vary in their arcs of vibration from 1 80° to 220°, 

 and as the glass spring was wound up 360°, and did not break until 

 it was shortened to seven coils, we may fairly conclude that it would 

 have performed the usual arc of 180°, even at the seven coils, with- 

 out fracture. It will be remembered, that the glass spring was ap- 

 plied to a previously adjusted chronometer in our few first experi- 

 ments ; and in adjusting it for variation of temperature, we found 

 that we had to deal with a substance, with the nature of which we 

 were altogether unacquainted ; for on applying the glass balance 

 spring, an excess of compensation was found, and, after repeated al- 

 terations to reduce this excess, we applied the least compensation 

 that could be afforded us by the usual balance. This was done by 

 placing the whole of the compensating weight at the end of the arm 

 of the balance, instead of attaching it in the usual way to the mova- 

 ble end of the arc of compensation, and, as the arm expands in a 

 direct line from the centre of the balance, we supposed that our ob- 

 ject would have been accomplished. 



Having now only the compensation rim composed of the usual la- 

 minae of brass and steel, and the weight being also placed at the ex- 

 tremity of the arm, where no curvature could be produced, to bring 

 it nearer the centre, we found that the laminae themselves, without 

 any weight, produced an excess of compensation ; we therefore de- 

 termined on finding the amount of error in time arising from the spring 

 when subjected to a variation in the thermometer from 32° to 100°, 

 To arrive at this, we made a solid disk of glass, and, having removed 

 the former balance, we applied the disk in its stead, and brought the 

 chronometer under this arrangement to mean time at a temperature 

 of 32°. Assuming that no error would arise from the glass balance 

 by its diameter being increased by heat, the variation shewn when 

 the thermometer was raised to 100° would be attributable to the glass 

 spring only. 



The temperature was raised to 100°, when to our astonishment, 

 the chronometer lost only the small quantity of forty seconds in twen- 

 ty-four hours : the experiments were repeated, and the same results 

 obtained. 



It next became an object of much interest to find the amount of 

 error of a steel balance spring under similar circumstances. With 

 this view, we removed a compensation balance from a chronometer, 

 and replaced it with a glass disk. Having brought it to mean time 

 at 32°, we raised the temperature to 100°, and the rate shewn was 

 losing 385 seconds in twenty-four hours. We then followed up these 



