334 Chronometers. 
As chronometers vary in their arcs of vibration from 180° 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 toa 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 are 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- 
minz 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 lamine 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 
f 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 7 
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 329°, we raised the temperature to 100°, and the rate shewn was 
losing 385 seconds in twenty-four hours. We then followed up these 
