Chronometers. 337 



c, c. These pieces were in length 0.5 in., breadth 0.052 in., thick- 

 ness 0.004 in. and weight 2 grains. The lightness and thinness of 

 these pieces only allowed of one method of compensating for tem- 

 perature, which was to cut off with a pair of scissors a small portion 

 from the top, and to draw out the timing screws d, d, to bring the 

 chronometer to mean time again, in consequence of the balance hav- 

 ing been lightened by cutting off a piece from the compensation la- 

 minaj. 



The compensation being completed, we next proceeded to test 

 the isochronism of the glass spring ; in which respect it was proved 

 to be as perfect as any metallic spring. The whole of the adjust- 

 ments being now completed, we finally tried its rate under different 

 degrees of heat, from 12° to 100° Fahr., and are of opinion, that 

 there was less error shewn than generallj'- attends metal springs, un- 

 der 'corresponding circumstances. The chronometer with this glass 

 spring and balance, was sent to the Royal Observatory, agreeably to 

 the order obtained from the lords commissioners of the Admiralty, in 

 October, 1833. 



There have been two causes why the investigation has been de- 

 layed; first, until the last winter, the thermometer did not fall suffi- 

 ciently low to enable us to draw a fair conclusion ; secondly, there 

 has been an acceleration, on a gaining rate, since it has been placed 

 on trial ; and this has been the sole obstacle to the complete success 

 of this experiment. This acceleration must, therefore, be a subject 

 of further research : that it arises from some latent cause, would ap- 

 pear from the circumstance, that the two chronometers, placed on trial 

 by order of their lordships, as well as others which we have had in 

 our possession for private experiment, have gained nearly in the same 

 ratio. Of the two chronometers tried by order of their lordships, 

 No. 616 has been stationary at the Royal Observatory, and No. 790 

 on board H. M. S. Fairy, Capt. Hewett, employed in surveying in the 

 North sea. On the close of the season, this latter chronometer was 

 returned to the Royal Observatory, until required ; therefore, (with 

 regard to this chronometer,) we have both a sea and land rate. The 

 annexed tables shew the going of the two chronometers. From 

 these tables it appears that the chronometers gained most at the ear- 

 liest period of their trials, and our object is to vi^atch the termination 

 of their acceleration. 



If we might venture to offer some practical remarks on the nature 

 of balance springs which have been subjected to the effects of fire 

 Vol. XXXIL— No. 2. 43 



