Chronometers. 



333 



salt mixed together. The whole is then placed in an outer wooden 

 vessel, which is subsequently incased in flannels. By this arrange- 

 ment a low temperature may be preserved for many hours. The 

 chronometer with the glass balance spring was placed in a vessel of 

 this description, and the thermometer soon fell to 12° of Fahrenheit, 

 at which it continued, with little variation, during the first twelve 

 hours ; at the end of twenty-four hours the thermometer was at 28°. 

 The result of this experiment we considered satisfactory as to the 

 power of the glass spring to resist the effect of a low temperature. 

 To enable us to determine the second question, namely, the power 

 of resisting the shock arising from the discharge of cannon, the lords 

 commissioners of the Admiralty were pleased to order the experiment 

 to be made on board H. M. S. Excellent at Portsmouth. The Rev. 

 George Fisher superintended these experiments, and their results are 

 shewn by the following. 



Having by these experiments established the conclusion, that the 

 fragility of glass was no, obstacle to its application to the purpose of 

 forming the balance spring of a chronometer, our next step was to 

 ascertain at what degree of tension a glass spring would break. We 

 made a glass spring for this purpose, and applied it to the balance of 

 a chronometer. It was wound up to 360°, detached from the es- 

 capement, and suddenly released, being shortened one coil at each 

 trial. The following table will shew the result : — 



Table II. 



