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. 
TaBe L 
Distance of Chronom-| Number! 
13 Sept. gel Situation be 7h gir with | Effect produced 
| 1833. fravlieyy a 24lb. gun, | aan | o gun . n Chronometer. 
Expt. | | 
l 42 feet | Bs Placed bare on the deck None 
ab 
2 32 — Oo. Do. 
3 20 — Do. Do. 
4 15 — Do. Do. 
5 12 — Do. on the carriage of Do. 
the adjacent gun. . 
6 Broadside from Do. the lower deck, Do. 
the main-deck 9 feet bel id-shi 
guns ; shotted. | main-deck gun 
Signed, Grorce Fisner. 
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 :-— 
Taste II. 
Degrees of Tension. - Number of turns in Spring. 
360° 12 None 
Do. il Do. 
Do. 10 Do. 
Do. 9 Do. 
Do. 8 Do. 
Do. 7 Broke 
