550 



Mr. Hartnup on testing Chronometers. 



[Dec. 14, 



The efficiency of the method will be seen by the following three ex- 

 amples, in which the test was repeated four times in succession. 



TABLE IV. Showing the mean daily rates, gaining, of three chronometers 

 tested in three definite temperatures four times in succession. 



The preceding examples have not been selected to show the large errors 

 in a ship's longitude which might result from the use of very bad instru- 

 ments, but rather that in what are considered good and carefully regulated 

 chronometers errors may, with adequate means for testing, be detected, 

 and tables of corrections supplied to the mariner. 



Examples 1 and 2, Table I., show how nearly it is possible to com- 

 pensate for change of temperature between 55 and 85. Some chrono- 

 meters so compensated, when exposed to a temperature of 40, change 

 their rates very much, while in others the alteration of rate is compara- 

 tively small. 



On ascertaining the chronometrical difference of longitude between the 

 Liverpool Observatory and the Observatory at Cambridge, Massachusetts, 

 the late Professor W. C. Bond at the commencement employed twelve 

 marine chronometers which had been used previously on several occasions 

 for obtaining differences of longitude. During the voyages between Liver- 

 pool and Boston, in the summer months, the sea and shore rates of these 

 chronometers were sensibly the same ; but during the winter months they 

 differed considerably. On testing these instruments in 40 and CO , the 

 following results were obtained : 



TABLE V. Showing the increase of gaining-rate of twelve chronometers 

 caused by changing the temperature from 40 to GO . 



The chronometers alluded to in this Table were made by the late Mr. 



