1851.] 



On the Rates of Chronometers. 



61 



Barometer and Thermometer at Calcutta at the Surveyor General's 

 Office. Bar. reduced to 32° Fahrt. 



Date. 



Sun Rise. 



9h. 50' 



A. M. 



Noon. 



2h. 40' 



P. M. 



4 P. M. 



Sunset. 



1850. 

 April 

 26th, 



29.770 



g 



78.3 



29.831 



g 



83.3 



29.809 



si 



S 

 87.9 



29.754 



g 

 87.3 



29.746 



5^ 

 g 



84.9 



29.755 



g 

 81. 



27th, 



.674 



77.8 



.657 



79.8 



.642 



30.0 



.568 



81.9 



.535 



81.8 



.571 



82.2 



28th, 



.760 



78.1 



.818 



85.3 



.800 



88.0 



.751 



86.8 



.735 



87.3 



.734 



84.8 



( To be continued. ) 



On the Rates of Chronometers, as influenced by the Local 

 Attraction of Ships, and by Terrestrial Magnetism. By Henry 

 Piddington, President of Marine Courts, Calcutta. 

 In the latest and best English treatise on Navigation, that of Lieut. 

 Raper, R. N. 3rd Edition, 1849, p. 174, after briefly referring to 

 various opinions as to the causes of the variation of rates in Chrono- 

 meters, such as motion, temperature, shocks from guns, thunder-storms, 

 magnetism, &c. the author says that, " it seems generally admitted that 

 the principal cause of the change of rate is variation of temperature" 

 and he adds that " as regards the local attraction (deviation) of the 

 ships themselves affecting the rates, no decisive experiments appear to 

 have been made on the point." I have thus thought that where good 

 experiments have been casually made, it becomes of much importance 

 to Nautical, Hydrographical and Geographical science to preserve the 

 records of them. 



Before detailing the particular instances to which this paper refers 

 it may be useful to give a brief sketch of what is known and has been 

 done to elucidate this most important question up to the present time, 

 so far as the limited means of Indian research enable me. 



The earliest accounts we have of the effect of Magnetism on Chro- 

 nometers, whether Terrestrial or Local, is I think that of Mr. Varley 

 in the Philosophical Magazine, Vol. I. (1798) who discovered that 

 the balances acquired polarity at two opposite points on the rim, and 

 thus that the going of the time-piece was affected by the position of 



