64 On the Rates of Chronometers, [No. 1. 



communication in No. 15 of the same work (to which I cannot refer) 

 in which it is stated by Messrs. Arnold and Dent as one of the results 

 of their experiments that the rate of a Chronometer was sensibly 

 affected by terrestrial magnetism when it was moved in Azimuth ; 

 details a series of experiments shewing clearly the effect of terrestrial 

 magnetism on Chronometers ; of which the rates were first ascertained 

 when the arms of the balances were nearly in the position of the XII. 

 and VI. on the dial plate, and then when these figures were alternately 

 placed towards the North and South and East and West ; the differ- 

 ences amounting to -f- 0.42 and -{- 0.35 ; when the North (XII.) 

 was reversed to South ; and to -f- 0.28 and + 0.22 when they were 

 changed from West to East ! 



The same paper also contains a communication from Mr. Northcote, 

 Master of H. M. S. Jupiter, shewing the influence of the ship's mag- 

 netism on the rates of her Chronometers in a voyage to and from the 

 East Indies. 



And finally, Professor Airy of the Royal Observatory at Greenwich 

 (Naut. Mag. for 1840, p. 231), after describing his observations and 

 experiments upon a Chronometer which had been sent to him from 

 Messrs. Brookbanks & Co., " as particularly magnetic," gives rules for 

 correcting the effect of terrestrial magnetism on a Chronometer by 

 simply placing it on the top of the glass of a compass box. No 

 experiments seem as yet to have been made as to obtaining any 

 correction for the ships magnetism. I do not find this subject referred 

 to by the editor of the latest edition (1848) of Bowditcb's American 

 Navigator ; and this then appears to be, from all the authorities to 

 which I can refer in India, the present state of our knowledge as to the 

 phenomenon itself, and the causes and means of correcting it. 



My friend Captain Hopkins, of Messrs. Green's ship the Prince of 

 Wales, called upon me in January to mention that he had experienced 

 in his outward bound voyage of 1850-51, a remarkable alteration in the 

 rates of his Chronometers ; which though first rate ones and always 

 performing well on former voyages he had found to be upwards of 

 forty miles wrong by his lunars on his arrival at the Floating Light ! 

 This he was at a loss to account for, as it had never occurred before, 

 the shore rates given in England having always been within a trifle 

 correct. 



