in the Rates of' Chronometers. 225 



depending upon some cause very distinct from that usually 

 operating on ship-board. 



This being the case, I beg to offer what I consider to be 

 by no means an improbable explanation of the irregularity in 

 question ; that is to say, Is it not probable, or at least possible, 

 that what Mr Fisher has attributed to an unusual acceleration 

 in the ship, was actually a corresponding retardation on shore, 

 occasioned by the action of some terrestrial magnetic power be- 

 low the surface of the earth at Fair Haven ? Such partial ter- 

 restrial action is by no means unknown, I might almost say not 

 uncommon ; some instances of the kind have, I believe, been 

 published, and two or three others have come to my know- 

 ledge, upon information on which I can place the greatest re- 

 liance. The first is a statement made to me by Captains Vi- 

 dal and Mudge, who informed me, that during their former 

 voyage, while they were engaged in surveying the coast of St 

 Mayo, one of the Cape de Verd islands, they found the land 

 on which they were carrying on their observations so strong- 

 ly magnetic that the needle of their theodolite became wholly 

 useless : the dip was so much increased that the needle would 

 not traverse till they had inclined the face of the instrument 

 at a very considerable angle, and even then the direction of 

 the former was found so variable and uncertain as to be whol- 

 ly inapplicable to the purposes of the survey. Now, let us 

 suppose that the chronometers of the ship had been brought 

 on shore in this place to obtain their rates, there can be no 

 question that they would have been found very different from 

 what they were on board, and as the difference would be all 

 that could be shown by the observations, and supposing no 

 partial action to have been suspected on shore, the change 

 would naturally be referred to some attraction in the vessel, but 

 the character assigned to it would be the reverse of that which 

 actually belonged to it ; viz. if the chronometers lost on shore, 

 they would be supposed to gain on board, but if they gained 

 in the former case, they would be supposed to be losing in the 

 latter. It is true, that, on the return of the vessel, the error 

 would be detected, provided the rates were still taken in Lon- 

 don ; and it is much to be regretted that Mr Fisher has not 

 informed us whether this precaution was taken in the cases be 



