1851.] On the Rates of Chronometers. 77 



I cannot now refer ; and it seems not improbable that as a change of 

 terrestrial magnetism also took place when the changes of tempera- 

 ture occurred with those ships' Chronometers which have supposed 

 their rates affected by temperature, the eifects of the one, as more 

 sensible and better known, or in other words nearer at hand, have been 

 compendiously attributed to the other. The causes seem to stand 

 rather in the following order as to the importance of their effects, the 

 whole of them being constant ones. 



I. The ship's local attraction. Sometimes that of the cargo in 

 merchantmen, or of warlike stores in a man-of-war : Alters rates also by 

 privation, as when cargo or warlike stores are discharged, or Chrono- 

 meters carried on shore.* 



II. Terrestrial magnetism, and the angle made by the poles of the 

 polarized balance with the magnetic meridian. 



III. Changes of temperature. 



It is evident also that all these may be under some circumstances 

 trifling, or that one may neutralise the two others if they should act 

 in opposite directions ; but it is also evident that they may be each 

 comparatively trifling in itself, yet, if the whole act the same way, 

 they may amount on a long voyage to a considerable error, against 

 which it behoves the careful navigator to be on his guard. The 

 scientific workman will consider, better than I can do, if it may not be 

 worth his while to produce on trial a Chronometer from the balance 

 of which magnetic metals should be wholly excluded. Glass balances 

 have, I know been tried, but found too fragile. Tough porcelain would 

 seem to promise better. 



* " The changes so frequently noticed to take place in the rates of Chronometers 

 moved from the shore to the ship and the reverse, are well known to be caused 

 partly by change of temperature and partly by change of situation," says Captain 

 Fitzroy, p. 326 of appendix ; and in a note : " This may be connected with magnet- 

 ism." The work is published in 1839, and Mr. Fisher's second paper appeared 

 in 1837, but Captain Fitzroy may not have seen it, since he refers only so cursorily 

 to a fact of such high importance shewn by direct experiment. 



