344 THE REV. G. FISHER'S ACCOUNT OF CAPT. SIR EVERARD HOME'S 



have been reduced to the same epoch, or mean period of observation, by the applica- 

 tion of a proportional part of this change ; and the corrected or concluded times of 

 completing a given number of vibrations for each needle, together with the corre- 

 sponding forces, which are inversely as the squares of these times, are given in sepa- 

 rate columns. These forces are again reduced, and compared with the horizontal 

 force at Plymouth, which is taken equal to unity. 



The magnetism of the needle B appears to have been subject to some occasional 

 changes during the voyage, but not of sufficient magnitude or regularity as to admit 

 of any correction being made on this account. The mean of all the experiments col- 

 lectively, that were made at the same place with this needle, is therefore taken as 

 one result, and compared with others obtained in a similar way at the other places. 



Some degree of uncertainty is attached to experiments of this nature when made 

 with various needles, the whole of which have only been occasionally used, and the 

 magnetism of these subject to various changes, on account of the difficulty of assign- 

 ing such values for the final results as the experiments themselves seem best to jus- 

 tify, since the means as deduced from the e^eriments at each place are on this ac- 

 count somewhat vitiated, being no longer strictly comparative ; and moreover the 

 same experiments may by different persons be differently compared with each other, 

 and thereby results somewhat different may be obtained. 



It is necessary to state, therefore, that the horizontal forces at Plymouth, Bermuda, 

 and Port Royal in Jamaica have been determined by the experiments made at Ply- 

 mouth in March 1834, immediately before leaving England, compared with similar 

 ones made at the two latter places in the months of June and July following, and by 

 applying a small correction for the diminution in the intensities of the respective 

 needles. The experiments at Chagres are compared with the previous and subse- 

 quent ones at Jamaica made at short intervals of times ; those at Para, Maranham, 

 and Demerara with those made at Bermuda ; and lastly those at Antigua with the 

 previous and subsequent ones at Bermuda. The experiments thus grouped and com- 

 pared together are contained between the horizontal lines in the Tables II., III., IV., 

 and v., together with the comparative horizontal forces between these places ; these 

 forces are then compared with the force at Plymouth, and the results given in a se- 

 parate column. 



Having thus obtained the horizontal forces at each place as compared with Plymouth, 



these are again reduced to the forces in the direction of the dipping needle by mul- 



... , 1 . X 1 I- I, xu r x- COS . dip at Plymouth .„, 



tiplymg the horizontal forces by the traction cos dip at Place ' comparative 



whole forces being deduced by means of the horizontal needles, and also by direct 

 experiment with needle B when vibrated in the dipping instrument, the final results 

 are given in Table VIII. 



G. F. 



