MAGNETIC INTENSITY. 



503 



change which the cylinder had undergone in the preceding 

 voyage, they persevered in diligently observing, and carefully 

 recording, its time of vibration, at most of the principal ports which 

 they visited in their voyage of five years' duration. Nor was it 

 until their return to the Cape Verd Islands, in September 1836, 

 that they could infer, from observations repeated at the same spot 

 as in their outward passage in 1832, that the cylinder had not 

 varied in any thing like the degree that it had done in the prece- 

 ding voyage, and that the care and pains they had bestowed were 

 therefore likely to be recompensed by success. 



This appears a fitting opportunity to remark, how much the 

 establishment in England of a depository for magnetic needles is 

 needed ; whence officers, and persons desirous of making such 

 observations, might be supplied with instruments, which had been 

 kept a sufficient time to have attained their permanent magnetic 

 state, and had been examined from time to time to prove that they 

 had done so. The correction for temperature should be ascertained 

 for each needle, and given with it; as well as the time of vibration 

 (or whatever else constituted the measure of intensity, — as, for 

 example, the angle of deflection in Mr. Lloyd's statical needles,) — 

 observed at the spot which should be selected as most suitable for 

 a point of general comparison ; and the observations should be 

 repeated at the same spot on the return of the needle. The want of 

 such an establishment has long been greatly felt ; and opportunities, 

 where nothing was wanting but proper instruments, have been lost 

 in consequence, where determinations of great value might have 

 been obtained, in parts of the world of the highest magnetic interest, 

 and where such opportunities are of rare occurrence. 



The corrections necessary to render the times of vibration at the 

 different stations strictly comparable with each other, are as 

 follows. 



1st, For the rate of the chronometer. 

 2d, For the temperature of the needle. 

 3d, For the arc of vibration. 



4th, For any change in the magnetic condition of the cylinder. 



In extensive voyages, the last-named correction, or that for the 

 change in the cylinder itself, is the one which requires principal 

 consideration. The corrections for temperature, and for the arc, 

 on the first of which particularly much stress has sometimes been 



