198 OBSERVATIONS OF THE TERRESTRIAL 



be 3'. The very close agreement of the results must of course be 

 regarded as in a great measure accidental. 



Observed Dip. Corrected Dip. 



Needle L (1), July, 1834, .. 7059'-5 . . . 719'-5 

 Needle S (1), Aug., 1834, . . 71 3-5 ... 71 9 -5 

 Meyer's needle, Nov., 1833, . 7111-7 ...719-7 



"We have hitherto spoken only of the needles whose poles are 

 changed in each observation, and which are used exclusively for 

 the determination of the dip. The necessity of a correction in the 

 results obtained with the other needles, whose poles are unaltered, 

 is obvious. By reason of the deviation of the centre of gravity of 

 the needle from the axle, the weight of the needle itself has in all 

 cases a certain moment acting with or against the directive force. 



Let , as before, be the inclination of the needle to the horizon 

 when unloaded, and the corresponding angle when the weight 

 is attached, and let p denote the ratio of the moment of the needle 

 itself to that of the added weight ; then the dip, S, will be given 

 by the equations* 



(1) 



in which t is the correction sought. 



The constant coefficient p in the expression for this correction 

 will be known when the corresponding values of the angles , , 

 and are known at some one station. Its value, in the case of 

 Needle IV., has been thus found to be 'O0205.t 



It will easily appear, from the second of the preceding formulas, 

 that when the coefficient is so small as that just assigned, the 

 variations in the values of e, resulting from moderate changes in 

 the angles on which it depends, will be inconsiderable. In the 

 observations in Ireland, for example, the entire change in the 

 amount of the correction is a small fraction of a minute. In this 

 and other similar cases, therefore, the correction may be regarded 

 as constant; and its value maybe inferred from any series of simul- 

 taneous observations made with the needle to be corrected, and 



* Transactions of the Royal Irish Academy, Vol. XVIL, p. 450. 

 . 451. 



