508 



MAGNETIC INTENSITY. 



The object of noting the time of every tenth vibration is to 

 check errors in the counting, which will sometimes occur in the 

 course of the 360;, particularly with the very short and quick- 

 moving needles of M. Hansteen's very portable apparatus^ and at 

 stations of low dip, where the horizontal force is greatest^, and the 

 needle consequently moves most quickly. Several such mistakes 

 evidently occurred. When the time of completing every supposed 

 tenth vibration is observed with tolerable exactness^ and the dura- 

 tion of each pair of vibrations decidedly exceeds any irregularity of 

 probable occurrence, apart from miscounting the number of vibra- 

 tions, such mistakes can be discovered with ease, and rectified 

 with certainty. This has been done in every case where no doubt 

 could exist of a mistake of the kind having occurred ; such as 

 when all the intervals are of nearly equal duration, with one or 

 two exceptions, which differ as much as three or four seconds from 

 the general body. There are two stations, however, Callao and 

 Keeling Islands, where the rectification is not so clear, or the true 

 result so obvious. At Callao there are three series of horizontal in- 

 tensities, each of forty observed intervals, which should be of ten 

 vibrations each. Several of these intervals are between 17,5 and 

 18,5 seconds, and several others between 20,5 and 21,5 seconds. 

 These can hardly represent an equal number of vibrations, because 

 the difference between them is greater than can easily be supposed 

 due to any uncertainty in seizing the particular beat of the chro- 

 nometer at which the vibration was completed ; it is, moreover, 

 about the time that would be occupied by two vibrations more or 

 less. The question then arises, do the longer intervals represent 

 12, and the shorter 10 vibrations, or do the longer represent 10, 

 and the shorter 8 ? In the former supposition the intensity at 

 Callao would be about L01(Paris=1.348) : in the latter about 

 0.75. The difference shews how great an error would be risked by 

 either assumption. If we take a mean of all the intervals as they 

 stand, the amount of error risked would be certainly lessened ; 

 but we should assuredly not have the true time of three hundred 

 vibrations, except on one supposition : namely, that the irregu- 

 larities in question are not errors in estimating the number of 

 vibrations, but that each interval really represented an equal 

 number, and that some unusual and accidental cause occasioned 

 the needle to difler so greatly in successive intervals. But this 



