504 



TNfAGNETIC INTENSITY. 



laid, are important when extreme accuracy is sought; as for 

 example, in comparing the force at stations which have served 

 as the unities of difierent observers : their effect is, however, of 

 little moment in observations which include great differences of the 

 terrestrial intensity. But when the magnetic condition of the 

 needle has varied, and interpolation becomes necessary, the in- 

 stances are rare in which it can be done with entire satisfaction. 



The time of vibration of this cylinder at Plymouth, with correc- 

 tions applied for the chronometer's rate, the temperature, and the 

 arc, was, in December 1831, 770,8 seconds, with a dip of 69° 27', 6, 

 and in October 1836, 777,3 seconds with a dip of 69° 17', 5. 

 The difference in its magnetic state, at the two periods, is shown 

 by the squares of the times of vibration multiplied by the cosine 

 of the dip observed at the respective periods. This makes known 

 what the time of vibration of the same cylinder would have been, 

 had it been free to move in the direction of the dipping-needle, 

 instead of being suspended horizontally ; and, consequently, if it 

 had been acted upon by the total magnetic intensity, instead of by 

 the horizontal component only. It is here assumed that the total 

 terrestrial intensity is constant at the same place. This is doubtless 

 not strictly true ; but the amount of the change must be too small 

 to require consideration in the period occupied by Captain Fitz- 

 Roy's observations. The horizontal component must, however, 

 necessarily vary with the changes in the dip : and it is, therefore, 

 from the time of vibration in the direction of the dipping-needle, 

 and not from the time of horizontal vibration, that the change, or 

 otherwise, in the magnetism of the needle is to be inferred. We 

 find, then, the equivalent time of vibration of this cylinder in the 

 direction of the dipping-needle to have been 456,4 seconds in 

 December 1831, and 462,2 seconds in October 1836; showing a 

 difference of 3,8 seconds in fifty-eight months. If we compare this 

 amount with the far greater loss of intensity sustained by this 

 cylinder in the preceding voyage, it seems a probable supposition 

 that, at the commencement of Captain Fitz-Roy's voyage, the cylin- 

 der had nearly attained its permanent magnetic state ; and that its 

 further loss of magnetism, occasioning an increase of 5,8 seconds 

 in the time of vibration, took place in the early part of the voyage : 

 supposing the loss to have been progressive, and not sudden, as 

 from accidents, of which the observations give no indications. Con- 



