504 MAGNETIC 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 different 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 5,8 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- 
