daily variation of the horizontal and dipping needles , &c. 337 
my experiments, the length to which I should carry them, I 
did not, from the first, register the temperature and state of 
the atmosphere ; for from certain notes of this kind made 
lately, it appears to me that the quantity of daily change de- 
pends in a greater degree on the intensity of the solar light, 
than on the mere temperature of the day ; although it is cer- 
tain, from some recent experiments by Mr, Christie, that 
the change of temperature of the air, during the day, has a 
much greater effect upon the intensity of action in the oppo- 
sing magnets, than I could possibly have imagined. 
On the daily variation of the dipping needle. 
Notwithstanding my observations on the daily change of 
this instrument have not been so successful as those on the 
horizontal needle, yet it will be proper to say a few words on 
the subject of the experiments, although I do not intend, in 
the present instance, to give any numerical results ; those I 
have obtained not being so uniform as I could wish, nor such 
as to justify their publication. 
The instrument I employed was made by Messrs. W. and T. 
Gilbert: it was remarkably free and accurate, and certainly 
gave results with greater uniformity than any dipping needle 
I ever used. The needle was only six inches in length, a 
quarter of an inch broad, and very thin ; it performed in 
the meridian forty-one vibrations in one hundred seconds, 
when under the usual terrestrial influence ; and when masked 
and adjusted by two magnets placed in the line of the dip, 
it made only fifteen vibrations and a half in the same time ; 
the power was therefore reduced about eight times. 
It is not necessary to explain here the means that I em- 
ployed, and the precautions I took to ensure stability ; it 
mdcccxxiii. X x 
