horizontal needle when under the influence of magnets. 361 
During this day and the following, until the observations 
were discontinued, I observed the barometer every hour, but 
as it was remarkably steady, (varying during the 26th only 
from 29.77 at 3 h A. M. gradually to 29.75 at 3 h P. M., and 
from that to 29.82 at n h P. M., and on the 27th from 29.92 
at 5 h A. M., to 30.0 75 at i h P. M.,) change of atmospheric 
pressure could have little influence in the deviations which 
took place. I regret much that I did not observe, what I 
now consider of more importance, the temperature both of 
the instrument and of the atmosphere. There was continued 
rain nearly through the whole day, no sensible change taking 
place in the temperature of the air ; and I am persuaded, 
from this circumstance and the situation of the place in 
which the observations were made, that no material change 
took place in that of the instrument. 
The observations from the 20th to the 27th of April, are 
exhibited [PL XXV/] in the same manner as were those at 
north. In the present instance, the easterly deviations were in 
a contrary direction to what they were in the former case, but 
I have still placed them towards the left, that the similarity 
of the curves in the two cases may be the more evident. 
The principal irregularities to be observed here take place 
in general from about three o’clock in the afternoon to seven 
o’clock ; and we might almost be led to infer from this cir- 
cumstance that there are two causes, in opposition to each 
other, producing the deviation, and that these being nearly 
equal at this time, alternately preponderate : but the obser- 
vations are too limited to allow of our drawing such general 
inferences from them. We shall see, from observations in 
other positions of the needle, that irregularities very fre» 
quently take place during the afternoon. 
MDCCCXXIII. 3 A 
