200 
MR. FOX ON THE VARIABLE INTENSITY 
contracting or expanding force*. The magnetic bar is eight inches long, half 
an inch wide, and one tenth thick : its south pole extends rather more than 
two inches beyond the centre of suspension of the wooden support, so that 
there is nearly an equal portion of the southern half of the needle on each side 
of the centre, the north end being extended as a lever to control, in great 
measure, the magnetic movements of the needle'f'. A counterpoise of lead is 
placed at the other extremity of the support, sufficient to allow the north pole 
to dip at an angle of about forty degrees from the horizontal direction. The 
foregoing description also applies to the needle in which the south polarity 
predominates, except that the poles are reversed, the counterpoise being at the 
depressed end of the support. The former I shall distinguish as No. 1, and 
the latter as No. 2. Each is inclosed in a box of slate ; as, if of metal, the 
action of the needles might have been disturbed : and the boxes are mounted 
steadily on bricks, in a room appropriated to them. A slip of glass at one end 
of each box enables me to observe the vibrations, which are from east to west, 
like the horizontal needle. 
I have observed that the magnetic intensity is subject to frequent slight 
variations; but these I have been mostly unable to refer to any obvious cause, 
except when accompanied by the appearance of the aurora borealis, which evi- 
dently affected the needles on many occasions. Their vibrations, I think, 
generally became less rapid with a moist atmosphere, and more so when it was 
very dry; but I do not speak with full confidence on this point, as I have only 
recently made any regular hygrometrical observations. I might, perhaps, also 
mention changes of the wind, and snow storms, as being sometimes attended 
with fluctuations in the intensity of the needles. If the times of their vibration 
at different periods are compared, differences in their relative intensity will be 
noticed ; that of No. 2. having on the average been diminishing, during the last 
three months nearly, in a more rapid ratio than No. 1. This must, I think, be 
considered an interesting fact, if confirmed by future observations. It does 
* If the rivets which attach the magnetic bar to the support are placed at a proper distance from 
the acting pole, the equipoise of the two extremities will not be affected by the contraction or expan- 
sion of the bar. 
t The neutralization of the pole would have been more fully effected had a smaller proportion of it 
extended beyond the centre of suspension ; to 1§ an inch would probably have been sufficient. 
