98 
REV. J. FARQUHARSON’S EXPERIMENTS ON THE INFLUENCE 
admits of the ready attachment of the silk ; so that the needle can be placed 
on the steel point or suspended with the silk, with its flat face either vertical 
or horizontal. 
This apparatus measures, with great accuracy, very minute changes in the 
declination of the needle. A change as small as 10" is quite sensible by it. It 
was placed on a firm-set table, in a room of my house, on the 21st of September 
hist ; and by shifting the whole apparatus, the cross wires of the micrometers were 
brought into a line with the needle, when at rest at 8 o’clock in the evening ; 
the index of the vernier being at the same time brought to zero on the divided 
circle. The readings of the variations of the needle are therefore reckoned 
from its position at that hour, which was made choice of as being that when 
the Aurora borealis was most likely to appear, and thus the diurnal variation 
would be brought to interfere as little as possible with any variation induced 
by that meteor. 
The needle, where it is stationed, is subjected to some influence from several 
tixt pieces of iron, but not to any from iron that is moved from place to place. 
There was a necessity for removing the apparatus temporarily from its station 
on the 24th of November ; but it was carefully replaced on the 2nd of Decem- 
ber ; marks having been made in the floor for the feet of the table, and on 
the table for the feet of the apparatus ; and the cross wires brought to the line 
of the needle at 8 p. m. as before. 
In making observations on the intensity, the time occupied by a certain 
number of horizontal vibrations of the suspended needle is measured by a stop- 
watch, the character of which it is necessary to describe. It is a time-keeper, 
on which, indeed, not much reliance ought to be placed, if it were necessary to 
have the intervals estimated in absolute mean time ; as, although it is adjusted 
to return nearly to mean time at the end of every twenty-four hours, when only 
wound up once during that period, yet it goes very unsteadily at many of the 
intermediate hours. I have found, however, that it keeps time nearly with a 
well-regulated pendulum clock, from 12 minutes after it is wound up till 
about an hour after that time. It is therefore always prepared for the ob- 
servations by being stopped 12 minutes after it is wound up ; and thus, 
although the intensities measured by it could not very safely be compared with 
those measured by a more correct watch, yet considerable reliance may be 
