and on the Dip of the magnetic Needle. 387 
3. The variation compass being placed in the house for 
observation, could not be supposed to be entirely out of the 
influence of iron ; I was therefore desirous to ascertain how 
far that influence might extend ; for the determination of 
which, the following method was adopted. 
Having caused to be sunk into the earth to some depth a 
strong post, in the wood yard of Somerset House, at a consi- 
derable distance from the influence of any iron, on which the 
compass might be placed, and from which station, there was 
a convenient mark at a proper distance to which its telescope 
could be adjusted ; I took the compass there at those times 
of the day when the needle was stationary, viz. morning and 
afternoon: before the compass was carried out of doors, obser- 
vations were made in the room ; then it was taken out of 
doors to the above mentioned station for observation there ; 
and the observations were again repeated after the compass 
had been restored to its situation in the room ; so that had 
any alteration taken place in the interval, such alteration 
would have been detected ; but during the whole series, no 
material difference occurred between the observations made 
in the house before, and after those taken in the yard. 
The observations therefore made in the yard, compared 
with those taken in the house, both before and after those 
taken out of it, formed the comparison for obtaining the error, 
or the effect of the iron w'ork of the room on the needle in 
the house, and there is reason to believe that considerable 
accuracy has been obtained. They are as follow. 
By a mean of 20 sets, or 200 observations taken with the 
compass in the yard, compared with twice that number taken 
in the house, before and after those taken in the yard, the 
