$86 Mr. Gilpin's Observations on the Variation, 
the compass was adjusted. To obtain the angle that this 
mark made with the true meridian, I fixed a transit-instru- 
ment on the mahogany board above mentioned, precisely in 
the same place where the compass had been placed, and 
having adjusted its telescope to the said mark, the transits of 
the sun and stars over a vertical circle passing through the 
zenith and this mark, were observed ; and the angle contained 
between the said mark and the true meridian, was found by 
computation to be 3 1° 8',8 as above. 
2. For the purpose of ascertaining what error there might 
be, from a want of parallelism between the line joining the 
indices and the magnetism of the needle, and thereby to de- 
termine whether in the usual method of observing, the in- 
dices shew the true angle which the direction of magnetism 
makes with the hrst division or zero, a great many observa- 
tions were made on both ends of the needle, and with both 
sides of the needle uppermost, (the cap of the needle being 
made to fit on readily on either face for this purpose,) viz. 
north end and south end in its upright position, and north 
end and south end with the needle inverted, and the mean of 
the four giving the angle greater by 2', than that shewn by 
the north end in the upright position of the needle, ( which 
was the end always used in these observations,) two minutes 
have been added to all the observations read from the instru- 
ment, as the correction for this error to angles on the east 
side of zero, and subtracted from angles on the west side, to 
obtain the true angle ; which error to angles on the west 
side, however, only occurred, when the instrument was taken 
out of doors to determine the effect of the iron work of the 
building. 
