Variation of the Magnetic Needle. 65 
great there as at Cambridge or Salem, it follows that the dif- 
ferences observed in New York are not too great to be ac- 
counted for by the diurnal motion alone, without resorting 
to the hypothesis of an irregular increase in the mean quan- 
tity of the variation. It may also be observed that the 
variation found at the same time and place with different in- 
struments will frequently vary half a degree or a degree; 
and, by changing the place of the instrument a few feet, 
the same effect will sometimes be produced. This is more 
particularly the case in compact places, when the observa- 
tions are made from the windows or on the top of a building; 
the nails and other iron used in constructing it, having fre- 
1805, 1808, and 1811. 
The observations in the year 1805 were made at a house 
in Summer Street, Salem, with a theodolite, furnished with 
a quadrant of altitude, telescope, &c. graduated to minutes. 
Atter making the usual adjustments, and fixing as nearly as 
possible the quadrant of altitude, and the north point of the 
needle at the commencement of the graduation of their re- 
spective circles, I estimated the errors of these a 
ments, ied them respectively to the observed alti- 
tudes an f the sun, ina similar way to the meth- 
evening, at the distance of two or three feet from the wall, 
(or farther when it could be done) in order to avoid as much 
as possible the effect of the iron in the walls of the building. 
Having obtained in this way the sun’s true altitude and mag- 
netic azimuth, the true azimuth was calculated and the va- 
Vor. XVE.—No. 1. 9 
