Mr. Bowditch on the variation of the magnetic needle. 339 
adjustments, and fixing as nearly as possible the quadrant of altitude, 
and the north point of the needle at the commencement of the gradu- 
ation of their respective’circles, I estimated the errors of these last ad- 
justmenis, and applied them respectively to the observed altitudes and 
azimuths of the sun, ina similar way to the method of correcting for 
the index error of an observation made with a quadrant of reflection. 
To ascertain these index errors. to a greater degree of accuracy, I 
generally took the mean of ten observations of the needle, and three 
observations of the quadrant, before and after eachset of observations. 
The instrument was placed within the house, at an eastern window in 
the morning, and at a western in the 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 
magnetic azimuth, the true azimuth was calculated and the variation 
deduced by the usual rules of spherical trigonometry. The observa- 
tions were as follows. | 
ad, 4,5 Ta OF Cae salle ig 
1805 November 18915 A.M. 4 observations | 5 58 W. 
4, Pp. M. 5 6 17 
19 9 A. M. 6 6 02 
Zz P. M. 6 5 56 
21:2 P.M. 10 6 15 
2379 A.M. 9 5 56 
2 80 P.M. & 5 45 
26 2 P. M. 10 5 51 
27 9 . M. 12 5 42 
3 P. M. 10 6 o1 
28 9 A.M. 10 5 43 
3 Po MM. iz 6 06 
299 A.M. 3 § a2 
30 9 A. M. 12 6 o1 
Mean of 115 observations 5 57 We 
& 
NS 
