Mr. Bowditch on the variation of the magnetic needle. 341 
needles; and, perhaps in part from the imperfection of the brass of 
which the instruments were made. ‘To obviate these difficulties I 
procured a needle 24 inches in length, suspended on an agate, and had 
it neatly fixed in a mahogany box, moveable at one end on a pivot by 
which the box was attached to a board, marked with a graduated arch 
of a circle, subdivided in such manner that minutes of a degree could 
easily be read by means ofa nonius. ‘The box was made wholly of 
wood and ivory, and when fixed in its place there was no iron near it. 
A table about three feet in height was fixed in the middle of a room 
of the building in the north part of Market-street, and by means of the 
theodolite and the sun’s azimuth, I marked on the table, with great 
care, a frue meridian line, and then placed the box on it, and observ- 
ed the differences between the true and magnetic meridian for every 
hour, when convenient, from 6 A. M. to 10 P. M. from April 1810, 
to May 1811. The greatest variation observed during this time was 
6° 44’ W. The least 5° 56’ W. To ascertain whether the building 
affected the needle, I fixed a true meridian line on a table in the gar- 
den adjoining the house, at 30 feet distance from any building, and 
nearly five feet from the ground, and by the mean of 48 observations, 
I found that the variation in the garden by this instrument was less by 
3' 25” than in the house, so that it was necessary to subtract this quan- 
tity from all the observations to obtain the true variation. The mean 
variation for each hour of the day, and for each month of the year, 
as deduced from these observations, and corrected for the error 3’ 25”, 
are given in the following tables. 
