338 Mr. Bowditch on the variation of the magnetic needle. 
drawn from them, since no noti¢e whatever is taken of the diurnal 
variation of the needle, which sometimes exceeds any of the changes 
that have been observed. For if we examine Professor Sewall’s ob- 
servations in the first volume of the Memoirs of the American Acade- 
my, we shall find that in an interval of two or three months, in the year 
1782, the declination changed at Cambridge from 6° 21’ W, to 7° 08’ 
W, varying 47’; and I have observed at Salem, in the year 1810, that 
the declination varied 48’ in a short period of time. lither of these 
diurnal changes exceeds the alteration observed at New York; and as 
there can be no doubt that the diurnal variation is nearly as great there 
as at Cambridge or Salem, it follows that the differences observed in 
New York are not too great to be accounted for by the diurnal motion 
alone, without resorting to the hypothesis of an irregular increase in 
the mean quantity of the variation. It may also be observed that the 
variation found at the same time and place with different instruments 
will frequently vary half a degree ora 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 observations are made from the windows or on the top of a 
building; the nails and other iron used in constructing it, having fre- 
quently a great effect on the position of the needle, Notwithstanding 
the difficulty of obtaining the correct values of the variation, it is of 
importance to ascertain it, at regular intervals, as correctly as possible, 
particularly in this country, where most of the boundary lines of lands 
are determined by the compass. ‘To assist in this object, 1 shall here 
give an abstract of my observations made at Salem in the years 1805, 
1808, 1810, and 1811, 
The obseryations in the year 1805 were made at a house in Sut 
mer Street, Salem, with a theodolite, furnished with a quadrant of al 
titude, telescope, &c. graduated to minutes, After making the usual 
