horizontal needle when under the influence of magnets. 345 
daily motion are considered minus, and those in a contrary 
direction plus, whatever may be the position of the needle. 
The first observations which I made, being principally with 
a view of ascertaining how my apparatus would answer, and 
likewise the general characters of the deviations, were neces- 
sarily somewhat imperfect ; but as they show the great extent 
to which the deviation may be rendered sensible, and first 
indicated to me distinctly, what I afterwards found so de- 
cidedly marked, the morning easterly deviation, I will not 
omit them. The needle which I made use of is a very light 
one, about ■— of an inch broad close to where the agate is 
centered, and diminishing from there to very fine points at 
the ends : its length is six inches. The rim within the com- 
pass-box is very accurately divided into degrees, and thirds 
of a degree ; so that, from practice, I can easily read off the 
directions of the needle to every two minutes. It may at 
first sight appear superfluous to mention any particulars 
respecting the room in which the observations were made, 
but as both Mr. Barlow and myself found some anomalies 
in the directions of the needle, when corresponding observa- 
tions were made in different situations, and as these anoma- 
lies may perhaps be attributable to such local circumstances, 
I shall always state them as nearly as possible. The obser- 
vations contained in the following table, were made in a 
room with a single window, facing E 40° S, magnetic, and 
having an iron grate in it, the situation of which was S 15 0 E, 
distant six feet from the needle ; but care was taken that no 
iron should be moved in the room, nor any small articles 
of iron brought into the immediate neighbourhood of the 
needle. 
In the first column of the table, the hours are marked at 
mdcccxxiii. Y y 
