on Magnetic Attraction . 5291 
centre of the ball is moved upwards or downwards ; and CB is a 
line drawn perpendicular to sCn , and meeting OZ in B. Accord- 
ing to the view of the subject which I have advanced, when the 
centre of the ball is in the point B, there should be no deviation 
of the horizontal needle ; when the centre of the ball is above 
the point B, the north end of the needle should deviate from 
the ball ; and when it is below B, the deviation of the north end 
should be towards the ball. 
To ascertain how nearly the experiments coincided with these 
ideas, I computed, for every position of the compass, the height 
of the point B above the plane of the table. The following are 
the results compared with each other. 
Angle from 
| the Meridian. 
Calculated 
height at 
which the 
deviation 
should — 0. 
Observed 
height at 
which devia- 
tion — 0. 
North to- 
wards west. 
Difference be- 
tween the ob- 
served and 
calculated 
heights. 
Observed 
height at 
which devia- 
tion — 0. 
South to- 
wards east. 
Difference be- 
tween the ob- 
served and 
calculated 
heights. 
10° 
+ 4.185 
— 3.95 
— .235 
+ 4.35 
+ ,165 
20 
+ 3.993 
— 3.90 
— .093 
+ 4.20 
+ .207 
30 
+ 3.680 
— 3.70 
+ .020 
+ 3.65 
+ .030 
40 
± 3.255 
— 3.30 
+ .045 
+ 3.25 
+ .005 
50 
+ 2.732 
— 2.80 
+ .068 
+ 2.90 
+- .168 
60 
+ 2.125 
— 2.15 
+ .025 
+ 2.10 
— .025 
70 
+ 1.453 
— 1.50 
+ .047 
+ 1.40 
.053 
80 
+ 0.738 
— 0.90 
+ .162 
+ 0-80 
+ .062 
I made similar observations, placing the needle at the several 
distances of 14, 16, and 18 inches from the centre of the table, 
for the values of <p 40° and 50°, as at these angles the changes 
in the deviation become very sensible. The following are the 
results obtained. 
