366 Mr. Barlow on the effects produced in the rales of 
time would serve, if we could obtain such results, to estimate 
the intensity of action of the attracting body. 
But although we cannot detach the balance for such an 
experiment, we may still form some idea of the intensity of 
action, by causing a small magnetized needle to oscillate in 
the place of the balance, and by counting the number of its 
vibrations as above described. Indeed there is not much 
difficulty in estimating, theoretically, the change of intensity 
due to a certain change in the position and distance of the 
attracting body ; but I prefer experiment, as more satisfactory 
to those who may not be able to follow out completely the 
mathematical investigation on which such a computation must 
depend. With this previous view of the subject, I began 
with first ascertaining the time in which forty vibrations were 
made with a small magnetic needle in different situations with 
respect to an iron shell eighteen inches in diameter, and at 
eighteen inches distance from its centre ; the weight of the 
shell being 49 6 lbs. 
But as the degree of intensity, as well as the quantity of 
deviation, occasioned by the iron ball, has reference, not to 
the plane of the horizon, but to the plane of no attraction * I 
proceeded with these experiments as follows : 
Let SQNO' (fig. 2. PI. XXV. ) represent the iron shell, 
or a sphere concentric with it; O O' its magnetic equator, 
or plane of no attraction, and ah , cd, ef, See. parallels of lati- 
tude answering to 6o°, 45 0 , 30°, &c. HH' the horizon, and 
SN the natural direction of the magnetic action in this place ; 
the circle S Q N O' denoting the plane of the magnetic me- 
* See “ Essay on Magnetic Attraction,” page 18. 
