TRANSACTIONS OF THE SECTIONS. 
21 
as possible. If we increase the stroke, by increasing the distance 
between the bar and the magnets, we diminish the power inversely 
as the square of the distance ; if by applying the power of the mag¬ 
net at the shorter arm of a lever, we diminish only in the inverse 
ratio of the distance: thus, if it is wanted to increase the stroke 
twelve times, the power in one case with the smaller distance, is to 
the power in the other with the larger distance, as the square of 
the larger is to the square of the smaller, as 12 2 :1 2 ::144: 1. 
With a lever, the power with the smaller distance is to the power 
with the larger as the distance in the latter case is to the distance 
in the former,:: 12 : 1. The power in any case is much diminished ; 
but as we can create it in any quantity, this is of little consequence. 
The repulsion of the magnet for the bar, though considerable, is 
much less than its attraction. 
The construction of the machine by which Mr. M’Gauley has 
exemplified the application of magnetism as a moving power is 
easily conceived. An oaken frame supports two magnets, A and B, 
horizontally. The bar P K, fixed in a strong pen¬ 
dulum of wood, of which S is a horizontal section, 
swinging on steel knife-edges, vibrates between the 
magnets, and has attached to its lower extremity a 
rod connected with the reversing apparatus and 
any other required machinery. The poles of the 
magnets are simultaneously reversed, and the bar 
driven with great force from one to the other, and with a ve¬ 
locity of two or three hundred vibrations in a minute. The ap¬ 
paratus for reversing the poles is simple, and can be adapted almost 
without increasing its weight to any combination of magnets. Let 
A B represent the axis upon which the wires rv rv and 
7v' w' f crossing each other under it, are turned; these dip into 
cups of mercury, c and c\ connected with each other by the 
wire Q, and with the zinc plate of the battery by the wire P; and 
into the cups c" and c connected with each other by the wire X, 
and with the copper of the battery by the wire D. w w and w' ?v' are 
connected with the cups V and R by the wires K and N, and with 
the poles of an electro-magnet M by the wires S and T. The wires 
K and N rise in the cups of mercury V and R, but do not leave the 
mercury. Let us suppose, as in the figure, the wires w w and w' w' 
to dip into the cups c c n : we shall trace the electricity. It flows 
