Mr Barlow's Electro-Magnetic Experiments . 375 
either, but both conspiring to give to the wire the rectilineal mo- 
tion which has been described. This experiment is also due to 
Mr J. Marsh. 
Exp. VI. — “ To exhibit a wheel and axle rotation by means of 
a horse-shoe magnet 
“ The machine by which this motion is produced, is represent- 
ed in Fig. 8.5 where AB is a rectangular piece of hard wood* 
CD an upright wooden pillar, DE a piece of stout brass or 
copper wire, and ab a somewhat smaller wire, soldered upon it 
at E, on the lower side of which the wheel W, of thin copper, 
turns freely ; hj is a small reservoir for mercury sunk in the 
wood, and gi a narrow channel running into it : HH is a strong 
horse-shoe magnet. Mercury being now poured into the reser- 
voir fg, till the tips of the wheel are slightly immersed in it, 
and the surface covered with weak dilute nitric acid, let the 
connection with the battery be made at i and D, and the wheel 
W will immediately begin to rotate with a great velocity. If 
the contact be changed, or if the magnet be inverted, the mo- 
tion of the wheel will be reversed ; but, in general, the best ef- 
fect is produced when the wheel revolves inwards.” 
Exp. VII. u To exhibit a compound wheel and axle rotation 
with two horse-shoe magnets . 
fi£ The machine for producing this motion is shown in Fig. 9, 
ABGD is a rectangular piece of board, having two grooves, 
about half an inch deep, cut in it parallel to its length. C p 9 
Zq are two wires having cups for connection at Z and C, and each 
passing into its respective groove ab , cd, filled with mercury; into 
which are slightly immersed the points of the wheels of W, W' ; 
these being fixed on an axle W W', and resting upon the two 
supports mn, rs , brought to a fine edge at n and s , in order to 
reduce the friction as much as possible, and to give the greater 
freedom of motion. N and S are two horse-shoe magnets, placed 
as in the figure, with the like poles interior and exterior of the 
wheels. 
“ The apparatus being thus prepared, and the contact made 
at Z and C, the wheels will begin to revolve, and in a very short 
