132 New Electro-Magnetic Engine. 
end projecting above the helix. When the bar is very light, for 
instance a tube of sheet iron, and somewhat longer than the helix, 
its upper end will project nearly as much above, as its lower end 
is below the helix. A variety of very pleasing experiments may 
be made with things thus arranged. If the battery circuit be 
broken rapidly, the bar will not drop, but exhibit a rapid vibratory 
or dancing movement. If the battery current be slightly dimin- 
ished without actual interruption, and there are various well 
known ways of doing this, the bar will sink, and rise again on 
restoring the full power of the circuit. The sensation is novel 
and peculiar when the bar is pulled down slowly through the 
helix, owing to the great space—at least three inches—through 
which the action is sensibly maintained. Ifa string be attached 
to the bar and the circuit broken by drawing the wire across 
a rasp or file, to a person holding the string, the sensation is pre- 
cisely that felt by the angler when the fish has seized his hook. 
As pleasing modifications of this experiment, I have contrived 
several instruments, one of which is called the watchman in his 
tower.* 'The helix is mounted upon a stand, and the connexions 
with its extremities so arranged, that when the connecting wires 
with the battery are made to touch the legs of the stand, the ar- 
mature or bar which is concealed within the helix, instantly starts 
up and exhibits the figure of a man upon its upper end, which 
falls back upon breaking the circuit. 
Another curious instrument is the galvanic or magnetic oun. 
Four or more helices arranged successively, constitute the barrel 
of the gun, which is mounted with a stock and breech. The 
bar slides freely through the helices, and by means of a wire 
attached to the end towards the breech of the gun, it makes 
and breaks the connexion with the several helices in succession, 
and acquires such velocity from the action of the four helices, 
as to be projected to the distance of forty or fifty feet. Among 
the useful results of this principle of action, are a galvanometer 
of great value to the experimenter, and the electro-magnetic 
engine. The galvanometer gives an actual measurement by 
weight of any combination of pairs, up to that number which 
is beyond the saturating power of the bar or magnet within 
the helix, that is to say, for an instrument with a given sized 
* This instrument and the magnetic gun will be particularly deseribed in the 
next number. 
