before one of the apertures, while at the same time an alarm begins to
ring. The same maneuver performed by the agent at the receiving-post
has the effect of causing the disk to disappear. The two contact
springs in communication at each aperture with the alarm and the line
are connected by a strip of ebonite, M, against the center of which
presses the button.

_Electrical Controllers for Water-Tanks._--The object of these
apparatus is to warn the person in charge of a water-tank that the
latter is full, and that he must stop the engine-pump; or, that the
tank is empty, and that he must at once proceed to fill it. The
Company of the North has on exhibition two such apparatus--one of them
Lartigue's, and the other Vérité's.

1. _The Lartigue Controller_ (Fig. 15).--This apparatus consists of a
long lever, A, which carries at one of its extremities a funnel, E,
having a very narrow orifice and which is placed under the overflow
pipe of the tank. The lever is kept normally in a horizontal position
by a counterpoise; but, as soon as the overflow runs into the funnel,
the weight of the water tilts the lever, and the mercurial commutator,
F, closes the circuit of a pile, which actuates an alarm-bell located
near the pump and engine. The two stops, a and _a'_, limit the play
of the lever.

2. _The Vérité Controller_ (Fig. 16).--This apparatus consists of a
float, F, provided with a catch, C, calculated in such a way as to act
only when the float has reached a certain definite height. At that
moment it lifts the extremity of the weighted lever, E, which in
falling back acts upon the extremity, a, of another lever, N,
pivoted at the point, O. The piece, P, which is normally in contact