th
the external armature of the condenser. Upon connecting the rings, _p_
and _t_, by a wire to which cartridges are attached, any number of the
latter may be ignited.

The parts that we have just enumerated are inclosed in a tin box
covered with a wooden casing, P. Between the two there is inserted a
sheet of hard rubber in order to prevent a loss of electricity; the
whole is held in place by strong springs.

In order to show the normal state of the condenser, a scale consisting
of 15 metallic buttons to give the dimensions of the sparks, is
arranged at X. This scale is capable of being connected with the
rings, _q_ and _t_, by means of chains; when the spark obtained after 15
or 20 revolutions considerably exceeds the intervals of the scale, it
is a sure thing that the machine is in a proper state.

In order to prepare the apparatus for carriage, the winch is taken off
and placed in the compartment, _m_, which is closed by means of a
door, Q.

Figs. 5 and 6 show the arrangement of the dynamite cartridges and
wires in the blast hole. Figs. 7 to 10 show different arrangements of
the igniting wires. Figs. 11 and 12 give the general arrangement for
igniting a number of cartridges simultaneously by means of the
electric machine. Fig. 13 shows the arrangement where powder is
employed. Fig. 14 shows the arrangement of a horizontal
hole.--_Annales Industrielles._

* * * * *

IMPROVED ELECTRIC FIRE ALARM.

The object of this apparatus is to close an electric circuit when the
temperature of a room rises above a certain point. Many devices have
been invented for effecting this object, each of which have their own
advantages or disadvantages. The invention of Mr. Pritchett enables
the required result to be obtained in a very satisfactory manner. The
apparatus consists (as shown by the figure) of a long glass vessel
containing air; connected to this vessel there is a glass tube filled
with mercury. The whole is mounted on a metal cradle, which turns on
pivots. According to the position which the glass vessel and its
adjuncts occupy in the cradle (this position being adjustable by means
of a thumb-screw, seen at the upper part of the cradle), so will the
same have a tendency to rock longitudinally over to one side or the
other. Now, if we suppose the position to be such that the right hand
end of the glass vessel is depressed, and the left hand end raised,
then if the vessel becomes