provided in the runway, so that the doors may not, as I have frequently
seen them, overrun the opening which it is to protect. Doors should
overlap the edges of the openings on all sides. Large projecting jambs
should never be used.

All doors contained in "fire walls" should have springs or weights
attached to them, so as to be at all times closed. Fire doors can be shut
automatically by a weight, which is released by the melting of a piece of
very fusible solder employed for this purpose. So sensitive is this
solder that a fire door has been made to shut by holding a lamp some
distance beneath the soldered link and holding an open handkerchief
between the lamp and link. Though the handkerchief was not charred, hot
air enough had reached the metal to fuse the solder and allow the
apparatus to start into operation.

These solders are alloys more fusible than the most fusible of their
component metals. A few of them are: Wood's alloy, consisting of:
cadmium, 1 to 2 parts; tin, 2 parts; lead, 4 parts; bismuth, 7 to 8
parts.

This alloy is fusible between 150° and 159° Fahr. The fusible metal of
D'Arcet is composed of: bismuth, 8 parts; lead, 5 parts; tin, 3 parts. It
melts at 173.3°. We can, therefore, by proper mixture, form a solder
which will melt at any desirable temperature. Numerous devices for
closing doors automatically have been constructed, all depending upon the
use of the fusible solder catch.

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