- 
On the means of safety in Steam Boats. 5 
metallic bodies—that steam is elastic and dense, and therefore a good 
conductor of sound: therefore, a bell fixed in a boiler above the water 
will ring. But I have ascertained, and it is obvious, that if its rim 
touch the water it will not vibrate or sound; so that when two bells 
are placed therein, the one, (for example,) an inch higher than the 
other, with suitable wires leading out from each tongue through pack- 
ing to the front of the boiler ; if the lower one touch the water it will 
not ring, while the upper one being above the water will sound and 
be heard ; thus making it known to the engineer that the surface is 
between them. In like manner any requisite number of bells, or 
sonorous bodies, of the proper size, may be placed the one, half an 
inch or an inch higher than one and Jower than another, so that the 
actual place of the surface through the vertical space occupied by 
the whole may be known within half an inch or less, whatever the 
temperature of the steam. And I give the bells a shape preferably, 
with perpendicular sides, and without flare, in order to prevent sedi- 
ment from lodging thereon, which might, if in extreme, lessen the 
sound ; and if the water used be very foul, I place a cover somewhat 
above and partly around them each, for the same purpose. 
The alarm bell (or other sonorous body) is of the largest size, 
preferably, which the upper part of the space, above the ordinary 
reach of the water, will admit. It is intended to ring spontaneously 
whenever the water shall happen to subside so much as to make bare 
and expose the furnace or flue to the action of the fire within: or, if 
a single cylindrical boiler, exposing some part of the sides to the ac- 
tion of the fire without, or under, whereby the flue or sides. unpro- 
tected by the water might become red hot, and impart great heat 
suddenly to any accession of water, causing, (as writers on the sub- 
ject say,) so great an increase of steam of high temperature and great 
expansive force, that the safety valves cannot vent it. Nor, for the 
same reason, could an opening made by a fusible plug relieve the’ 
boiler instantaneously of so great a volume. And this js manifest 
from the law of increment of expansive force, compared with in- 
crease of heat, as exhibited in the following extract from the table 
of results of Frei Dulong, Gerard and Arago, on the expansive 
force of steam.* 
* Ann. de Ch. et de Ph. Vol. XLIL, p. 74. 
