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 rm 

 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 lower 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 n fusible plug relieve the 

 boiler instantaneously of so great a volume. And this is 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 Prony, Dulong, Gerard and Arago, on the expansive 

 force of steam. '^ 



Aim, de Cli. cl tie Ph. Vol. XLHl, p. 74. 



