5. Might of the in the Boiler. As it has been ascertained, that the greater part of the mortality from steam explosions proceeded from inattention to the great point of keeping up the due supply of water in the boilers, this should never be. lost sight of. Captains of boats ought not to trust this business solely to the engineer or fireman.
To ascertain the height of the water in the boiler, it is well known that the original expedient, and the one commonly used, consists of two stop-cocks in front of the boiler, one of which communicates with the water, the other enters the boiler above it. The issue of steam from the first indicates the deficiency of water, and the discharge of water from the last, shows that it stands too high. But these cocks do not upon all occasions answer the end intended by them. Mr. Potts observes," when an engine runs regularly, it is not difficult by this means to make the supply of water to the boiler by the hot water pump equal to the evaporation; but every variation that takes place with regard to the load with which the engine labours, or the velocity with which it runs, tends to disturb the equilibrium between the supply of water and the evaporation, attained by any previous adjustment of the pump. Again, when a temporary stoppage of an engine is necessary, as for taking in, or discharging Passengers, the fire is rarely abated to such a degree as to prevent all further evaporation. The supply from the pumpbeing cut off, the water sinks rapidly." Cases have even occurred in which explosions have happened in the boats of the United States, when going at their regular speed, as was shown by the steam gunge, if the solemn asseverations of the cap tains are to be believed, and immediately after the water-cocks had been opened and indicated the supply. The boats Etna, United States, and Chief Justice Mar shall, are cases in point, as has already been men tioned under the third head of the causes of ex plosions.
This fact shows clearly, that implicit reliance ought not to be placed for safety, upon the guage-cock emit ting water. But further, " even where the water is not so far reduced in the boiler, as to be below the top of the tube, I am by no means inclined to consider this boiler in a safe state. The plates of a boiler urged by an intense fire, and covered with only a thin stratum of water, become very considerably hotter than the steam and water above them. The unequal expansion resulting therefrom, renders the parts more disposed to give way, when further stress comes upon them.'" A boiler or flue therefore, repeatedly thus treated, may give way upon the occasion of an extra strain from high steam, although the guage-cocks will show water in the boiler. This deficiency in the guage-cock may
be remedied in low pressure stationary engines, by the means mentioned by Mr. Farey as having been used by Mr. Smeaton, viz. " A pipe going down beneath the surface of the water in the boiler, and at the upper end of the pipe, at the top of the house, a whistle mouth-piece is formed; then, if the water sinks too low, the steam will issue at the pipe, and give the alarm."t Professor Hare proposed that a hollow globe, swim ming on the water of the boiler, should open a small cock so as to produce a jet of steam in front of the boiler, whenever the water should sink too low.' This contrivance is applicable to high and low pres sure engines; an objection to it has been raised, viz. that by frequent expansion and contraction, the parts soldered together have opened so as to cause the vessel to fill with water and sink. A little attention may certainly obviate this difficulty. Pumice stone is used as a float, for Watt's feeder at the Globe Mill. The glass tube water-guage will afford additional means to determine the height of the water.
With the same object Mr. J. L. Sullivan has in vented an alarm bell-foot, and phonic gauge for boil ers; the object of which is to cause the water itself to give the alarm. For this purpose he uses bells or metallic triangles within the boiler, to ascertain where the surface of the water is within certain limits. The principle of their operation is founded in the fact, that bells emit a louder sound in compressed air, than in the atmosphere. Two bells are to he placed in a boiler, one an inch higher than the other, with suit able wires leading from each tongue through packing 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 that the surface is between them. It is in tended 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 action of the fire without, or under, whereby the flue or sides unprotected by the water might become red hot, and impart great heat suddenly to the water, and causing so great an increase of high steam, that the safety-valves cannot vent it. A specification of this plan is contained in Silliman's Journal for 1831, and deserves an experiment by all those to whom passengers entrust their lives, on board steam-boats.