In France, a royal ordinance requires that every boiler shall be provided with two fusible plates of unequal sizes. The fusing point of the smaller is 10° centigrade (18° of Fahr.) al.mve the temperature of steam having an elasticity equal to that which the steam to be used in the engines, should have. The second plate fuses at 10° (18° Fahr.) above the first.
" The plate does not approach the point of fusion without being softened : it is therefore to be feared that it might give way under a tension much less than that which produces its fusion. At the outset this actually did take place, but the difficulty has been obviated, by covering the plate with a wire gauze with small meshes, before it is fixed by bolts to the aperture which it is to close. Even now parts of the plates yield partially, swelling out in different places as the fusing point approaches, but experience has shown that it is only very near to this point that the metal yields entirely. opening a free passage to the steam. When the fusible plate has been melted, all the steam escapes through the opening which it closed, and it may take some time to replace it, to fill anew the boiler, and to heat the water, during which the engine stands still. In certain cases, this sudden absence of the moving powers might occasion serious accidents. This is a real and a great difficulty,* and perhaps is the reason why our neighbours (the Eng lish) have not adopted the fusible valve, but give a preference to the ordinary safety valve, which never opens, except when the elasticity of the steam within has passed a certain limit, previously fixed by the en gineer, and falls, closing the aperture, when the elasticity has returned within this limit without the moving power failing entirely."t Gualtier directs, that" these fusible disks should be composed in such proportions, that they will melt at a temperature but little higher than that at which the steam boiler of the machine ordinarily works. 2. 1 hat they should be placed in a proper situation, i as it is known that in different points of the surface of the boiler, the temperature is not perfectly equal, and that a disk which would melt at one point will remain solid in another; the best pos:tion for them is over the fire place."1 10. Da771pCrS. To enable the fireman to have full command of the fire, to increase it when i and to diminish it when too intense, and more steam is generated than is wanted, dampers, at the junction of the flues with the chimney, should never be omitted.
These dampers should be and be connected with the safety-valve as proposed by the Chevalier Edelcrantrz; for nothing should be left to the vigilance of the fireman.§ Mr. Renwick suggests that " there should be another, to be worked by hand as occasion may require; and in order to place the fuel under the con trol of the firemen, the passage by which the air is ad mitted to the ash-pit ought also to be capable of being opened and shut at pleasure. Doors and valves for this purpose should therefore be provided, and the apparatus is called a Register."il Too many precau tions to prevent the loss of lives cannot be adopted.
11. Attention to the working of the Engine. As it has been mentioned in the preceding pages, that just before an explosion has taken place, the en gine has been observed to labour, or work more slowly, the engineer should bear this fact in mind, and prevent the calamity by timely attention. The
cause of this slow motion has already been explained, p. 443. The means of prevention therefore is, to avoid opening the safety valve, or steam guage cock, which would cause the water to rise suddenly, and come in contact with the over-heated metal; to with draw the fire as speedily as possible; to check the draught of the chimney by the damper, and to avoid a sudden change of position of the boat.
12. Strengthening the partitions between the Boilers and Passengers. A great means of security to passengers in case of an explosion, is to strengthen the between the boilers and the passengers, and to weaken the other parts of the enclosure, so that the steam should issue out at the place of least resistance.11 Prcfessor Silliman recommends that this bulwark of timber should be made so strong as to resist not only water and steam, but also the fragments that may be pro jected, or even the entire boiler should it be thrown from its bed;** that two boilers be placed on the guards of the vessel over the water, and that the side next to it should be open. This bulwark has been many years adopted in the English steam packets.ft 13. The only infallible means of prevent ing danger from explosions remains to be mentioned. As it has happened, that explosions have occurred on board of boats, when, according to the declaration of those concerned in their management, they could not be ascribed to any of the causes before enumerated, and as similar cases may again take place, the ONLY CERTAIN MEANS to insure perfect safety to travellers, is to place the machinery in one boat, and to tow another containing the passengers. This was one of the measures which the unfortunate Fitch intended to adopt, if he succeeded with his boat, and mentions it in his manuscripts bequeathed to the library com pany of Philadelphia. It has recently been again pro posed by Air. J. R. Sullivan, and demands the serious attention of the proprietors of steam-boats. The at tempt was indeed made three years since by the owners of the Commerce, on the North River, but, owing to the deficient power her speed was much slower than that of other boats, and therefore not much patronized. Mr. S. proposes to use the strong est form of boiler, and to have the Follower of a light sharp construction, with one deck, that the resistance of the water might be small. Professor Silliman re marks that " there seems to be no objection to the plan of any weight, upon waters generally smooth. The excessive speed now aimed at is of no im portance; no reasonable man will be dissatisfied if (sleeping and waking) he can go ten miles an hour. This degree of speed, and probably more, is attain able in tow boats." Ile adds this important consider ation; " the proprietors of steam•boats must answer to their country and to God, if they neglect any prac ticable means of defending their fellow creatures from the most awful calamity to which the confiding travel ler is exposed." But this argument will have little avail with them; the concluding one used by him will have much effect, viz. that '' the firs! boat which is as certained to afibrd absolute security will be a fortune to its proprietors." On the Economy of using high.pressure Engines acting expansively, and condensing.