Mr. Dodd, an eminent British steam engineer, "disapproves of those safety-valves which have a co nical seat, as they arc liable to be jammed in, and also fastened by contraction : and also those that are pressed down by a weight on a lever, as accident or design, by altering the position of the weight, may increase or decrease the pressure, and they are pecu liarly inapplicable to the motion of vessels at sea. With lever safety-valves even the most cautious man may, in the hurry of business, place the weight too far out on the lever, and if the lever be to short to admit this, rash men frequently put on an extra weight, and thus endanger the property and them selves, and those on board. The best inaccessible valve" is formed by fixing round the hole or orifice on the boiler, a circular brass ring flat on the upper surface. From the centre of this ring, and secured to the under side thereof, arises a spindle or pivot, passing into a cylinder closed at the top in the mid dle of the valve and weight. This pivot should be grooved on the sides, to allow the entrance of steam; and should not extend to the top of the cylinder, nor fit tight by the tenth of an inch. This cylinder has a brass flat bottom resting on the flat circular ring, and both surfaces so finely ground as to be air and steam tight: valves similar in this respect have been long and successfully used. A leaden circular weight, hav ing a hole up the centre, is placed over the brass tube or cylinder, and rests on its base. The whole is surrounded by an iron box rivetted to the boiler, and has a lid fitting tight and locked. From the side of this box passes a pipe of sufficient bore to convey away the escape steam. The steam from the accessi ble or common valve is conveyed by a pipe through the vessel's side into the water, and to prevent a va cuum in this pipe, and the cold water rising up to the safety valve, connected to this steam-escape-pipe is a reverse or atmospheric valve. That it may be known when the accessible safety valve is out of order, we pass the escape-steam-pipe of the inaccessible valve into the paddle boxes, from whence the steam will be heard and seen to issue in the event of that valve be ing forced to act; and all on board may know that something is wrong with the other valve. Yet this circumstance should create no alarm, for however much he may urge the fire, the steam will escape by that valve, and foil his utmost effort to raise it to a dangerous strength."t These particulars are given, as Mr. Dodd's work is not common in the United States.
8. Steam Guage. Ample and afflicting experience having demonstrated, that safety-valves do not on all occasions afford security against explosions, no steam engine on shore, or in a boat, should be without a mer curial Steam Guage. For, "although the load on the safety-valve makes a sufficient regulation of the strength of the steam, to avoid any danger of bursting the boiler, it is not a sufficiently accurate indication, to enable the engine man to keep up the steam always to the same elasticity. Mr. Watt, therefore, employed a steam guage, which operates by a column of mercury. This steam guage consists of an inverted syphon, or bent tube of glass or iron ; one leg of which communicates with the boiler, being joined to the steam-pipe, and the other is open to the atmosphere. A quantity of mercury is poured into the tube, to occupy the bent part which joins the two legs ; and the mercury in merely being exposed to the pressure of the steam, while the external air acts upon the other, it is evi dent that the difference of level of the two furnaces, will express the elasticity of the steam above or below the atmospheric pressure, by the height of a column of mercury it will support. When the tube is of glass, this difference of level may be seen and measured on a scale of inches ; but when an iron tube is used, a small light wooden rod, floating on the surface of the mer cury in the open leg, points out the height of the col umn against a scale of half inches, fixed above the open end of the tube. In this case, the divisions,
which arc numbered for inches, must be only half inches; because the mercury will descend in one leg, as much as it rises in the other; so that the scale must be doubled, to show the real difference between the two surfaces.
" The tube is made of wrought iron, in the same man ner as a gun-barrel, but with the two ends bent par allel, like the letter U : the interior of the tube ought to be bored, in order that both legs may be precisely of the same diameter, otherwise the guage will not show the pressure correctly, because the mercury will not sink so much in one leg as it rises in the other. A steam guage of this kind is usually attached by two screws to the steam pipe, or else to the end of the boiler, or at any part having open communication with the boiler, and in a convenient place for the en gine keeper to see it, because this should be his con stant guide for the regulation of the fire and the damper." If this steam guage be properly fixed, so as to be always kept upright, and of a diameter adapted to the size of the boiler, it can never fail to point out the elastic force of the steam within it; as by the rising of the mercury, which is shown on the scale attached to the tube, the fireman can ascertain at once, whether the pressure be greater than the boiler is cal culated to bear. If the mercury becomes stationary, it would instantly point out that the tube was stopped, and give time to guard against danger. "It is also a capital counter security to the valves, for if this guage indicate a higher pressure of steam than that at which the valves ought to rise, the engineer may know that they are impeded, and rectify the error. It is the duty of the man frequently to look at this guage, that he may know when to increase the fire in the furnace, and occasionally to tap the guage, that by the excited action of the mercury, and the indicating rod (where the syphon is not of glass but of metal), he may be assured that the action of the guage is free and uninterrupted. Every steam packet on the Thames is provided with one of these guages."§ The utility of the steam guage was shown in the case of the Legislator of New York (p. 445). The rod rose as high as the deck would permit, thus proving an immense pressure on the boiler.
9. Plates of Fusible Metal. "As soon as it was found that the common safety-valves sometimes got out of order, and did not present a certainty of security, it was proposed to replace them by an entirely different contrivance. the action of which should never be uncertain. This was the fusible metal valve, described in the ' Annuaire du bureau des longitudes' for t829 and 1830. To understand rightly the use of these valves, we should know, that it is possible for steam to have a very high temperature, with but little elasticity, but not possible, that a great degree of elasticity, should not be accompanied by a high temperature. Experimenters have determined the lowest temperature necessary for steam to acquire a tension of one, two, three, ten or more atmospheres. By using these results, we can know what tempera ture the steam must not surpass after we have fixed on the pressure. If we then cover an opening in the boiler with a plate made of an alloy of lead, tin and bismuth, in proportions such that the alloy will melt at the limit of the temperature fixed upon beforehand, this temperature can never be exceeded, for on reach ing it, the plate melts and gives vent to the steam.