The boiler, besides the danger of bursting from over-pressure of the steam within it, is also liable to injury by the external pressure of the air, if the steam within be condensed, as it must be on cooling, when the fire is let out, thus causing a comparative vacuum in the boiler. To guard against this, there should be a safety-valve to act in the direc tion opposite to the usual one, which, yielding to the pressure of the atmosphere, would allow of the entrance of the air, when this pressure exceeded the one exerted by the steam to keep the valve closed.
Another source of accident, which should be guarded against most sedulously, is the formation in the boiler of a deposit of the earths, &c. chemically united with the water or held suspended in it, and which are deposited from all water when long kept boiling in any vessel, as is illustrated and proved by the furring of old tea-kettles. This deposit is intensely hard, and adheres so closely to the metal, that it requires a chisel and hammer to detach it when accumulated to any thickness. Being a bad conductor of heat, it prevents the rapid generation of steam, and by not allowing the water to be in contact with the metal, so as to carry off the heat imparted to the latter, the metal gets red-hot and is burnt, or, in chemical language, becomes oxidised by long exposure to a high temperature. If, under these circumstances, a fissure should be produced in the earthy crust, the water, suddenly admitted to contact with the red-hot iron, is converted instanta neously into steam of such high pressure as to risk the bursting of the boiler.
It is one of the advantages held out as an inducement to their adoption by the inventors of tubular boilers, that, owing to the in equality of the temperature of the liquid at different distances from the source of heat, a circulation is continually going on, which mechanically prevents the formation of a deposit, while in large open boilers no such cause can operate to any extent. In these the remedy appears to be the frequent cleaning out of the boiler, to prevent the accumulation of the deposit, and the admission of the water which supplies the waste to the part not over the furnace, which should be separated from the rest by a partition extending upwards nearly to the level of the water ; the fresh water is thus allowed to deposit its sediment in the part where it can be least productive of the evils alluded to, and whence it may be removed collectively from time to time.
To indicate the actual pressure at any time of the steam within the boiler, this is furnished with a gauge (fig. 12), consisting of a bent tube, open at both ends, one orifice A opening into the boiler. This tube con tains mercury, which will obviously be at the same level in both branches when the steam in the boiler is of the same pressure as the atmosphere, but will rise in the longer leg as the pressure of the steam increases, and will thus by its altitude indicate that pressure. If the longer leg
be of a certain length, the mercury would flow over, or be blown out; altogether, if the steam were suddenly to increase in its elastic force, the gauge would in this case act as a safety-valve. The loss of the mercury may be guarded against, under these circumstances, by a cistern placed round the orifice of the gauge to receive it, as shown in the figure.
A water-gauge is often used instead of a mercurial one, to save the expense of this metal ; but then the tube must be long enough to allow a sufficient column of water to balance the pressure of the steam ; and by making the tube of sufficient diameter, this water-gauge then constitutes an efficient safcty.valve. As these gauges cannot, for obvious reasons, be made of glass, to allow of the height of the fluid within them being directly observed, this height is indicated by a light wooden rod projecting beyond the cnd of the gauge, which floats on, and therefore rises and falls with, the mercury or water.
In locomotive-engines, where the use of a mercurial and still more of a water-gauge is impracticable, the same end is attained by a ther mometer, on the well-known principle that the temperature of steam is always in a constant relation to its pressure ; or by Bourdon's steam gauge, in which the steam acts upon a diaphragm, as in the aneroid.
If there be not water in contact with that part of the boiler exposed externally to the direct action of the flame and hot air in the furnace and flues, the iron would become red-hot, and so suddenly increase the pressure of the steam in contact with it, that an explosion would pro. bably ensue ; and if not, the iron in that part would be more oxidised or burnt. To prevent this it is necessary that there should be always water in the boiler above the level of the highest part of the flues ; and to enable the engineer to ascertain whether this is the case, there are in all boilers two pipes with cocks, one of which dips down into the water, while the other reaches only to within a few iuches of its surface when at the right height in the boiler ; the consequence of this arrange ment is, that if the cock of the shorter pipe be opeued steam will issue from it, and water from the other when that is opened in its turn ; but if steam escapes from both cocks, the engineer is warned that there is not sufficient water in the boiler, and therefore directs his attention to remedy the deficiency.