In fact, a spare boiler should be provided wherever stoppages are of serious importance in a concern. The strength of low-pressure boilers should be twice the regulated pressure on the safety-valve; but high-pressure boilers should be proved to at least three times their working pressure. Upon the form of boilers much of their strength and their efficacy in the generation of steam depend. The earliest boilers employed for generating steam, and applied as a motive force, with which we are acquainted, were either globular or hemispherical, with flat or concave bottoms; and many of these are still in use for the supply of high-pressure engines. In situations where fuel is so abundant as to render needless any economy of it, they may continue to be used with advantage, as their form adapts them to withstand steam of great elastic force. The waste of fuel caused by them, elsewhere, led to a very general substitution of boilers of an oblong form; and those known by the term of " waggon boilers," from their shape, formed one of the many improvements of the steam engine introduced by Watt.

A longitudinal section of a boiler of this kind is represented by Fig. 1 in the subjoined engraving, fitted up with all the appendages now generally applied, and set in a furnace of the usual construction, a a a is the boiler, having a cylindrical return flue b throughout its length; the form of these parts will be best understood by considering them with reference to Fig. 2, which shows the figure of the boiler in its cross section, b being the return flue; this portion of the boiler should be always placed near to the bottom, and be constantly kept covered with the water, as seen at c in the drawing. The flame and smoke from the furnace d first passes under the boiler, then returns through the flue b to the front, where the current is divided, and passing to the right and left through lateral flues in the brickwork (one of which is brought into view at e e) before it enters the chimney f f. The water is supplied to the boiler by the feed-pipe g, which is made to contain a column of water equal to the amount of pressure in the boiler.

On the top of this pipe is a cistern h; i is a float, made of stone, suspended by a wire passing through a stuffing-box j, which is attached to one extremity of a lever, whose fulcrum is at k, a weight f being suspended to the extremity of the other arm of the lever sufficient to counterbalance the difference between the specific gravity of the stone and the water, causing the former to lie on the surface of the latter. By this arrangement, when the water sinks below the proper level in the boiler, the stone float descends with it, causing the attached wire to operate upon the c c lever above, and lift the valve shown in the cistern h, which then affords a fresh supply of water. The feed pipe likewise contains an iron bucket, hung by a chain that passes over two pulleys o o; the other end of the chain is attached to a plate of iron p, called the damper, which is used to enlarge or contract, and, when required, entirely close the throat of the chimney.

By this contrivance, when the steam in the boiler is urged to too great an extent, its pressure forces the water in the feed pipe upwards, thereby raising the bucket m, and causing its counterbalance, the damper, to descend in the throat of the chimney, and reduce the intensity of the fire. At q and r are two guage-cocks, by which the proper height of the water in the boiler may be always ascertained. If q discharges water, and r steam, the water is at the proper height; if both cocks discharge water, it is above its proper height; and if both discharge steam, it is below its proper height. This latter circumstance, which can only take place by the defective action of the last described apparatus, is of serious moment, and requires an immediate remedy; the most safe is, in our opinion, the reduction of the fire, by stopping up, by the readiest means at hand, all access of air to the grate, after which, attention may be paid with confidence to removing the cause of the defect. For the purpose of showing at all times the pressure acting upon the boiler, a steam-guage is employed.

This is usually made of an iron tube, bent into the form represented, one end communicating with the steam-room of the boiler, and the other open to the atmosphere.

This tube is partly filled with mercury, and into the externally open end is put a slight rod of wood, which floats perpendicularly in the mercury, and shows by its altitude, on a divided scale of inches affixed to that leg of the tube, the pressure of the steam. If the steam raises the mercury or rod one inch, it proves that the pressure is one half pound per square inch on the internal surface of the boiler, tending to burst it; for if the section of the bore of the pipe was just one inch, the pressure would be supporting one cubic inch of mercury, which will be found to weigh early half a pound; therefore, for every two inches rise, one pound pressure may be reckoned; and as condensing engines seldom work with more than three or four pounds pressure upon the inch, the scale need not be longer than eight or nine inches For preventing the pressure becoming greater than the boiler is calculated to sustain without incurring danger, a safety-valve u is provided. This is usually a circular piece of metal, with a conical periphery ground to fit into a conical seat, and kept there by the pressure of a lever, loaded with a determinate weight, that will not suffer it to rise until the steam has acquired an excess of force above that required for working the engine, or would endanger the bursting of the boiler.

In large boilers, especially those in steam boats, it is usual to have another safety valve, inclosed in a box under lock, the key being kept by the chief manager, to prevent the possibility of improper interference with it, by ignorant or imprudent persons: by the neglect of which precaution many serious accidents have occurred. The steam which escapes from safety valves is generally conducted by a pipe from the valve boxes into the chimney. At v v is the steam-pipe by which the engine is supplied, the quantity being regulated by a throttle-valve at w, and a screw-down valve at x. For the purpose of cleaning out or examining the condition of the boiler, a circular or oval hole y is made sufficiently large to admit a man to pass through, and therefore called the man-hole.