will thus be more effectually dried. The steampipes are made either of iron or copper; and of these, iron best withstands the high temperature of the smoke-box and the impact of the cinders, but it is liable to internal corrosion. The steam-pipe, after entering the smoke-box, divides into two branches, one passing down each side of the smokebox so as to leave a free space for cleaning the tubes, and also to avoid as much as possible the impact of the hot air and cinders; but in some engines the steam-pipe descends vertically, which is somewhat inconvenient in practice. The area of the steam-pipe is one-sixth to one-eighth of the area of cylinder, and the branch steam-pipes are each about one-tenth of the area of cylinder.

The admission of the steam from the boiler to the cylinders is regulated by a valve or regulator, which is generally placed immediately above the internal fire-box, and is connected with two copper pipes, one conducting steam from the highest point of the dome down to it, and the other conducting the steam that has passed through it along the boiler to the upper part of the smoke-box. Regulators may be divided into two sorts, viz. those with sliding valves and steam ports, and those with conical vaives and seats, of which the latter kind are the best. The former kind have for the most part hitherto consisted of a circular valve and face, with radial apertures, the valve resembling the out

stretched wings of a butterfly, and being made to revolve on its central pivot, by connecting links between its outer edges or by a central spindle. In some of Stephenson's engines with variable expan-sion geer, the regulator consists of a slide-valve covering a port on the top of the valve-chests. A rod passes from this valve through the smoke-box below the boiler, and, by means of a lever parallel to the starting lever, is brought up to the engineer's reach. Cocks were at first used as regulators, but were given up, as they were found liable to stick fast. A gridiron slide-valve has been used by Stephenson, which consists of a perforated square plate moving upon a face with an equal number of holes. This plan of a valve with a small movement gives a large area of opening. In Bury's engines a sort of conical plug is used, which is withdrawn by turning the handle in front of the fire-box; a spiral groove of very large pitch is made in the valve-spindle, in which fits a pin fixed to the boiler, and by turning the spindle an end motion is given to it which either shuts or opens the steam passage according to the direction in which it is turned. The best regulator would probably be a valve of the equilibrium description, such as is used in the Cornish engines.

Safety-valves and fusible plugs.-The safetyvalves are placed upon the dome, in Bury's and Stephenson's engines; but it has been found much

better to place them on the cylindrical part of the boiler, because when an engine commences to prime, the water projected from the blast-pipe generally causes an unusual generation of steam, which escapes at the safety-valve, and in its passage of course accumulates and lifts the surfacewater and foam at whatever point of the boiler the safety-valves are situated; thus the further they are placed from the steam-dome the better, as they will then diminish the evil of priming, which, if placed upon the steam-dome, they would only aggravate. Indeed, if the safety-valves are properly situated, an engineman has the great advantage of being able to check or stop the priming of the boiler on the instant, by causing his safety-valves to blow off strongly. It is requisite to place the safety-valves upon a tubular pillar, of such altitude as to prevent the escaping cloud of steam from obscuring the look-out of the engineman. Bury's 14-inch engine contains a pair of safety-valves of 21 inches diameter, exclusive of the mitre; and Stephenson's 15inch engine contains a pair of 4-inch diameter. The latter dimension is preferable, as large safetyvalves are much less liable to adhere to their seats than small ones. Safety-valves require to be tested. occasionally; and the best method consists in attaching the valve joint-pin to one end of an ordinary pair of scales, when the overbalancing weight at the reverse end will indicate the real pressure upon

the valve, which exceeds the nominal pressure by the weight and friction of the lever, with its joints and spring balance, and the adhesion of the valve to its seat. To bring this adhesion to a minimum, it is a good plan to make the lip of the valve-seat somewhat flatter than a mitre, that is, at a less angle than 45° with the horizon: 30° answers very well.

The safety-valve is pressed down by means of a lever, and a screw at its extremity is attached to a spiral spring balance. To find the pressure per square inch, multiply the weight indicated on the scale, by the ratio of the two arms of the lever, and divide the product by the number of square inches in the area of the valve; but to save the trouble of

་

calculation, the ratio of the arms of the lever is made so as to be expressed by the number which represents the area of the valve, so that the weight marked on the balance is the pressure per square inch upon the valve. Some allowance must be made for the weight of the valve itself, and part of that of the lever. It is expedient to put a stop. upon the screw by which the lever is screwed down. or the tension of the spring increased, so as to prevent the pressure from exceeding a safe amount. Lock-up valves, which were intended as a precaution against the recklessness or neglect of the engineer, have fallen into disfavour, as from such valves. being inaccessible and seldom being required to act, they became fixed in their seats; but it is an easy thing to make a valve which can be raised, but cannot be forced down by the engineer, and such valves are in general use in steam vessels. In the engines of Cave, Hick, and Jackson, one of the valves is permanently loaded a little above the usual pressure, and enclosed in a chest; it is usually made with bent, flat, steel springs, pressing against one another, and guided by standards screwed to the valve-seat. One of these valves is shown by fig. 55.

A plug of lead is usually fixed in the furnace crown, which melts if the boiler becomes short of water, and gives notice of the danger. In some engines a cock is attached to the top of the steamdome, against which a small disk of fusible metal is