through the valve and pipe for every hundred pounds of water evaporated; or, put in a more convenient form,

Area through main feed-valve in square inches

= total heating surface in square feet ÷ 240;

and area through donkey feed-valve in square inches

=

total heating surface in square feet ÷ 300.

As the feed-valves cannot always be placed on that part of the boiler best suited to receive the feed-water, and also in order to distribute that water so as to avoid its affecting the boiler plates, and internal pipe should be always fitted. To avoid the necessity of blowing the boiler down in case of accident to the feed-valves, it is a very common practice to fit these valves high up on the boiler, even in many cases above the water-level. This plan also has the advantage of providing a means of warming the feed-water, than which nothing is more essential for the preservation of the boiler; the heating is effected by the passage of the water through a long internal pipe of brass or copper, which leads it to where there is a down current of water, so that the comparatively cold feed-water may not interfere with the circulation.

Some engineers prefer to inject the feed-water in the form of spray, either above or a little way

beneath the surface of the water in the boiler; this avoids all chance of injury to the boiler plates, as any gaseous matter mechanically mixed with the feed-water is at once given up and mixes with the steam.

Great care should be taken in any case that the internal feed-pipes "run full;" that is, that they are never filled with steam, but always with

water.

The dynamic effect of the steam in the feedwater, when mixed inside the pipe, is very startling; every stroke of the feed-pump produces an explosion, and in a very short time both external and internal pipes are damaged seriously.

To avoid this, the internal pipe should, when discharging above the water-level, be turned upward at the end, so as to always remain filled with water; and when turned downward to discharge under water, the end should be well below the lowest working level.

An additional means of safety is sometimes afforded by fitting inside the boiler a clack valve, so arranged as to close over the end of the internal pipe or on the spigot of the ordinary check valve; when this is provided, the latter can be examined when steam is up. A cock is also sometimes fitted close to the check valve, so that the supply can be regulated by it, instead of by interfering with the lift of the check valve.

Blow-off Cock.-A cock should be fitted at or

near the bottom of the boiler, to answer the double purpose of admitting sea-water before getting up steam, and to blow off some of the water when required. This cock should be a very strong one, as it is liable to rough usage, and being out of sight and not easily got at, it is very apt to be neglected. For this reason, as well as because a large cock is difficult to open and shut, some engineers prefer a valve to a cock. If a cock is fitted, it should be so arranged that its handle or spanner cannot be removed when it is open.

The clear area through a blow-off cock should be = I square inch + 0.2 square inch for each ton of water in the boiler.

FUNNEL OR SMOKE STACK.

Funnel. This is usually of circular section, but sometimes, to minimize the transverse size of the boiler hatch, it is made of oval section. The funnels of men-of-war are often made of oval section for the same reason, but instead of the section being an ellipse, as is generally the case in the mercantile marine, it is like that of an oval boiler.

The best height to look well is four to five diameters above the taffrail, the latter when there are high bridges or boats in wake of the funnel. For the same reason, the ring for the shrouds should be the diameter from the top.

Naval ships generally have funnels with a sectional area equal to one-eighth the area of the grate. In the mercantile marine a somewhat larger funnel usually obtains, the area being from one-fourth to one-sixth that of the grate; in general practice a funnel, whose sectional area is one-fifth to one-sixth that of the grate, and whose top is at least 40 feet from the level of the grate, will give a very good result. The objections to a large funnel, beyond that of space occupied and cost, are resistance to the wind and large surface exposed to the cooling action of both wind and water, whereby the hot column within is partially cooled, and the draught thereby checked. On the other hand, a small funnel is liable to become excessively hot, and when the fires are freshly charged to become choked with smoke, and at all times it tends to check the draught. The funnel of a war-ship may be small, because it is so seldom that the boilers are urged to the utmost, and it must be as small as possible for obvious reasons. When the draught is forced either by a blast, or by other artificial means, the funnel may be short, and of comparatively small diameter. The area

at the base of a locomotive boiler is seldom more than one-tenth the area of fire grate, and often as small as one-twelfth.

CORRUGATED STEEL BOILER FURNACES.

The use of corrugated cylindrical furnaces for internally fired boilers, that is, for boilers in which the pressure is upon the outside of the furnace, has resulted directly from the use of very high pressure steam as a necessity; the pressure now carried in marine and other internally fired boilers being impossible without the employment of these furnaces; and this pressure is continually on the increase by reason of the greater fuel economy realized from its use, and the less weight and bulk of the steam machinery using it for the development of equal power.

With the pressure of present practice, and in a still higher degree with the much greater pressure that will be needed in the immediate future, any other form of furnace is out of the question. Safety, convenience, economy in first cost and after repairs, economy of fuel, ease and rapidity of management and cleaning, all the financial, engineering and practical requirements combine to make the cylindrical corrugated furnace the only one that can be adopted in the construction of the internally fired boilers:

In fact, at the present moment, no other kind of furnace is used, unless in some cases of low pressure, which has now become the rare exception. Even in such case, however, a corrugated cylindrical furnace is both cheaper in the