side of the same, and furnished with a cock to afford a means of regulating the admission of water to the condenser. The action of this arrangement is as follows. Before starting the engine, a tank, in which the condenser is fixed, is filled with cold water, and subsequently kept full; steam is then admitted to the condenser to expel the air which formerly filled it, which it does either through a conical valve placed at the top of the condenser, called a snifting valve, or otherwise through a valve at the foot leading to the airpump. As soon as all the air is displaced, the cock attached to the pipe which terminates within the condenser is opened, and water flows into the condensing vessel, where it comes in contact with the steam within the same, and reduces it at once to a liquid condition, leaving a vacuum approaching more or less nearly to an absolute vacuum, according to the management of the apparatus. Fig. 88 represents a vertical section of one form of condenser. It

Fig. 88.

is a species of swelled pipe; the upper and lower extremities or necks are bored out so as to be of greater diameter at their inner than at their outer ends. Into these necks are inserted the extremities of pipes of corresponding form, as shown, the surrounding

.

interstices being filled up to make good the joints. The upper pipe brings the exhaust steam from the cylinder, and the lower one communicates with the air pump.

Another form of condenser consists of a cast-iron cylinder fitted with a cover, and having the passage which serves to communicate with the air-pump cast on it. The condensers used in marine engines are not usually immersed in water, on account of the confined space in which they are employed.

Condensers of the second class are far more intricate and varied in their forms than are those which we have just described, the object in using these being to recover pure and unmixed the water resulting from the steam which has passed through the engine, which is very desirable when clean water cannot be obtained, or when the water contains much mineral matter, such as is the case with sea-water. Attempts have been made from a very early period down to the present time to produce a surface condenser which should be efficient, the first consisting of two thin cylinders of large diameter, placed concentrically one within the other, water being allowed to circulate around the outer tube and within the inner one, the steam to be condensed being introduced into the annular space bounded by the peripheries of the two concentric cylinders. This contrivance was, however, found in practice to be unequal to the duties required of it; it was consequently abandoned, and condensers of the first class were for a time exclusively employed. Subsequently a form of surface condenser was introduced, which has proved more successful. It consisted of a number of tubes of small diameter, around which water was permitted to flow, and into these tubes the exhaust steam was passed and there condensed, and from the condenser the water thus formed may be pumped directly back to the boiler. This apparatus is that invented by Samuel Hall. Since the production of this form of surface condenser, a great variety have been introduced to public notice, and it has also been attempted, in some instances with success, to use air as the cooling medium in the place of water, in order that the principles of condensation might be applied where sufficient water for the ordinary condensing apparatus could not be obtained. Craddock's condenser, intended for use with either air or water, consists of a number of small tubes attached at the top and bottom to vessels which serve to afford com

munication amongst all tubes. This condenser, being caused to revolve rapidly in air or water, is found tolerably efficient, as with the former medium a vacuum equal to nine pounds pressure per square inch may readily be obtained. Surface condensers, to be used with air, have also been formed of thin plates fixed parallel to each other, between which is passed the steam to be condensed. When air condensers are attached to locomotive carriages they may be fixed, as the velocity of the carriage itself is sufficient to cause the required circulation of the air. With regard to the construction of the various forms of condensers, it is only necessary here to observe that those of the first class merely require to be turned, planed, or faced at the joints, and those of the second class are somewhat similar in their construction to multitubular boilers. An account will be hereafter given.

The next element to which our attention is directed is the airpump, by means of which the condensed steam, the water used for condensing it, and the air which is always contained in the latter, is withdrawn from the condenser, together with any portion of steam which may escape condensation, so that the vacuum produced previous to starting the engine may remain unimpaired while it continues in action.

The air-pump consists of a cylinder accurately bored, within which the piston moves air-tight. There is a valve at the foot of the air-pump, opening in a such a manner as to allow of the passage from the condenser to the air-pump of such matters as are to be withdrawn from the former; whilst another valve at the top of the pump allows of the exit, and prevents the return of the same from and to the pump. The water, &c., below the piston is allowed to pass through it by means of valves opening upwards, the action of the apparatus being as under. As the piston, or bucket as it is called, of the air-pump rises, the water, air, and vapor in the lower part of the condenser pass through the bottom valve into the lower part of the air-pump on the descent of the air-pump bucket, the water, &c., beneath it, forces the valves formed in it open, and passes through to the upper side, and when the bucket again ascends, the water upon it is raised and forced through the upper valve into the hot well. This constitutes what is termed a singleacting air-pump. When the pumps are made double-acting the bucket is replaced by a solid piston, and two sets of valves are

employed, so that when the piston ascends, it draws in water below, and forces other water out above, and on its descent it draws above and forces below, so that the pump works during both the up and down-stroke; whereas, in the former case the pump is only effective during the up-stroke. The material of which the cylinder of the air-pump is formed is frequently cast-iron, but it should be lined with brass or Muntz metal; and this is especially necessary when sea water or foul water of any description is used; the same material should also be applied for air-pump rods. Iron rods, covered with brass, are very frequently used, but they are found to waste away where they are joined to the bucket. The method of constructing air-pumps is as follows: The cylinder, if of solid brass, is simply bored out in precisely the same way as a steam cylinder; but when it is lined it is first bored out, and the lining then bent, introduced into it, and made to fit firmly by hammering it on the inside, whereby the lining is expanded, so that the casing takes a firm grip; the lining is then bored out to the required size. The piston or bucket is accurately turned to fit the cylinder of the pump, and packed generally with hemp, which is tightened up by means of a junk-ring. Metallic packing has occasionally been used, but the vacuum obtained is not nearly so good, and, on the whole, it is inferior to hemp. The valves, if of metal, are accurately planed or turned, and then faced up or ground; various descriptions are used, but we shall not pause here to describe them, as a complete account will be given in a subsequent chapter.

With regard to the feed-pump, it is only necessary here to observe that what are called plunger-pumps are most generally employed; for a description of which the reader is referred to the chapter on pumps.

In concluding the present chapter, we may observe that it is the practice of some manufacturers to grind the steam-cylinders and the piston rods and similar parts; it is advisable to draw-file throughout their length, and polish.

In this chapter we have endeavored to explain the form and mode of manufacture of the principal elements of the various kinds of steam-engines, omitting, however, various minor arrangements which require no special comment, and the action of which will be illustrated by examples.

CHAPTER XIV.

ON PUMPS AND VALVEŠ.

It is proposed in the present chapter to give a general account of those pumps which are most commonly employed in practice to raise water, avoiding any thing further than a mere reference to such forms as are of doubtful efficiency. The first class which we shall consider is that which includes bucket or piston pumps, which are those most commonly used for the ordinary purposes of life; the principle of their action is as follows.

Let us suppose that we have a cylinder fitted with a piston, in which there is a valve capable of opening upwards, so as to allow of the ascent of a fluid through the piston, but effectually preventing its return. Let the bottom of the cylinder be closed, and also furnished with a valve opening upwards, at the termination of a short pipe, of which the lower extremity is immersed in water; let the piston fit the cylinder water-tight, and let it be at the bottom of its stroke. If the piston be now raised, there will evidently be a vacuum beneath it, into which the water will be forced up the short pipe, by reason of the external pressure of the atmosphere. The piston having arrived at the top of its stroke, is stopped, when the valve at the bottom of the cylinder will close by reason of its own weight; and if the piston be now caused to descend, the water beneath it will raise the valve in the same, and pass through to the upper side, and when another up-stroke is made, this valve having closed, the water above the piston will be raised, and will flow over the top of the cylinder, the lower part of which will again be filled. The upper end of the cylinder or pump-barrel may be closed, and supplied with a valve to allow of the exit of the water raised, and to prevent its return upon the piston.

There is, of course, a limit to the height of the pipe which effects the communication between the bottom of the pump-barrel