vessel into the sea governed by a cock F, which is likewise put in connection with the lever H, so that it shall be opened when the lever H is drawn to the position F', the cock D' being closed in all positions of the lever between K and F'. Thus, whenever the cock F communicating with the sea is open, the cock D communicating with the boiler is closed, and vice versâ, both cocks being closed when the lever is in the intermediate position K. By this arrangement the boiler cannot, by any neglect in blowing off, be left in communication with the sea, nor can more than a ton of water be discharged except by the immediate act of the engineer. The injurious consequences are thus prevented which sometimes ensue when the blow-off cocks are left open by any neglect on the part of the engineer. When it is necessary to blow off, the engineer moves the lever H, to the position D'. steam in the boiler on the surface the salted water or brine up the pipe open cock c into the tank, and this continues until the tank is filled when that takes place, the lever is moved from the position D' to the position F', by which the cock D is closed, and the cock F opened. The water in the tank flows through the pipe E into the sea, air being admitted through the valve v, placed at the top of the tank, opening inwards. A second ton of brine is discharged by moving the lever back to the position D', and subsequently returning it to the position F'; and in this way the brine is discharged ton by ton, until the supply of water from the feed which replaces it has caused both the balls in the indicator to sink to the bottom.

The pressure of the of the water w forces BA, and through the

(212.) A different method of preserving the requisite freshness of the water in the boiler has been adopted by Messrs. Maudslay and Field, and introduced with success into the Great Western and other steam-vessels. Pumps called brine-pumps are put into communication with the lower part of the boiler, and so constructed as to draw the brine therefrom, and drive it into the sea. These brine-pumps are worked by the engine, and their operation is constant. The feed-pumps are likewise worked by the engine, and they bear such a proportion to the brine-pumps that the quantity of salt discharged in a given time in the brine is equal to the quantity of salt

introduced in solution by the water of the feed-pumps. By this means the same actual quantity of salt is constantly maintained in the boiler, and consequently the strength of the solution remains invariable. If the brine discharged by the brine-pumps contains parts of salt while the water introduced by the feed-pumps contains only part, then it is evident that five cubic feet of the feeding water will contain no more salt than is contained in one cubic foot of brine. Under such circumstances the brine-pumps would be so constructed as to discharge of the water introduced by the feed-pumps, so that of all the water introduced into the boiler would be evaporated, and rendered available for working the engine.

To save the heat of the brine, a method has been adopted in the marine engines constructed by Messrs. Maudslay and Field similar to one which has been long practised in steamboilers, and in various apparatus for the warming of buildings. The current of heated brine is conducted from the boiler through a tube which is contained in another, through which the feed is introduced. The warm current of brine, therefore, as it passes out, imparts a considerable portion of its heat to the cold feed which comes in; and it is found that by this expedient the brine discharged into the sea may be reduced to a temperature of about 100°.

This expedient is so effectual that when the apparatus is properly constructed, and kept in a state of efficiency, it may be regarded as nearly a perfect preventive against the incrustation, and the deposition of salt in the boilers, and is not attended with any considerable waste of fuel.

(213.) About the year 1776, Mr. Watt invented a tubular condenser, with a view to condense the steam drawn off from the cylinder without the process of injection. This apparatus consisted of a number of small tubes connecting the top and bottom of the condenser, arranged in a manner not very different from that of the tubes which traverse the boiler of a locomotive engine. These tubes were continually surrounded by cold water, and the steam, as it escaped from the cylinder passing through them, was condensed by their cold surfaces,

whence it was drawn off by a pump in the same manner as in engines which condensed by injection. One of the advantages proposed by this expedient was, that no atmospheric air would be introduced into the condenser, as is always the case when condensation by injection is practised. Cold water, which is injected, has always combined with it more or less common air. When this water is mixed with the condensed steam, the elevation of its temperature disengages the air combined with it, and this air circulating to the cylinder, vitiates the vacuum. One of the purposes for which the air-pump in condensing steam-engines was provided, and from which it took its name, was to draw off this air. If, however, a tubular condenser could be made to act with the necessary efficiency, no injection water would be introduced for condensation, and the pump would have no other duty except to remove the small quantity of water produced by the condensed steam. That water being subsequently carried back to the boiler by the feed-pumps, a constant system of circulation would be maintained, and the boiler would never require any fresh supply of water, except what might be necessary to make good the waste by leakage and other causes.

This contrivance has been of late years revived by Mr. Samuel Hall of Basford, near Nottingham, with a view to supersede in marine engines the necessity of using sea-water in the boilers. Mr. Hall proposes to make marine boilers with fresh water to condense the steam without injection, by a tubulated condenser, and to provide by the distillation of sea-water the small quantity of fresh water which would be necessary to make good the waste. These condensers have been introduced into several steam-vessels: in some they have been continued, and in others abandoned, and various opinions are entertained of their efficacy. I have not been able to obtain the results of any satisfactory experiments on them, and cannot therefore form a judgment of their usefulness. Mr. Watt abandoned these condensers from finding that the condensation of the steam was not sufficiently sudden, and that consequently at the commencement of the stroke the piston was subject to a resistance which inju

riously diminished the amount of the moving power, whereas condensation by jet was almost instantaneous, and the efficiency of the piston throughout the entire stroke was more uniform.

Mr. Watt also found that a fur collected around the tubes of the condenser, so as to obstruct the free passage of heat from the steam to the water of the cold cistern; and that, consequently, the efficiency of the condenser was gradually impaired, and could only be restored by frequent cleansing.

It is stated by Mr. Hall that a vacuum is preserved in his condensers as perfect as that which is maintained in the ordinary condensers by injection. It is objected, on the other hand, that without the injection water and the air which accompanies it being introduced into his condensers, Mr. Hall uses as large and powerful an air-pump as those which are used in engines of equal power condensing by injection; that, consequently, the vacuum which is maintained is produced, not as it ought to be altogether by the condensation of steam, but by the air-pump drawing off the uncondensed steam. To whatever extent this may be true, the efficacy of the machine, as indicated by the barometer-gauge, is only apparent; since as much power is necessary to pump away any portion of uncondensed vapour as is obtained by the vacuum produced by the absence of that vapour.

A tubular condenser of the form proposed by Mr. Hall is represented in fig. 126.; a is the upper part of the condenser to which steam is admitted from the slide after having worked the piston; k is the section of a thin plate, forming the top of the condenser, perforated with small holes, in which the tubes are inserted so as to be steam-tight and water-tight. Water is admitted to flow around these tubes between the top k and the bottom d of the condenser, so as to keep them constantly at a low temperature. The steam passes from a through the tubes to the lower chamber ƒ of the condenser, where it is reduced to water by the cold to which it has been exposed. A supply of cold water is constantly pumped through the condenser, so as to keep the tubes at a low temperature. The air-pump g is of the usual con

similar valves in the cover of the pump also opening upwards. The water formed by the condensed steam in ƒ is drawn

through the foot-valve, and after passing through the pistonvalves, is discharged by the up-stroke of the piston into the hot well. Any air, or other permanent gas, which may be admitted by leakage through the tubes of the condenser, or by any other means, is likewise drawn out by this pump, and when drawn into the hot well is carried from thence to the feeding apparatus of the boiler, to which it is transferred by the feed-pump.

A provision is likewise made by which the steam escaping at the safety-valve is condensed and carried away to the feeding cistern.

(214.) One of the remedies proposed for the evil consequences arising from incrustation is the substitution of copper for iron boilers. The attraction which produces the adhesion of the calcareous matter held in solution by salt water to the surface of iron has no existence in copper, and all the saline and other alkaline matter precipitated in the boiling water in