Chapter XXVI (2)

"As it is possible to blow so much cold air into a fire as to put it out, by first heating the air it would burn all the stronger, and whatever heat is taken out of the condenser into the fire-place from the steam that has been made use of, half this extra heat will go into the boiler again, or in other words, but half the quantity of cold will be put into the fire, being the same in effect as saving fuel. Taking heat from the condenser through the boiler sides is an additional new principle in this engine. I find by blowing through tubes that the condensation of a surface foot of air-tube against a surface foot of boiler fire-tube is greater than the fire that passes through the boiler sides, where the common chimney draught is used, by nearly double; but I expect when both air and fire tubes are forced by a strong current of air it will be nearly equal, and the increase of steam and of condensation can be increased by an increased current of air, so as to cause a surface foot of fire and of air sides to do perhaps five times as much; and of course the machine will be lighter in proportion. I think air sides condensation preferable to water sides, as so small a space does the work, and is always convenient, and its power uniformly increasing with its speed, by the increased quantity of air, without the weight of water vessels. This kind of engine can be made to suit every place and purpose, and I think such an engine of the weight of a Boulton and Watt engine will perform twenty times the duty.

"Air sides condensation will be advantageous on board ship, because there are holes for the passage of water through the bottom and sides of the ship.

"I am anxious to have your opinion on this plan of returning the hot air from the condenser to the fire-place, and what you think the effect will be.

"The Comptroller of the Navy has not yet returned from Plymouth, therefore no answer has been given to me.

"You will see by the sketch how very small and compact an engine is now brought without complication or difficulty; each surface foot of boiler and condenser is equal to one-third of a horse-power, weighing 20 lbs., or 60 lbs. weight for each horse-power. The consumption of fuel is so small when working a differential engine, that I expect it will not exceed 1 lb. of coal per hour for each horse-power.

"The cost of erection and required room are so small from its simplicity that it will be generally used. As I am very anxious that every possible improvement should be considered prior to making a specification for a patent, I must beg that you will have the goodness to consider and calculate on the data I have given you. I am sorry to trouble you, but I am satisfied this will be to you rather a pleasing amusement than a trouble. The warming machines will take a very extensive run, and I believe will pay exceedingly well.

"I am almost in the mind to take a ride down to see you in a few days, but am now detained here about the American mining concerns.

"I remain, Sir,
"Your very humble servant,
"RD. TREVITHICK."

The letters and foot-note are the only changes made by the writer in Trevithick's original sketch so descriptive of a wonderful application of varied and improved principles of long-known difficulty and importance; the beautifully compact tubular boiler for giving high-pressure superheated steam, surface condensation, absence of feed and condensing water, and return of the heat, in other engines wasted in condensation, to the fire-place; though there is little or no mention of the mechanical or moving parts of the steam-engine, yet its vital principles are grasped with the hand of a master. The sketch in the letter hastily made forty years ago is more ingenious than any portable engine since constructed, though there may be no sufficient proof of its practical success. The propeller to be worked by this novel engine was of course his long-idle screw.

_Steam Engines, 21st February, 1831._

"NOW KNOW YE, that in compliance with the said proviso, I, the said Richard Trevithick, do declare that the essential points in my improved steam-engine, for which I claim to be the first and true inventor, are:--

"Firstly, the placing of the boiler within the condenser, in order to obtain the additional security of the strength of the condenser to prevent mischief in case the boiler should burst, and also by the same arrangement to conveniently make the condenser, with a very extensive surface, enabling me to condense the steam without injecting water into it.

"Secondly, the enclosing of the condenser in an air or water vessel, by which the intention of safety from explosion is further provided for, and my engine really rendered what I denominate it, a high-pressure safety engine.

"Thirdly, the condensing of the steam in the condenser by means of a current of cold air or cold water forced against the outsides of the condenser.

"Fourthly, the returning of the condensed steam from the condenser back again into the boiler, to the end that sediment and concretion in the boiler may be prevented; and,

"Fifthly, the blowing of the fire with the air after it has been heated by condensing the steam.

"In forming my improved steam-engine I employ several or all of these points according to convenience, in combination with the other necessary parts of steam-engines in common use.

"These, my essential points, will admit of various modifications as to form and proportions such as must be and are quite familiar to every competent steam-engine manufacturer, and therefore it will be sufficient for the perfect description of my improved steam-engine that I explain some of the modes of forming and combining the essential points of my invention with the other parts of steam-engines in common use. In my most favourite form of engine in which I condense by a current of cold air, the fire-place and flue, the boiler, the condenser, and the air-vessel, are made of six concentric tubes, standing in an upright position. The inner or first tube forms the fire-place and flue, and at the same time the inner side of the boiler. This tube is conical, having its small end upwards. The next or second tube is cylindrical, about 6 inches larger in diameter than the lower end of the first tube, and forms the outside of the boiler, leaving a space all round of about 3 inches at the bottom, and so much more at the top, as the flue is taper for holding water and steam between the two tubes. The third tube is about 2 inches larger in diameter than the second, in order to allow a space of about an inch for powdered charcoal or some other slow conductor of heat. This tube also constitutes the inner side of the air-vessel. The fourth tube is about 2 inches larger than the third, and forms the inner side of the condenser. The fifth tube, about 2 inches larger than the fourth, forms the outside of the condenser; and the sixth tube, about 2 inches larger than the fifth, forms the outside of the air-vessel, and at the same time the outside of the whole of the generating and condensing apparatus, consisting of fire-place, flue, boiler, condenser, and air-vessel. These tubes are made of wrought-iron plates riveted together, and are all cylindrical, except the first, which is conical, the bottom or fire end being the largest. The first or inner tube is closed at bottom, but has an opening on one side near the bottom, through which the fire-bars are introduced, and the ashes and clinkers taken away. To this opening a neck-piece about 3 inches long is riveted, having a flange to fit against the inside of the second tube, when the two tubes are concentric, through, the side of which second tube is an opening corresponding with that in the first tube, and the flanch is screwed to the second tube so as to make one opening through the sides of the two tubes. The second tube extends downwards about 5 inches below the first tube, and has a flanch turning inwards, to which a second round plate of iron is screwed, forming the bottom of the boiler. The first tube has an external flanch at the top, and the second tube an internal flanch, both of the same height, and screwed to a cast-iron circular plate or cap-piece, which extends wide enough around the boiler to form also the cover for the air-vessel. This plate has a hole in the middle as large as the flue. The sides of the condenser and air-vessel are formed of four concentric tubes, each about 2 inches larger than the one within it. The inner and outer of these tubes constitute the sides of the air-vessel, and are each furnished with an external flanch at the top by which they are screwed to the cap-piece. The two intermediate tubes constituting the sides of the condenser are riveted together at the top, leaving a space of about an inch between their upper ends and the cap-piece, so as to allow of a free communication over them between the outer and inner parts of the air-vessel. The inner tube of the air-vessel extends downwards about an inch below the boiler, and is closed by a flat plate screwed on to a flanch projecting inwards from the tube; the two tubes of the condenser descend about 3 inches lower than the boiler. The inner tube has an internal flanch, to which a flat circular plate is screwed to close up the tube. The outer tube of the condenser is of the same length with the inner, and is provided with an external flanch about 3 inches broad. The outer tube of the air-vessel has an external flanch 2 inches broad, and is just long enough to come down upon the broad flanch of the condenser last described, and these two flanches are together bolted upon a bottom piece of cast iron, which is a dish of 4 inches deep, and equal in diameter with the diameter of the outer tube, and having a flanch the same breadth as the flanch of the outer tube, and the bottom piece is secured to the air-vessel and outer tube of the condenser by bolts going through all the three flanches. An opening is made through the sides of all the four tubes of the condenser and air-vessel opposite to and as wide as the fire-place opening through the sides of the boiler. The upper part of both openings to be of the same height, but the outer opening is made as low as the bottom of the boiler, in order to allow room for a pipe to enter that part of the boiler for forcing the water into it, and also another pipe and cock for drawing off the water or sediment, in case foul water be used by accident or carelessness. These two openings through the condenser and air-vessel, and through the boiler, constitute one fire doorway through all the six tubes for access to the fire-place; a ring is placed between the two tubes of the condenser around the fire doorway, so as to cut off all communication of the steam in the condenser with the air in the doorway; another similar ring is placed between the condenser and the outer tube to prevent the escape of air into the fire doorway, and a half ring is placed in the lower part of the fire doorway between the condenser and the inner tube of the air-vessel, to prevent ashes from falling into the air-vessel, and yet allow a free passage for the air from the inner part of the air-vessel into the upper part of the fire doorway. These two rings and the half ring are secured in their places by rivets passing through all of them and through the tubes, and uniting all firmly together, the interstices being filled with iron cement. A ring is also placed between the boiler and the air-vessel around the fire doorway, against the outside of which ring the charcoal powder is tightly rammed, and will hold the ring in its place without the necessity of either rivets or screws. That part of the fire doorway which is above the fire-bars is supplied with an inner door, to shut the fire-place even with the outside of the boiler, and exclude all access of air to the fire, except through the grating. The whole of the fire doorway is enclosed by an outer door even with the outside of the air-vessel, to exclude all air, except that which comes through the air-vessel; a pipe is fixed in the bottom or dish-piece leading to a forcing pump to draw the water out of the condenser and force it into the bottom of the boiler through the pipe before described. A blowing cylinder of about ten times the content of the main cylinder is screwed against the outside of the air-vessel, and opposite to the two outlet valves of the blowing cylinder two apertures are made in the air-vessel, through which the air is forced in. The main cylinder of the engine, of the usual dimensions according to power wanted, is also screwed against the outside of the air-vessel high enough above the blowing cylinder to allow room for the main-crank shaft to work between them. The forcing pump before mentioned is also screwed to the outside of the air-vessel, and thus my improved steam-engine becomes more compact and convenient than any preceding steam-engine. For the purpose of supplying the boiler with distilled water, in case there should be a deficiency in it, a small vessel made of two upright tubes, one within the other, is placed on the cap-piece. The inner tube is of the same diameter as the flue, and forms a continuation of it. The outer tube is about 6 inches larger than the inner, and the space at the top and bottom between the two tubes is closed by two ring-shaped pieces. This vessel may be about 18 inches high; a cock is fixed in the top of this vessel, to which a bent pipe is fastened, leading to and united with a pipe which arises from the top of the condenser and passes through a hole in the cap-piece, and thus a communication between the supplying vessel and the condenser may be opened or shut at pleasure; another pipe, also furnished with a stop-cock, arises from the vessel, and communicates with a water-cistern to receive its supply of water when required; a third pipe, having a cock in it, opens into the vessel near the bottom to let out the sediment; a small cock to let the air out is also fixed in the top of the vessel, which cock may also be used for letting air out of the condenser. In order to supply the boiler with water by means of this vessel, the stop-cock leading to the condenser is shut, and that leading to the cistern is opened, and at the same time the air-cock is opened to allow the air to escape that the water may fill the vessel. When the vessel is nearly full of water, the air-cock and the cock from the cistern are shut, and that in the pipe leading to the condenser is opened. The water being then heated by the flue is converted into steam, which, passing into the condenser, is there reduced to water again, leaving the sediment or salt in the supplying vessel, which sediment or salt may be occasionally blown out through the bottom pipe by filling the vessel with, water, shutting the water, steam, and air cocks, and opening the cock of the outlet pipe at a time when the steam in the vessel is strong. But the supply of water from the condenser being always equal to that converted into steam and used in the engine, there is no tendency to a variation in the height of the water in the boiler, except there be leakage or waste of steam in some part of the engine. An upright glass tube, having an iron tube of communication with the lower part of the boiler and another iron tube of communication to the upper part of the boiler, is conveniently placed against the outside of the air-vessel to indicate at all times the height of the water in the boiler; as is usual in steam-boilers, a valve is placed on the top of the air-vessel to allow of the escape of a portion of the air in case that the quality of the fuel should not require so much air for perfect combustion as the steam requires for good condensation. The degree of the condensation of the steam may be increased at pleasure, by increasing the velocity of the air passing into and through the air-vessel. The other parts of my improved steam-engine, such as the steam-pipes, the throttle-valve, the safety-valve, the vacuum-valve, the working valves, crank, connecting rods, cross-heads, pistons, piston-rods, and various other minor parts common to engines in general use may be made in the usual forms, and placed in the most convenient situations; they cannot, therefore, need any description. When it is intended to use water for condensing instead of air, my improved steam-engine must be made as before directed, except that the communication between the air-vessel and the fire-place must be closed, which may be done by a perfect ring of iron surrounding the opening leading to the fire-place, instead of the half ring before described, and a forcing pump must be employed to draw water from a reservoir, and force it into the vessel which I have hereinbefore denominated the air-vessel, but which in this mode of working would more properly bear the name of water-vessel. In this case a blowing cylinder, the dimensions of which must be calculated according to the quality of the fuel to be used, may be worked to blow the fire through a pipe leading into the ash-pit. This, however, will not be necessary where there is a chimney high enough to create a strong draught. In respect to proportions, my improved steam-engine admits of considerable latitude, and it will be sufficient direction to any practical engineer to say that for engines working with steam of 120 lbs. to the inch, used expansively till it be nearly reduced to atmospheric strength and then condensed, a 10-horse engine may have a fire-place of 20 inches diameter, the flue at the top 10 inches diameter, and a boiler of 20 feet high; a 60-horse engine, a fire-place of 36 inches diameter, a flue of 16 inches diameter, and a boiler of 20 feet high. In boat-engines, and in other cases where height cannot be allowed, the diameter must be increased. The thickness of the two tubes constituting the boiler sides of a 10-horse engine may be 1/8th of an inch, that of a 60-horse a quarter of an inch, and so in proportion for engines of other power. The tubes constituting the condenser and inner tube of the air-vessel may in all cases be 1/8th of an inch thick. The outer tube may be 3/8ths of an inch thick, to afford stability to the working cylinder, the blowing cylinder, and the forcing pump fastened to this tube, and as an ultimate perfect barrier against explosion. The respective distances of the other tubes constituting the outside of the boiler, the condenser, and air-vessel, will be the same as hereinbefore given, and therefore their diameters will depend upon the diameter of the fire-place. The cap-piece in small engines may be half an inch thick, and in large engines an inch. The bottom of the ash-pit and bottom of the boiler must have about half an inch of thickness for every foot diameter, or they may be cast with ribs to afford equivalent strength. The fuel is supplied through a door in the flue, at the top of the boiler, consisting of coke or coals the least liable to swell with heat. The flue may be filled to about one-third of the height of the boiler, and the water fill about three-fourths of the boiler, leaving one-fourth for steam.

"Having clearly explained my improved steam-engine so that any person competent to make a steam-engine can from this description understand my invention and carry the same into effect in as beneficial a manner as myself, I proceed to observe that the extreme safety of my improved steam-engine will be seen, from considering that in case the boiler should explode inwards into the flue, the power of the steam would be first reduced by filling the flue and fire-place, and could not escape through the chimney and fire doorway faster than it would diffuse itself and be condensed by mixing with the surrounding air, and thus lose all its force. But should the outside of the boiler burst, part of the force of the steam would be spent in filling up the interstices between the particles of the charcoal, and would then probably be too weak to effect a breach through the inner tube of the air-vessel; and should such a second breach be effected, the space within the air-vessel would allow the steam to expand and partly condense, and a portion to escape into and through the fire doorway, where it would divide itself, and proceed harmlessly up the flue, and out at the doorway; so that the outer case being a reserve of strength, would to a certainty withstand the force remaining in the steam after the before-mentioned successive reductions of power."

The patent of February, 1831, perfects the sketch in his letter of July 27th, 1829, which in its turn made more perfect the plans put into practice in 1815, just before leaving England for America.[175] The prejudice against the use of his high-pressure steam-engine he tried to meet by calling it "a high-pressure safety engine." The boiler was of six wrought-iron upright tubes, one within the other. The inner one was the fire-tube, surrounded by a tube of larger diameter, forming the water and steam space. This was again surrounded by another tube, 2 inches larger in diameter, the space being filled with charcoal or other non-conductor of heat; another tube, 2 inches more in diameter, formed the inner circle of the condenser, having an inch space for the passage of cold air from the blowing cylinder, carrying the heat from the condensing steam back to the fire-place. Still another tube, 2 inches more in diameter, giving a space into which the used steam from the cylinder passed to be condensed. Then came the outside tube, 2 inches more in diameter, forming a second space for the passage of air, taking heat from the condenser into the fire. The steam-boiler had its heat retained by a coating of charcoal; next to it came a current of cold air an inch thick, carrying back to the fire any heat that had passed through the charcoal coat, and also the heat from the inner surface of the condenser. Then came the inch-thick circle of steam, on its exit from the cylinder, to be condensed; and finally an outside circle of cold air, performing the same functions as the inner circle in condensing the steam and carrying its heat back again to the fire.

[Footnote 175: See Trevithick's letters, July 8th, 1815, vol. ii., p. 80, and 7th, vol. i., p. 364; and 16th May, vol. i., p. 370; and patent of 1815, vol. i., p. 375.]

The object or principle of this engine was to avoid the loss of heat, and the necessity for either condensing water or feed-water, as described in the letter and drawing of August 19th, 1830, but the detail was changed, mainly to facilitate construction. As in practice it might be impossible to fully attain those objects, preparation was made to get rid of the salt from such water as might be required as feed-water to make good the loss from leakage or other defects in the working of marine steam-engines. The specification states: "For the purpose of supplying the boiler with distilled water, in case there should be a deficiency in it, a small vessel made of two upright tubes, one within the other, is placed on the cap-piece. The inner tube is of the same diameter as the flue, and forms a continuation of it. The water being heated by the flue is converted into steam, which, passing into the condenser, is there reduced to water again, leaving the sediment or salt in the supplying vessel."

Where water condensation was preferred the surface-air condenser could be converted into a surface-water condenser by a current of cold water in place of the air; in which case the air from the blowing cylinder was taken direct in to the fire-place or other means used for giving the necessary draught. Steam of about 135 lbs. to the inch was to be so expansively worked as at the finish of the stroke, on its escape to the condenser, to be no more than atmospheric pressure, or 15 lbs. to the inch--just the strength with which Watt preferred to commence his work in the cylinder.

The most prominent feature in Trevithick's numerous modifications of the steam-engine was the boiler. In the 'Life of Watt,' though his commentators have been numerous and eminent, little or nothing is said about the boiler or the steam pressure. He left that all-important part of the steam-engine just as he found it, resisting the increase of steam pressure, which was the mainspring of Trevithick's engine. The boiler of the high-pressure engines of 1796[176] sheltered the steam-cylinder from cold; and the used steam from the cylinder circulated around the exterior of the boiler, on its way to the blast-pipe, while the condensed portion was returned as feed-water in the patent engine of 1802.[177] In 1811 he proposed to force air into the fire-place, hoping thereby to reduce the amount of heat lost by the chimney.[178] His various forms of tubular boilers, as at the Herland Mine,[179] and at Dolcoath,[180] and the upright multitubular boilers patented in 1815.[181] followed up in 1828. "I shall have a small portable engine finished here next week, and will try to heat steam independent of water, in small tubes of iron, on its passage from the boiler to the cylinder, and also try cold sides for condensing." In 1829 a simple boiler and condenser composed of three tubes was made, the inner or fire-tube being 2 feet in diameter and 15 feet long, "for the express purpose of experimenting on the working the same steam and water over and over again;"[182] and on the same subject, "By making the condenser of 4-inch copper tubes 1/32nd of an inch thick, it would stand in one-twentieth part of the space of the boiler:"[183] and finally the sketch of the tubular boiler and tubular condenser of 1830, in its boiler portion similar to the best portable boilers of the present day, and the patent specification of 1831. Surely therefore to him belongs the credit of having invented and perfected the tubular boiler and surface condenser.

[Footnote 176: See vol. i., p. 104.]

[Footnote 177: See patent specification, vol. i., p. 128.]

[Footnote 178: See Trevithick's letter, 13th Jan., 1811, vol. ii., p. 6.]

[Footnote 179: See vol. ii., p. 71.]

[Footnote 180: See chap. xx.]

[Footnote 181: See Trevithick's letters, 8th July, 1815, vol. ii., p. 80; and 7th and 16th May, vol. i., pp. 364, 370.]

[Footnote 182: See vol. ii., p. 332.]

[Footnote 183: See vol. ii., p. 336.]

Smiles has written:[184]--

[Footnote 184: See 'Life of George Stephenson,' by Smiles, p. 279; published 1857.]

"For many years previous to this period (1829), ingenious mechanics had been engaged in attempting to solve the problem of the best and most economical boiler for the production of high-pressure steam. Various improvements had been suggested and made in the Trevithick boiler, as it was called, from the supposition that Mr. Trevithick was its inventor. But Mr. Oliver Evans, of Pennsylvania, many years before employed the same kind of boiler, and as he did not claim the invention, the probability is that it was in use before his time. The boiler in question was provided with an internal flue, through which the heated air and flames passed, after traversing the length of the under side of the boiler, before entering the chimney.

"This was the form of boiler adopted by Mr. Stephenson in his Killingworth engine, to which he added the steam-blast with such effect. We cannot do better than here quote the words of Mr. Robert Stephenson on the construction of the 'Rocket' engine:--'After the opening of the Stockton and Darlington, and before that of the Liverpool and Manchester Railway, my father directed his attention to various methods of increasing the evaporative power of the boiler of the locomotive engine. Amongst other attempts, he introduced tubes (as had before been done in other engines)--small tubes containing water, by which the heating surface was materially increased. Two engines with such tubes were constructed for the St. Etienne Railway, in France, which was in progress of construction in the year 1828; but the expedient was not successful; the tubes became furred with deposit, and burned out.

"'Other engines, with boilers of a variety of construction, were made, all having in view the increase of the heating surface, as it then became obvious to my father that the speed of the engine could not be increased without increasing the evaporative power of the boiler. Increase of surface was in some cases obtained by inserting two tubes, each containing a separate fire, into the boiler; in other cases the same result was obtained by returning the same tube through the boiler; but it was not until he was engaged in making some experiments, during the progress of the Liverpool and Manchester Railway, in conjunction with Mr. Henry Booth, the well-known secretary of the company, that any decided movement in this direction was effected, and that the present multitubular boiler assumed a practicable shape. It was in conjunction with Mr. Booth that my father constructed the 'Rocket' engine.

"'In this instance, as in every other important step in science or art, various claimants have arisen for the merit of having suggested the multitubular boiler as a means of obtaining the necessary heating surface. Whatever may be the value of their respective claims, the public, useful, and extensive application of the invention must certainly date from the experiments made at Rainhill. M. Seguin, for whom engines had been made by my father some few years previously, states that he patented a similar multitubular boiler in France several years before. A still prior claim is made by Mr. Stevens, of New York, who was all but a rival to Mr. Fulton in the introduction of steamboats on the American rivers. It is stated that as early as 1807 he used the multitubular boiler.

"'These claimants may all be entitled to great and independent merit; but certain it is, that the perfect establishment of the success of the multitubular boiler is more immediately due to the suggestion of Mr. Henry Booth, and to my father's practical knowledge in carrying it out.'

"We may here briefly state that the boiler of the 'Rocket' was cylindrical, with flat ends, 6 feet in length, and 3 feet 4 inches in diameter. The upper half of the boiler was used as a reservoir for the steam, the lower half being filled with water. Through the lower part twenty-five copper tubes of 3 inches diameter extended, which were open to the fire-box at one end and to the chimney at the other. The fire-box, or furnace, 2 feet wide and 3 feet high, was attached immediately behind the boiler, and was also surrounded with water."

Stephenson knew of Trevithick's patent of 1802,[185] in which a three-tubed boiler is shown; and it was after that time that Oliver Evans and Fulton tried their experiments, and also the numerous engines with single or return double tube, at work in the principal towns of England prior to 1804,[186] and near his residence in childhood and in manhood.[187]

[Footnote 185: See vol. i., p. 128.]

[Footnote 186: See Trevithick's letter, Sept. 23rd, 1804, vol. ii., p. 2.]

[Footnote 187: Mr. Armstrong's note, vol. i., p. 184.]

George Stephenson's Killingworth boiler, "to which he added the steam-blast with such effect," was a copy of Trevithick's boiler and blast, working since 1804 in Newcastle-on-Tyne, and was precisely the boiler described by Stephenson; "in other cases the same result was obtained by returning the same tube through the boiler." This is an admission from Stephenson that Trevithick's patent boiler was the best in use up to about 1828.

A further proof of the indirect public gain from the use of Trevithick's return-tube boiler over a period of thirty years is their having supplied high-pressure expansive steam in the first experiments made with such steam by the Admiralty, at whose request Mr. Rennie and others examined the duty of the Cornish high-pressure expansive engine, and Captain King, R.N., in charge of the Admiralty Department at Falmouth in 1830, gave an order to Harvey and Co. to construct high-pressure steam-boilers for the Government vessel 'Echo'; in 1831 the machinery was put on board the 'Echo' in the Government Dockyard at Plymouth, and included three of Trevithick's return-tube boilers, made of wrought iron, each 5 feet 6 inches in diameter and 24 feet long, with internal return fire-tube 2 feet 2 inches in diameter. The fire-place end of the boiler was 6 feet 9 inches deep by 5 feet 6 inches wide, to give room for the fire-place and ash-pit. The steam pressure was 20 lbs. on the inch above the atmosphere, worked by double-beat valves, 6 inches in diameter, with expansive gear.

This new machinery was fixed under the superintendence of the writer, after which the Government engineers took charge of the vessel, and the writer who had, as the mechanic in charge, worked like a slave, though receiving but 1_s._ 6_d._ a day and expenses, was not invited to take any part in the experimental trials, nor ever heard of the result except in the ordinary rumours of Admiralty bungling on board the 'Echo.'

Those boilers were similar to the Trevithick boiler that had served the locomotive in Newcastle and elsewhere from 1801 to 1828, the first steamboat experiments in England, in Scotland, and in America, and the numerous high-pressure engines then at work.

The enlarging the fire-place end of boilers or fire-tubes has led to many forms. Trevithick's model of 1796[188] had an oval tube giving a greater spread of fire-*bars; the same is seen in the 1808 steamboat;[189] the Dolcoath boilers of 1811[190] show the oval and also the bottle-neck fire-tube; the Welsh locomotive of 1804[191] had the fire-tube contracted at its bend or return portion; the Tredegar puddling-mill fire-tube of 1801[192] tapered gradually from the fire-bridge to the chimney end; in the London locomotive of 1808[193] the fire-tube took the bottle-neck shape close to the fire-bridge. The accompanying sketch shows the bottle-neck contraction, only on the top and sides of the fire-tube was to give breadth to the fire-bars _d_, and thickness to the fire at bridge _c_, after which the flue portion of the fire-tube was contracted: this boiler was for many years a favourite in Cornwall. The bottle-neck contraction of the 'Echo' boiler was similar to the above, except that the enlargement of the fire-place was downwards instead of upwards, and the fire-tube, instead of going through the end of the boiler, returned to near the enlarged fire-place, when it passed out through the side of the boiler to the chimney, just as in the Tredegar puddling-mill boiler; all those variations were with the object of increasing the fire-grate, and at the same time keeping down the gross size and weight of boiler and its water.

[Footnote 188: See vol. i., p. 104.]

[Footnote 189: See vol. i., p. 335.]

[Footnote 190: See vol. ii., p. 169.]

[Footnote 191: See vol. i., p. 181.]

[Footnote 192: See vol. i., p. 223.]

[Footnote 193: See vol. i., p. 207.]

In 1805, Lord Melville failed to keep his appointment with Trevithick, on his proposal to construct a high-pressure steamboat.[194] Rennie, a pupil and friend of Watt, and familiar with Trevithick's high-pressure steam-dredgers on the Thames, was employed by Lord Melville and the Admiralty on the Plymouth Breakwater, where in 1813 Trevithick proposed the use of his high-pressure steam locomotive and boring engine.[195] In 1820 Rennie wrote to Watt, that the Admiralty had at last decided upon having a steamer; at that time fifteen years had passed since Trevithick's offer to propel the Admiralty by steam-puffers, and ten years more were to pass before they could make up their minds to venture on high-pressure steam from his boilers. The Steam Users' Association are equally hesitating, judging from words just spoken by an engineer, the son of an engineer:--

[Footnote 194: See Trevithick's letter, 10th Jan., 1805, vol. i., p. 324.]

[Footnote 195: Ibid., vol. ii., p. 24.]

"Sir William Fairbairn said he had come to the conclusion, after many years' experience, that it was in their power to economize the present expenditure of fuel by a system which might not be altogether in accordance with the views of the members of the association or the public at large, and that was to increase the pressure of steam. He would have great pleasure in stating a few facts which might some day tend to bring about a change, if not a new era, in the use of steam. From the result of a series of experimental researches in which he had been engaged for several years on the density, force, and temperature of steam, he had become convinced that in case we were ever to attain a large economy of fuel in the use of steam, it must be at greatly-increased pressure, and at a rate of expansion greatly enlarged from what it was at present. Already steam users had effected a saving of one-half the coal consumed by raising the pressure from 7 lbs. and 10 lbs.--the pressure at which engines were worked forty years ago--to 50 lbs., or in some cases as high as 70 lbs. on the square inch."[196]

[Footnote 196: 'The Engineer,' March 15th, 1872: remarks by the Chairman at a meeting of the Manchester Steam Users' Association.]

Dear me! would have been Trevithick's exclamation had he read this; did I devote my whole life to the making known the advantages of high-pressure steam, and did I, seventy years ago,[197] really work expansive steam of 145 lbs. on the inch in the presence of many of the leading engineers of the day! Of course this short extract of a speech made by a member of a practical society, may not be taken as conveying fully the speaker's views, but it illustrates the immense difficulty Trevithick encountered in making his numerous plans acceptable to the public.

[Footnote 197: See Trevithick's letter, August 20th, 1802, vol. i., p. 154.]

Another modern statement bearing on inventions originating with Trevithick, but wearing new garbs with new names, shows the same tendency to ignore old friends, or, to say the least of it, to pass them by:--

"The trial of No. 36 steam-pinnace was made at Portsmouth yesterday. Her peculiarity consists in the arrangement of her propelling machinery, in the adaptation of the outside surface condenser, and a vertical boiler, both patented by Mr. Alexander Crichton. The condenser is simply a copper pipe passing out from the boat on one quarter at the garboard strake, and along the side of the keel, returning along the keel on the opposite side, and re-entering the boat on that quarter. The boiler is designed for boats fitted with condensing engines, and which, therefore, are without the acceleration of draught given by the exhausted steam being discharged into the funnel. It is of the vertical kind, and stands on a shallow square tank, which forms the hot well. The tubes are horizontal over the fire, the water circulating through them. The condensed steam is pumped into the well at a temperature of 100°, and being there subjected to the heat radiating from the furnace, is pumped back into the boilers at nearly boiling point. It is estimated that, under these conditions, the pinnace would run for nearly 48 hours without having to 'blow off' or carry a supply of fresh water, the waste water being made good by sea water."[198]

[Footnote 198: 'The Times,' November 24th, 1871.]

The peculiarity of this steam-pinnace of 1871, on which a patent was granted, is stated to be a metal surface condenser exposed to the cold water at the bottom of the boat, returning the condensed steam at about boiling temperature to the boiler, and a vertical boiler with horizontal tubes through which the water circulates, both of which in principle, if not in detail, are seen in the surface condenser of Trevithick's iron-bottom ship of 1809, and his vertical boiler of 1816,[199] and further illustrated in the inventions spoken of in this and the following chapter; and yet on so all-important a subject, dealt with in various ways by Trevithick from 1804 to 1832, his plans are reproduced as discoveries in 1871.

[Footnote 199: See vol. i., pp. 336, 364, 370.]

About 1828, Mr. Rennie, Mr. Henwood, and others, reported on the advantages of high-pressure expansive steam in Wheal Towan engine,[200] on the north cliffs of Cornwall, near Wheal Seal-hole Mine on St. Agnes Head, where in 1797 Trevithick had worked his first high-pressure steam-puffer engine in competition with the Watt low-pressure steam-vacuum engine. Captain Andrew Vivian was then his companion, and the Cow and Calf, two rocks of unequal size, a mile from the land, were from that time called Captain Dick and Captain Andrew, or the Man and his Man, and there they still remain in the Atlantic waves, fit emblems of their namesakes and their still living inventions. The stir made by those expansive trials led to the experiment in the 'Echo,' of which Mr. Henwood[201] thus speaks:--

[Footnote 200: See Mr. Henwood's report, vol. ii., p. 185.]

[Footnote 201: Residing at Penzance, 1871.]

"Captain William King, R.N., Superintendent of the Packet Station at Falmouth, attempted to impress on Viscount Melville, then First Lord of the Admiralty, the advantage of using high-pressure steam expansively in the Royal Navy, to whom Lord Melville replied that he had been taught by his friend, the late Mr. Rennie, that the danger attending such a course was very great, and that it would be difficult, if not impossible, to persuade him to the contrary."

]

Twenty-five years of precept and example caused the Admiralty to follow suit, and to request Mr. Ward, a Cornish engineer, to construct boilers and expansive valves for the Government steamboat 'Echo.' The writer was entrusted with fixing the machinery in the vessel at the Plymouth Dockyard, and before starting with it from Harvey and Co.'s foundry, waited on Captain King, R.N., at Falmouth, for his instructions, in happy ignorance of the fear of the Lords of the Admiralty to tread on Cornish high-pressure. After eying the applicant as captains in Her Majesty's service are apt to do when dealing with boys in the civil service, he vouchsafed to say, "Mind, young man, what you are about, for if there is a blow up, by ---- you'll swing at the yard-arm."