(206.)
In fact, a number of thin plates of water are exposed on both sides to the most intense action of flame and heated air; so that steam of a high pressure is produced in great abundance and with considerable rapidity. The plates forming the boiler are bolted together by strong iron ties, extending across the boiler, at right angles to the plates, as represented in the figure. The distance between the plates is two inches.
There are ten flat chambers of this kind for water, and intermediately between them ten flues. Under the flues is the fire-place, or grate, containing six square feet of fuel in vivid combustion. The chambers are all filled to about two thirds of their depth with water, and the other third is left for steam. The water chambers, throughout the whole series, communicate with each other both at top and bottom, and are held together by two large bolts. By releasing these bolts, at any time, the chambers fall asunder; and by screwing them up they may be all made tight again. The water is supplied to the boiler by a forcing-pump, and the steam issues from the centre of one of the flues at the top.
These boilers are constructed to bear a pressure of 400 or 500 lbs. on the square inch; but the average pressure of the steam on the safety valve is from 60 to 100. There are 100 square feet of surface in contact with the water exposed to the fire. The stages which such an engine performs are eight miles, at the end of which a fresh supply of fuel and water are taken in. It requires about two bushels of coke for each stage.
The steam carriage of Mr. Hancock differs from that of Mr. Gurney in this—that in the former the passengers and engine are all placed on the same carriage. The boiler is placed behind the carriage; and there is an engine-house between the boiler and the passengers, the latter being placed in the fore part of the vehicle; so that all the machinery is behind them. The carriages are adapted to carry 14 [Pg438] passengers, and weigh, exclusive of their load, about 31⁄2 tons, the tires of the wheels being about 31⁄2 inches in breadth. Mr. Hancock states, that the construction of his boiler is of such a nature, that, even in the case of bursting, no danger is to be apprehended, nor any other inconvenience than the stoppage of the carriage. He states that, while travelling about nine miles an hour, and working with a pressure of about 100 lbs. on the square inch, loaded with thirteen passengers, the carriage was suddenly stopped. At first the cause of the accident was not apparent; but, on opening one of the cocks of the boiler, it was found that it contained neither steam nor water. Further examination proved that the boiler had burst. On unscrewing the bolts, it was found that there were several large holes in the plates of the water-chamber, through which the water had flowed on the fire, but neither noise nor explosion, nor any dangerous consequences, ensued.
(207.)
"The base of the boiler and the summit are composed of cross pieces, cylindrical within and square without; there are holes bored through these cross pieces, and inserted through the whole is an air tube. The inner hole of the lower surface, and the under hole of the upper surface, are rather larger than the other ones. Round the air tube is placed a small cylinder, the collar of which fits round the larger aperture on the inner surface of the lower frame, and the under surface of the upper frame-work. These are both drawn together by screws from the top; these cross pieces are united by connecting pieces, the whole strongly bolted together; so that we obtain, in one tenth of the space, and with one tenth of the weight, the same heating surface and power as is now obtained in other and low-pressure boilers, with incalculably [Pg439] greater safety. Our present experimental boiler contains 250 superficial feet of heating surface in the space of 3 feet 8 inches high, 3 feet long, and 2 feet 4 inches broad, and weighs about 8 cwt. We supply the two cylinders with steam, communicating by their pistons with a crank axle, to the ends of which either one or both wheels are affixed as may be required. One wheel is found to be sufficient, except under very difficult circumstances, and when the elevation is about one foot in six to impel the vehicle forward.
"The cylinders of which the boiler is composed are so small as to bear a greater pressure than could be produced by the quantity of fire beneath the boiler; and if any one of these cylinders should be injured by violence, or any other way, it would become merely a safety valve to the rest. We never, with the greatest pressure, burst, rent, or injured our boiler; and it has not once required cleaning, after having been in use twelve months."
Dr. Church of Birmingham has obtained a succession of patents for contrivances connected with a locomotive engine for stone roads; and a company, consisting of a considerable number of individuals, possessing sufficient capital, has been formed in Birmingham, for carrying into effect his designs, and working carriages on his principle. The present boiler of Dr. Church is formed of copper. The water is contained between two sheets of copper, united together by copper nails, in a manner resembling the way in which the cloth forming the top of a mattress or cushion is united with the cloth which forms the bottom of it, except that the nails or pins, which bind the sheets of copper, are much closer together. The water, in fact, seems to be "quilted" or "padded" in between two sheets of thin copper. This double sheet of copper is formed into an oblong rectangular box, the interior of which is the fire-place and ash-pit, and over the end of which is the steam-chest. The great extent of surface exposed to the immediate action of the fire causes steam to be produced with great rapidity.
Various other projects for the application of steam engines on common roads were in a state of progressive improvement, [Pg440] when the greater advantages attending railways were considered so manifest, that considerable doubts were raised, whether, supposing the problem of the application of the steam engine on common roads to be successfully solved, it could ever be attended with the same economy and effect, as by the adoption of a railway. Among the projects which promised a successful issue, may be mentioned the locomotive engines contrived by Messrs. Maudslay and Field, by Colonel Maceroni, and by Mr. Scott Russell. These and others have, however, been abandoned, mainly, we believe, from the impression, that wherever traffic can exist, sufficiently extensive to render the application of steam power profitable, a railway must always supersede a common road; and that, even in the limited traffic to be expected on branches to the great railways, horse power applied to railways would be attended with more economy than steam power applied on stone roads.