[458] Though lightning from the heavens has never yet been usefully employed, and is not likely to be so, the electricity generated in galvanic batteries and used for telegraphy is precisely the same as lightning.
[459] Among other advantages the projectors offer an almost absolute safety of boilers from explosion, as they are made of 3-inch wrought-iron tubes ⅜ inch thick; the boilers when put together are proved to 2500 lbs. hydraulic pressure on the square inch, are worked from 300 lbs. to 500 lbs. per square inch as desired, and their bursting pressure is 20,000 lbs. per square inch.
[460] Messrs. Perkins and Son base their calculations for speed on the fact that the vessel they propose will have 30 horse-power to a foot of midship section, the best Atlantic steamers having only from 4 to 5 horse-power to each foot of midship section.
I have submitted these particulars to a gentleman of great scientific and practical knowledge of marine propulsion, who remarks: “This large steamer is, I fear, a wild idea, until the form of the present steam-ship is very much improved. It will require a great deal more power than what Mr. Perkins proposes to drive such a vessel 30 knots an hour; and marine engines must be very much improved to get anything like this power in a ship, and to maintain it for 100 hours on a consumption of 500 tons of coal.” But as everybody admits that we have not yet reached perfection, it is solely with the object of furthering improvement, that I furnish my readers with the plans and proposals of Mr. Perkins.
[461] It will be remembered that the earliest application of the steam-engine was for the purpose of pumping water; hence, when applied to turn machinery, the great lever of the pumping engine was retained. The same thing took place on the application of the steam-engine to navigation; and, even now, the beam or lever engine is in common use both here and in America.
[462] A practical engineer, with whom I had recently some conversation on this subject, informed me that when, many years ago, he was superintendent of one of the oldest Steam Navigation Companies, it was scarcely possible to maintain a pressure sufficient to keep the air out of the boilers, and the hissing noise it made, when rushing into the boiler through the reverse valve, was a not unfrequent tell-tale of the slackened efforts of the over-worked fireman.
[463] To convert a quantity of water at 32 degrees into 10 lbs. of steam, requires 1 cwt. of coal; but to convert it further into steam at 40 lbs. pressure, would only require 1·012 cwt., and to raise it into even 90 lbs. not more than 1·024 cwt. of coal would be required.
[464] The following illustration is from a photograph furnished by Messrs. T. Richardson and Son of West Hartlepool. It is exactly the same in principle as those supplied by Messrs. J. and G. Thompson of Glasgow, to the Bothnia and Scythia, belonging to the Cunard Company, which I have already described, and represents the usual construction of modern marine engines of the best class.
[465] See details in the Transactions of the Institution of Naval Architects, 1874.
[466] It may be stated, generally, that 1 lb. of coal can, under the most favourable circumstances, be made to evaporate from 12 to 16 lbs. of boiling water, the evaporation of each pound being equivalent to 745,800 foot-pounds of mechanical work. At this rate 1 lb. of coal ought to give out from nine to twelve million foot-pounds of work, while, in reality, no steam-engine does so much as two million foot-pounds for a pound of coal, so great is the loss from the want of proper means of utilizing the whole work produced by the combustion of the coal.—Vide Text-Books of Science, p. 174; by C. W. Merrifield, F.R.S.