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Another proposal, which has been much before the public, is that of Mr. S. J. Mackie, C.E. This is a double-ended and flat-bottomed boat, 400 feet long, 90 feet broad, and drawing 6 feet 6 inches of water. A section of it is given in Fig. 3. B and B’ are rectangular waterways going right fore and aft, difl'ering from that in Captain Dicey’s plan by having a bottom as well as a top and sides, and in there being two of them, instead of one central canal. Mr. Mackie’s mode of propulsion is by two or three pairs of paddle-wheels working in the rectangular waterways. He claims, as the advantage of his design, great steadiness at sea, ample and well-distributed space for the accommodation of passengers, great longitudinal and transverse strength, and the absence of any projections which could receive injury from waves or piers. He also expects to attain high speed. I believe his vessel would realise all that he claims, except speed, but that no steam pmver whatever could give her speed with such propellers. A paddle-wheel working in a closed tube would do more churning than propulsion; and as to placing two or more paddle-wheels behind the first, I think they are more likely to impede it than to do any useful work of their own. I have the highest personal respect for Mr. Mackie, and I have come to this udgment regarding his scheme with much regret; but I am sure he would be the last person to wish that I should suppress my opinion, advisedly formed.

At the annual general meeting of the Institution of Naval Architects in March, 1871, Mr. Michael Scott, C.E., brought forward a plan for controlling‘the motion of vessels for the channel passage, by the judicious use of water ballast. He proposed that the vessels should leave and enter the ports in light trim; but that, as soon as they got into deep water, their displacement should be altered by the admission of sea-water into closed compartments, in such measure as to make the ship’s behaviour as easy as possible. This plan may seem rather bold, on the face of it; but there is very little doubt that it could be successfully worked, with a little attention and experience. The quantity of water required could all be pumped out in a few minutes, onapproaching the harbour. It must he confessed that it is rather an artificial way of meeting the difficulty, and that it falls under the suspicion of being “too clever by half.” Still the plan is theoretically right, apart from the somewhat ticklish character of the adjustment. I do not think I should have recourse to such a plan, for myself, until the more direct method of Mr. Bessemer had been found to fail ; but, failing that, it may be worth reverting to.

At the same meeting, Mr. Evan Leigh, of Manchester, brought forward a proposal for a channel steam-ship, which may be said to bear a rough likeness to Mr. Mackie’s proposal.

There was the same double channel port and starboard, in each of which worked a paddle-wheel; only the channel, instead of being closed at the bottom and open from end to end, was segmental in profile, and open ~at the bottom. The paddlewheels were moreover arranged as drums instead of open wheels. This plan is just as inefficient in respect of propulsion as either Captain Dicey’s or Mr. Mackie’s, and Mr. Leigh has moreover encumbered it with a ridiculous plan for harbours, and with bad steering apparatus. The device is not worth much on its own account, and these appendages are quite enough to prevent its floating, either mechanically or commercially.

Mr. John West, of Liverpool, has published plans‘ of a “Channel F erryboat,” 345 ft. long and 43 ft. beam; breadth over all 68 ft., and draught 6 ft. 6 in. The propellers are two pairs of paddle-wheels, external, as in Mr. Bessemer’s ship; but the guards extend the full breadth of the paddles throughout the whole midship length, as in American river steamers; and, as in those, are utilised for cabins and smoke- boxes. The boilers, however, are inboard, instead of on the guards. Apart from the Bessemer saloon, the plan only differs in detail from that designed by Mr. Reed for Mr. Bessemer.

I have had before me a large number of schemes, of the most varied character, but the greater part of them hopelessly and irretrievably bad. Very few inventors will take the trouble either of acquiring the knowledge requisite to form an exact idea of the problem which they have undertaken to solve, or of ascertaining what steps have already been taken towards its solution. Hence we get schemes which never could be likely to work; schemes which have already been tried, and found to fail; schemes brought forward as new, which are already in common use. From among this chaos I have endeavoured to select those which are of the most immediate practical interest, and to lay before my readers, frankly and impartially, what I conceive to be the advantages and disadvantages of these.

My readers may now reasonably turn round and say, “ What does the writer mean to indicate as the best ship for the purpose ?”- My answer is :—

1. For mail service, in very rough weather, a small fullpowered vessel, which will make everybody sick whose inside has not been well salted.

2. As a mere preventative of sea-sickness, without special mechanism, the longest and broadest boat possible.

3. With reference to safe work, in entering and leaving port,

' See “ The Engineer ” for August 30, 1872.

in ordinary rough weather," as well as to reasonable ease and comfort at- sea, a boat 350 ft. long and 45 ft. broad or thereabouts, and drawing not more than 8 ft. of water.

4. As to the easiest thing likely to be obtained in our days —the Bessemer saloon steamer.

I give this to my readers as the best judgment which I can form on a subject which I have studied with great care. Whether right or wrong, it is to that extent worthy of attention, and it is wholly unprejudiced ; as I have neither interest nor share in, nor retainer for, any one of the schemes which have been brought forward.

' Boulogne harbour is 70 metres or 225 feet wide, and Calais harbour 100 metres or 330 feet wide. The entrances Of both are mere canals open to seaward.

ON EXPLOSIONS OF FIRE-DAMP IN COAL MINES; THEIR CAUSES AND MODE OF PREVENTION.

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HEN the old Hebrew poet wished to fix upon a class of men who were exposed more than their fellows to danger, and who had to face nature under her most terrible aspects, he selected, with admirable fitness for his day, those “that go down to the sea in ships and occupy their business in great watersz” should a Tennyson or a Browning wish nowadays to handle a similar theme, he would find illustrations still more apt in the perils of those who pass their lives in the chambers and galleries of deep mines. And coal-miners are exposed to a larger amount of risk than any others of the craft'; for, while they share ,with the rest of the mining population perils from falls of rock overhead, from sudden rushes of water, from accidents in shafts, and other similar sources of danger, they have in addition, always hanging over their heads, a hazard peculiarly their own, in the possibility of a sudden outburst of the gas known popularly as fire-damp, to be followed, should any one of a series of delicate precautions go wrong, by an explosion, which will first burn and shatter whatever comes in its way, and then leave behind it a deadly vapour that stifles out any life that the previous fire and havoc may have spared. We propose to attempt a short description of this everpresent source of danger to the coal-miner, and of the methods which have been suggested for guarding against it; to point out where these methods have succeeded, and where they have failed ; and to add a few hints as to how failure may have been caused, and how it may be avoided for the future.

It may be as well first to give our readers a few figures, showing the number of lives lost every year in Great Britain by explosions of fire-damp. The table below is taken from the Reports of the Colliery Inspectors.

TABLE snowme THE LOSS OF LIFE IN BRITISH COLLIERIES FROM 1864 TO 1871. ‘

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These results are far from reassuring. The percentage of lives lost from accidents of all kinds seems to be slightly increasing every year, and last year amounted to very nearly three in every thousand :‘the proportion lost by the cause of accident we are now specially considering varies very much from year to year, but last year it was almost exactly one quarter of the whole.

Fire-damp, the source of Such a large part of the accidents incidental to coal-mining, is a mixture of carburetted hydrogen, nitrogen, and carbonic acid gases. Of these carburetted hydrogen is by far the largest ingredient, and, as the cause of explosions, the one we are more specially concerned with ; it is a compound of carbon and hydrogen (CH4), and in a pure state burns with a faint bluish-yellow flame : it becomes explosive when mixed with air, the violence of the detonation depending on the proportions of the mixture : if the amount of gas be not more than T15-th of the whole, the mixture burns without explosion, though the presence of the gas is distinctly indicated by changes, well known to miners, produced in the flame of a candle or lamp held in it. The maximum of explosiveness is reached when the proportion of gas is from Y,l-fth to J6th : if a light be brought into such a mixture, the whole flashes into flame with fearful violence, and the unlucky miner whom it comes across is either scorched to a cinder or blown to pieces, or perhaps suffers both fates: when the gas is present to the extent of ith, or a larger proportion, the mixture again burns without explosion: so large a proportion of gas, however, makes an atmosphere in which breathing is carried on with difliculty, and when the proportion of gas reaches g-rd, respiration becomes impossible.

The products of the explosion are steam and carbonic acid, the latter a heavy gas, the presence of a small quantity of which in air is immediately fatal to life. This floods slowly the passages of the mine, and effectually stifles out any life that may have escaped the flame and concussion of the explosion.

'_ This was the year of the Oaks explosion, in which 361 men were killed ; in the same year explosions occurred at Tslk-on-the-Hill and Dukinfield, causing respectively 91 and 38 deaths.

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