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high in the centre, which might have been available for spare sets of engine work (if it be intended to carry such,) or other heavy stores, which would compensate for the centre of gravity of the boiler being raised, should that be considered an object.

It will probably be said, that this boiler space is necessary for the reception and subsidence of the salt, and that less would not do. But it would still appear that, were the whole of the bottom raised up to about a level with the top of the bridges, some advantage in heating would be obtained, a considerable space be gained for other purposes, and ample room be left for the subsidence of the heavy saline water, combined too with an equally effective arrangement for "blowing off."

The Engines.

I find it is still believed by some persons that the engines are to be constructed on the "trunk principle" of the late Mr. Humphreys; but such is not the case here adopted, but rather approaching the patent plan of Sir Mark Brunel, at least in the position of the cylinders, except that instead of the cylinders making a right angle, or 90° with each other, they stand at an angle of 60°, or thereabouts. From the dimensions and particulars already given, your readers will have so far become acquainted with this branch of the subject as to render it superfluous to do more than advert to such additional details as appear to possess novelty, and may therefore be interesting. It will be observed, that the foundation plate has a conical depression of about 12 inches, into which the piston dips; this depression fits into the bend of the ship, and is therefore taken advantage of in depressing both faces of the piston, and also dishing the cylinder cover to about eight inches at the centre, thereby affording the connecting rod to be that much longer. The piston is cast with its top and bottom face, arms, and outer ring, in one piece; and for the purpose of fitting in the keys to fasten the rod there are two holes, into one of the spaces between the arms, through which the fitting and fastening is performed, and which holes are then stopped by circular plates, with valve mitre edges, and made fast. The rubbing, or "metallic" surface of the piston, is one ring of cast iron, cut open at one point, with a half-lapped joint,

depth 7 to 8 inches, to be packed behind. The nuts for holding down screws for the packing ring are turned cylindrical, and inserted into holes of 2 inches diameter, drilled in the top of the piston. The holes to be expanded by heat, and the nuts inserted cold, so as to be held in by friction, and probably farther secured by a tap screw, but this I did not notice. How much better this plan may be than either mortice or dovetailed nuts, I am not prepared to say.

The piston rod is forged with an eye at the top for the connecting pin, and the lower end in the piston is parallel, or nearly so. The guide frames for the top of the piston rod are fixed to the cylinder

cover.

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The shells of the piston valves are brass cylinders with steam openings, as shown by the sections, having a twist," apparently for the purpose of causing the wear to be more uniform. On the outside of this brass casing there is an annular chamber leading to the cylinders. The piston valves are also each furnished with a cast iron expanding ring, as before described, for the cylinders. (Had my old friend, the late talented Arthur Woolf, adopted an arrangement of this kind in his "steam plug" to his small cylinder, he would have been more successful.) This form of valve, side pipes, &c., makes a convenient arrangement, but still leaves the question open as to whether it is better, if as good, for large engines, as the lantern valve of another scientific gentleman (of whose kindness and urbanity I have a very grateful recollection,) the late Jonathan Hornblower.

These valve pistons are to be worked by eccentrics in the usual way; but I understand the "reversing" is to be effected by an 8 feet spur-wheel being attached to the eccentric, with a pinion to slide into gear with the wheel, on the spindle of which pinion there is to be a spoke-wheel, similar to a tiller-wheel, and the eccentric is to be driven by the main shaft through the intervention of a "sliding clutch."

The expansion valves are similar to the sliding ventilators used for admitting hot air into buildings, and there being four openings in the length of the valve, the orifice opened for the admission of steam to the valve pistons will be four times the length of stroke of the valve, whatever

hat may happen to be. Throttle valve of the usual kind. The air pumps (brass lined) are to stand partly in the condensers, with the foot valves at their lower ends. Air pump rods (iron, brass cased) have also an eye at their upper end for the connecting pin. The connecting

joint of the air pump is kept in its right line by parallel motion rods, by one set of which are moved the boiler and bilge pumps. The connecting rods of two cylinders and of one air pump are attached to the same crank pin, and the bearings on the crank pin are turned spherical.

The most extraordinary part of the whole machinery, and more particularly deserving notice than any other, is the wrought iron main shaft, made at the Mersey Iron Works, Liverpool. In the rough it appeared quite sound, and nearly as well hammered as an anchor. I have also seen it since it was turned, and partly bored, but did not understand there were any serious defects in it, though it was said to be "hollow" in places.

The model exhibited to the public has its cranks in opposite directions from the centre of the shaft, and which the exhibitor said was correct; I have therefore drawn them so, although it is probable a better position will be found. It also exhibited a plain strap driving-wheel on the main shaft, with a pulley below on the screw spindle, which, it was said, is to make 80 revolutions per minute; but it appears that the method intended to communicate the power from the engines to the screw, as also the construction of the screw, is not yet decided on, although it was said to be "all settled."

The whole of the work appears extremely well executed, the details to have been considered with great care and judgment, and the proportions, with some exceptions, are well maintained. When finished, and set to work, I have no doubt they will prove good engines.

In all combinations of machinery, there are reasons for preferring particular arrangements; and should the screw propeller prove as effective as has been spoken of, the reasons for this arrangement of machinery will be sufficient to entitle it to public approbation; but, otherwise, I believe it will be admitted, that any position for a large steam cylinder, out of the vertical, makes but a second-rate affair.

The condensers being of more than the usual proportions, may be expected to make a more uniform vacuum than is generally found to exist; and to produce a better test card, though perhaps not higher, or even so high as the " patent best" of 36 inches.

I saw none of the endless schemes of which so much was said yesterday, and so little will be known to-morrow. Still, if schemes be not introduced, we cannot reap the benefits. For instance, the patent condenser, with its 14 miles of 4-inch pipe, and 40,000 joints, saving 15 per cent.-patent boilers, 25 per cent.35-rotary engines, 40-patent engines patent fuel, 30 per cent.-smoke burners, with slots in the sides and trunks in the middle, besides all that never-ceasing variety of patent paddles and patent propellers, (one screw that is to be excepted,) calling into use so many high-sounding names, and flourishing more geð. metry than Euclid ever dreamed of. Think of the boundless wealth which must accrue to that man who would be bold enough to amalgamate all these elements into one vast machine, that would require so much less than nothing to keep it going, and have "oceans" of power to spare!

Here would be a "mammoth" indeed! I will not prolong this paper by discussing the merits of the screw as a propeller, or trouble you with an opinion; besides, neither you nor your readers want it, for you have them of all shapes fancy. and sizes, and to suit every purpose and

There are decided disadvantages inseparable from the use of the paddle-wheel system, such as the great weight of upper works, shafts and engine framing, wheels and framing, paddle-cases, and a variety of ponderable matter, all tending to keep the centre of gravity in an elevated position; greater exposed front surface to head winds, or side surface with wind on the beam, and with the lee paddle immersed to a great depth, and the other frequently whirling in the air; loss of steam power, arising from the obliquity of paddles in entering and quitting the water, forcing some below and throwing much above the surface; from the great immersion of paddle-wheels on leaving light; strain and tremor from machinery port with a full cargo, and too little when running through the upper part of the

vessel, and consequently the necessity for greater strength in the upper works; besides the inconvenience to passengers and crew, arising from not having a clear deck and clean sides.

The disadvantages of the screw are generally of a different kind, perfectly free from those attendant on the paddlewheels. Until an efficient rotary-engine be forthcoming the reciprocating piston must be used, and as the number of revolutions produced by reciprocating engines will be found nearly in the inverse ratio of their nominal power, it is clear that the revolving speed will be much less in large engines than small ones; hence, a difficulty will arise in producing such a combination of machinery for converting, say 18 alternations of great intensity of power into a great number of revolutions as shall be effective, lasting, and not objectionably unpleasant to the passengers. Spur or bevel geer wheels will effect the purpose; but then, there is the continued clatter of iron tongues. Straps will do, provided they be wide enough, or if narrow, strong enough: but let us see how the matter stands.

Assuming as data:

Power given to main shaft 1,000 horses, or 33,000,000 pounds.

Piston to travel 216 feet, producing per minute 18 revolutions.

Circumference of driving wheel 80 feet, will give for the velocity at circumference of driving wheel per minute, 1,440 feet.

Strain at working or pitch line, 22,900 pounds, will require, if iron spur geer, according to good practice, and if a 4 inch pitch be adopted, that the width of the wheel or wheels (if divided) be about 45 inches. If by strap, belt or combination of ropes of any description on plain surfaces, the width being 5 feet, and circumferential contact on pulley 6 feet, that the tightness be such as to produce an adhesive contact on each square foot of 766 lbs. (The atmosphere is said to assist in the adhesion of straps, if so, in what way, and how much ?) Or it would require, if of common leather strap, according to good durable manufactory practice, that its width should be, 67 feet 6 inches, which is not much above half the width for single straps.

Have you not lately written, Mr. Editor, that there is some new plan for such a purpose, by the use of plain surface con

tact?

He ought to be a wise man who

will say it is impossible, yet perhaps there are such daring spirits.

Although I apprehend there never has been a band, or strap, used in the slightest degree approaching to what is here required, still I am of opinion that such a combination of materials may be produced as would accomplish the purpose: I will therefore dismiss the question as being one of surmountable difficulty.

The stern stuffing-box cannot be said to be a difficulty, though it has been urged as an objection and disadvantage consequent on the plan, and so, indeed, it is, to a small extent. I presume it will be considered advisable rather to permit a small quantity of leakage (which under such a head of water need be very trifling) than by making it water tight, thereby increase the amount of friction. The whole available power of 1000 horses being communicated to the spindle, and from the spindle to the screw, causing it to revolve, but which is given back to the spindle again in another shape, that of pushing forward, it is clear that the whole force is transmitted from the "inboard" end of the screw to the vessel, and as both the power and velocity will be considerable (making not less than 14 million turns in a fortnight's steaming), the surface of the end of the spindle, as also what it pushes against, are matters of no trifling amount, to be free of undue friction and of durable materials. The journal of the spindle as well as the socket of the rudder stern post must be of such materials as are the least likely to suffer by abrasion in sea water, and with as little friction as possible.

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And last, though not least," but by far the greatest, comes the screw. order that a vessel may be propelled 10 miles an hour by the screw, I understand it is found that the progressive velocity of the screw must be 12 miles an hour, or forward faster than the vessel, and supposing such to be correct, or thereabouts, and that the screw makes 80 revolutions per miniute, the pitch of the screw (or, perhaps, the base of the inclined plane will be an expression as

well suited) must be

(

5280 × 12

60 x 80 =13 feet 2 inches, whether it be a complete one entire thread, similar to the first Archimedes, two half-threads, similar to the present Archimedes (which I find is 6 feet diameter by 5

feet long each), or similar to the Great Britain model, and shown in figs. 7 and 8, still it but slightly affects the present inquiry. Allowing the diameter of the Great Britain screw to be 15 feet, the diameter of the circle of effect would be about 12 feet 6 inches, or 39 feet 6 inches circumference; therefore the mechanical constructions, if developed to a straight wedge, would be represented by A B, fig. 9, line of axis; C D, distance passed over by one revolution (13 feet 2 inches); D E, circumference of circle of total effect; and CE, acting face of the screw. The amount of resistance caused by the friction or adhesion of the water on the face of the screw will very much depend on the smoothness of the surface; or, probably, a thin disc of water will be carried round with the screw, and the friction take place amongst the particles of water at some slight distance from the face. F G, fig. 10, shows the divergent lines of the cone of motion communicated to the water, and, if the above premises be correct, it appears to promise a greater effect than has generally been expected, inasmuch as the direction of impact of the screw does not make so great an angle from the line of the axis.

I think this mechanical construction will prove to be not far from the truth. As, however, I am merely describing the character and quality of the drawbacks on each plan, I will leave the question of their amount to other and abler heads, but I consider that one practical fact, one fair experiment, with both plans, in which every possible circumstance and contingency shall be as much as possible alike, so that the result cannot be questioned, is worth more, for settling a matter involving so many intricate conditions and calculations, than all the algebra that was ever strung together.

All the machinery is in a forward state, and a great part ready for putting into the vessel. The boiler is nearly finished, and in the joiners' work, of cabins, berths, &c. considerable progress has been made, but it is impossible to give an opinion as to the time when she will be ready for sea.

In daily life occurrences happen which are called accidents, but I am a little sceptical that such is a proper term for most of them. I know there are many acts of carelessness from which fre

quently disastrous consequences ensue. Accidents at sea, and land, too, there are, but they are from elements beyond our control, and therefore

"The best laid schemes o' mice an' men,

Gang aft a-gley."

I know not either that the "vasty deep" could furnish forth from Vulcan's workshop, even with mermaids' help to boot, such appliances as would repair an accident to screw, shaft, or bearing, for be it remembered there can be no "tightening up bolts," no examining bearings, and other little attendances which all machinery either has or ought to have; but if any thing be wrong the vessel must go into dock, and be laid dry as low as the spindle at least, except there be means provided for slinging, unshipping, and lifting it over the stern quarter, though the whole of the stern machinery is so simple, and of such few parts, that derangements ought to be few, if any, and should such occur may probably be examined by the use of a diving-dress and rope.

The Great Britain is being constructed in a dock excavation, to which an entrance is in progress of building, which is to be closed by a floating caisoon. The bottom of this dock is 12 feet below the surface of the water in the harbour, and as her bottom stands a working height above the bottom of the dock, it is purposed to pump water to a higher level than the harbour, so as to remove the supports and allow her to drop and to be floated out.

It is contended by many nautical men, and some eminent in the profession, that the situation of the propelling force being at the stern will cause the vessel to run very wild in a head wind, and to counteract which the rudder will be in such constant requisition as to cause a considerable loss of power; but, as I said before, one sound and settled fact is worth a thousand opinions.

Taking all circumstances into consideration, it does appear that if by the use of an equal weight of fuel the "duty" performance of the screw be nearly equal to that of the paddle-wheel, and that the whole of the machinery be so constructed as to be lasting, and not unpleasant to passengers, it has the merit of being free from some serious inconveniences of the paddle-wheel, such as, great top-heaviness, opposition of the paddle-boxes to

the wind, &c., and possesses these advantages besides, namely, that strength in the upper part of the ship is not required to support machinery, and that the deck is clear-a great comfort to passengers, and of great convenience in management of sails and working the ship.

I understand it is the intention of the Directors to use wire standing rigging, which appears admirably adapted for the purpose from its being less in size, and therefore presenting less surface when under "bare poles," from its being lighter than rope, strength for strength, from its greater durability-for if oxydation be prevented there seems no limit to it, and from its maintaining nearly a permanent length, and not requiring frequent ting up," as is the case with rope, and probably it is more particularly applicable to iron vessels than wooden ones, from the rigidity of the former not requiring the elasticity that may be serviceable in the latter.

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There being two plans lately brought forward for the protection of ships from lightning, I may be permitted to say a word or two on that subject, as it is by no means an unimportant consideration. Mr. Snow Harris, of Plymouth, proposes to let in a slip of copper to the side of the masts, from the highest point of the maintop-mast, and to continue the same to the deck, and through the ship to the water. Mr. Andrew Smith, of London, uses copper wire rope, from the same point aloft, but brought down by the sides of the shrouds and crosstrees, &c., direct to the ship's side, and then, if in wooden vessels, to the water, or to the copper sheathing. The advantage of the latter plan over the former appears to be as follows. Electric fluid is conducted over the surface of metallic substances; and, as a slip of copper may be so divided into wires as to have its original surface increased from ten to one thousand fold, it is clear that a very much less quantity of material, formed into wire, will be equal in its conducting power. The continuity of a rope is much easier maintained, at the interruptions of caps and crosstrees, &c., and by its being led direct over the side by the shrouds, there is not that liability of interruption to the electric "circuit" which would ensue to slips of copper at the masts, crosstrees, decks, &c. We have lately witnessed serious "accidents "

VOL. XXXVII.

by

lightning to buildings; but may not some part of it be found to have had its origin in the conductors being broken, of an insufficient surface, and perhaps scarcely ever thought of? On land, there are innumerable points and projections forming conductors (though mostly imperfect) which assist to restore the equilibrium between the clouds and the earth, when in contrary electrical states; but it is quite easy to imagine millions of acres of clouds highly charged with electricity, either positive or negative, floating over the sea, ready to commence war with the waters beneath, if in the opposite electrical condition; and should a solitary ship be near, unprovided with good and sufficient conductors, the probability is that an accident" would happen.

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On commencing this task, I intended to confine myself to a faithful, straightforward, though concise description of this vessel, without departing either to the right or left in extraneous matter; but I find I have rambled a little out of the line of description, and into comparisons and investigations; though, having no prejudices or interest either way, my only aim has been to afford your readers a correct account of the magnificent Great Britain.

I remain, Sir, your obedient servant,
J. R. HILL.

98, Chancery-lane, August, 1842.

IRON LIGHTHOUSES.

Sir, It is just six years ago since I addressed you on the subject of an improved iron lighthouse, on a principle altogether different from any which had ever before been presented to the public; and the advantages which were proposed to be gained by it, were stated to be manifold and of the very highest importance, inasmuch as by its adoption, thousands upon thousands of pounds worth of valuable property, as well as numbers of our fellow men, who brave

"the dangers of the seas,"

might be annually saved from perishing. The great novelty of the idea, which was then presented for the first time to the public (as might be best gathered from the very rude sketch which accompanied my letter, and which you inserted at the commencement of your 26th volume), was the adoption of a mode of construction by which all the enormous force

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