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The stem is 12 inches deep at the forefoot, by 5 inches thick, and at the 8 feet water mark, it is 16 inches by 2} inches; thence it diminishes gradually to 12 inches by 14 inch. It is welded in one piece 18 feet long.

The ribs or frame are formed principally of angle iron, 6 inches by 34 inches by gths inch, at distances of 18 inches from centre to centre, but inclining gradually to 24 inches at the extremities, where, also, angle iron, 6 inches by 21 inches, and 4 inches by 3 inches, is used.

In that part of the body of the ship which is occupied by the engines, the ribs are doubled, by having a similar angle iron riveted to them, with the web inside, or, as it is termed, “reversed."

The outside plating commences with plates 6 feet to 6 feet 6 inches long, and 3 feet wide, by 1ths inch thick; of these plates there are four courses; these are followed by several courses of gths inch thick, which is the strength of the whole of the immersed part, up to the deep load water line.

Above that height the same thickness is preserved amidships, but it is gradually reduced to sths inch thick quite high up, and at the extremities, with a view to lighten them.

The longitudinal floor sleepers are ten in number; they are 3 feet 3 inches in depth, and 1 inch and 7ths inch thick.

The middle sleepers extend throughout the length of the vessel ; those on the sides are level on their upper surface, and consequently are terminated by the rising of the bottom of the ship.

These sleepers are tied to the bottom, and are preserved in their vertical position by inverted curves of strong angle iron, which are riveted to the ribs and also up their sides.

Along the upper edge of each, there is an angle iron, and over the whole is riveted an iron deck iths inch in thickness.

There are two bilge keels, consisting of a middle plate, 14 inch thick, and two angle irons, 5 inches each way by 1 inch thick.

These bilge keels are 110 feet long, and their under edges are on the same horizontal level with the under side of the keel, so that in docking the ship, if long baulks of timber are extended across the dock by way of blocks, the weight of the body of the ship (where the boil. ers and machinery are placed,) is supported at given parallel distances on both sides of the keel, all risk of straining it, or the machinery, is avoided, and the vessel is not obliged, in the usual manner, to rest upon her keel, until the bilge shores can be got under. The upper cargo deck forward is made of plate iron, 1 ths inch

i thick in the middle, and ths inch thick round the sides; it is riveted together throughout, as well as to the iron deck beams, and to the sides of the vessel.

The main deck is made of pine timber 5 inches thick, and the planks are cross-bolted at distances of 4 feet apart.

As this deck is situated on the load floatation plane of the vessel, where transverse stiffness is of more importance than longitudinal strength, the planks are placed athwartships, and their extremities firmly bolted down, through two longitudinal stringers of Baltic tim

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Civil Engineering. ber, to the shelf plates which are 3 feet wide by five-eights of an inch thick, and are very securely fixed to the sides.

The middle or promenade deck is also of pine timber 4 inches thick. placed length wise of the ship; it has also strong iron shelf-plates 3 feet wide by 4 inch thick, and Baltic stringers to attach it to the side of the ship.

The upper deck is of red pine timber, and is also placed lengthwise. As the sides of the vessel at this height, and also this deck, may be considered as the truss, which is to resist longitudinal deflection, or drooping of the extremities, the outside plates are there inch thick, and they have been strengthened by an outside moulding-iron sirar, 6 inches by 1 inch, and by additional straps of iron 7 inches by 1 inch, welded into lengths of 60 feet, and riveted to the inner sides of the upper line of plates.

The shelf-plate of the deck is 3 feet wide by 4 inch thick, and upon this, outside of the water-way plank, which is 4 inches thick, there is a course or tie of Baltic pine tinber 340 inches in section, carefuly scarphed and securely bolted to the ribs, and to the shelf-plate, throughout the length of the ship. There are three rows of timber piliars, or stauncheons, which are fixed to the bottom of the ship, passing up between longitudinal ties at each deck, and are secured to the upper one.

The beams of all these decks are made of angle iron, 6 inches by 31 inches by 4 inch, and their ends are bent down, and riveted to the ribs on each side.

Upon them, the shelf-plates before mentioned are riveted, and thus form a horizontal band 3 feet wide at each deck.

A crutch or strut is introduced at each end of nearly every deck beam, which is riveted to it, and to the ribs at about 3 feet from the angle of junction.

One of the most important improvements which has recently been introduced in the construction of vessels (particularly those of iron,) is the water-tight bulkhead; as in the greater number of cases, when an injury may be sustained in one compartment only, it may absolutely preserve a vessel from sinking; several instances of this bare already occurred, and even where it may not suffice for this purpose, it at least separates the leaky and injured from the secure parts, and gives time either to attempt to stop the leak, or to make other preparations.

In iron vessels, these bulkheads can be rendered much more effectual than in wooden ones, by their exact contact with the bottom and sides, while at the same time they form admirable ties and stitfeners.

In the Great Britain there are five such bulkheads.

The first separates the forecastle from the forward passengers' cabin and the hold, and as it is in the forepart of a vessel that injury is most likely to be sustained, this partition is made particularly strong and secure.

The next bulkhead divides the forward cabin from the engine room or more properly, from the fore-hold for the coal and the stokers, at the forward end of the boilers.

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The third bulkhead is abast the engine-room, but in this, there is inecessarily a hole for the screw-shaft to pass through; this is secured

by a well-fitted collar, and there is also a door, which is so arranged has to be shut and bolted quickly.

These three bulkheads pass up to the upper deck; there are also i two others; one separating the after coal-hold from the after cargo

hold, and another nearly at the stern ; both these terminate under the saloon deck.

The minute detail of the construction of the hull of the vessel would be too voluminons to be given here, and it would be unnecessary, as it will shortly be published. It is better, therefore, to proceed to describe the action of the screw propeller, which has now become an object of such deep interest to all who are engaged in marine engineering; and to the machinery by which it is to be put in motion.

At an early stage in the construction of the Great Britain, but not until her sides had assumed the form adapted for paddle wheels, the small steamer Archimedes, belonging to the Company owning the patent of Mr. F. P. Smith for the application of the Archimedean screw, visited Bristol, and amongst other parties invited to make an excursion to the Holmes, on board of her, were some of the Directors of the Great Western Steam-ship Company.

The performance of the screw on that occasion, induced the author to request permission of Mr. Smith and Captain E. Chappell, R. N., who was officially appointed by the Admiralty to report upon her, to proceed in her to Liverpool.

On the passage, enough rough weather was encountered to show that the screw possessed several good points, and was not so absolutely impracticable as had been asserted; and although far from venturing to give a decided opinion, on the author's return, he wrote such a letter to the Board of Directors as induced them, after some days of deliberation, to decide upon suspending, during three months, the progress of the machinery for paddles, and also of that part of the vessel which might be affected by the change, to call upon Mr. Brunel during that period to investigate the subject.

At the end of the proposed delay, the report which Mr. Brunel made was so favorable, that, undaunted by the novelty and vastness of the experiment, the Directors resolved to adopt this mode of propulsion, of the success of which they have now such cause of congratulation.

From that period, until it became necessary to decide on the cxact form of screw to be used, all possible means were taken, by experiment and observation, to arrive at the best shape and angle of inclination of the blades, or as it is commonly called “the pitch."

Amongst others, the proprietors of Mr. Smith's patent liberally lent the Archimedes to the Great Western Steam-ship Company, for a period of several months, which afforded ample opportunity of trying the performances of the several forms of screws recorded in the Table given in the next page.

These experiments were made in the Bristol Channel, under circumstances of weather, as nearly as possible similar, and the distances were very carefully measured by two of Massey's Logs, whose accaracy had been previously tested.

It will be observed, that the greatest velocity of vessel, 8.375 knots, was attained by Mr. Smith's screw of five feet 9 inches diameter, the angle of which was 193 degrees, and the slip was 21 per cent.; that is, the ratio of speed of the vessel to that of the screw, was as .787 to 1,

Particular attention is due to experiments Nos. 5, 6, and 7.

Reasoning upon the assumption, that the effort of the entering edge of each blade must cause the water to recede, and that each succeeding portion of blade should so increase in pitch as to impinge with uniform force against the water, which was so receding, a screw of this description was made and tried before it was discovered that it was the subject of a patent by Mr. Woodcroft.

The first trial served to show that the curvature or increase of pitch which had been given to it was too great, since the speed of the ressel was greater by two per cent. than that due to the mean pitch of the screw, whence it was evident that the entering edge was realiz retarding, and the terminating portion alone was doing the duty.

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5 20. 57.30 8.1751 8. 1.020 Woodcroft's increasing pitch, 7 0

3 blades, made of cast iron,

as first made. 6 21.50 62.60 8.10 8.10 1. The same, with 3 inches cut 7 0

off the entering edge of the

blades. 7 22.501 62.12 8.20 8.730 .940 The same, with 4 inches cut 7 0 7 5

off the entering edge of the

blades. 8 20.50 51.401 7.490 8.566 Four wrought iron arms, with 7 018 0

0180 blader, each 2 feet 9 inches

long by I foot brood. On the second trial, when a radial strip 3 inches in width had been cut off the after part of each blade, the speed of the vessel was exactly that due to the screw; whence it was also evident, that the front edge still did not assist.

On the third trial, after a second radial slip of 4 ivches had been cut off the entering edge of each blade, the vessel attained a speed of 8.2 knots, and the ratio of speed of the vessel was as .94 to 1 of the screw.

The horse-power employed on this trial, was by indicator, 62.12, and the speed of the vessel 8.2 knots, against 67.1 in the before-named trial, with the original screw of the Archimedes, when the speed she attained was 8.375 knots.

Although on neither of the trials numbered 5, 6, and 7 with this screw, was so great a speed of vessel attained as on that first named, it is important to draw attention to the fact, that the slip was reduced to a very small quantity.

But the horse-power exerted was also much less than in the first trial, arising from some imperfections in the cutting down of the screw and other causes, which would probably have been remedied had there been time to cast a new screw of this description; but unfortunately, just at this period, the Propeller Company required the Archimedes for service, and the experiments ceased.

This screw was afterwards tried by Mr. Barnes in the Napoleon, a very beautiful French Post-Office vessel, built by M. Normand, or Havre, when the following result was obtained :

Horse Power exerted, 95.5.
Speed of vessel, in knots, 10.15.
Speed of screw, 11.2.
Speed of vessel to 1. of screw, .895–104 per cent.

In the two cuttings down, this cast-iron screw with three blades, 9 feet in diameter, which was originally very slight, had been so much reduced in substance, that it weighed only 833 lbs. Mr. Barnes, therefore, could not venture to permit the engines to exert their full power, otherwise it is probable that a higher speed would have been attained.

The commencing angle is 17°, and the terminating one 191°; the increase of pitch is therefore ith, or 8 per cent.

The screw of the Great Britain, which is of wrought-iron, consists of six arms, formed by placing and riveting together four distinct forgings, or centre pieces, with arms welded to them, each of which is 6 inches thick.

Upon the extremities of these are riveted palms of plate iron, which are 4 feet 44 inches long on their circumferential edge, by 2 feet 9 inches in height, and į inch thick.

The diameter is 15 feet 6 inches, and the pitch or helix of one revolution is 25 feet, which equals an angle of 28 degrees.

Its weight is 77 cwt.

The area of the six palms, which may be considered as the effective part of the screw, is 56.25 feet; but the area, calculated as a plane perpendicular to the axis, that is as portions of a disc, is only 47-4 feet, and the portions of the arms within the blades, present a similar area of 26.88 feet.

As the rotary velocity of the outer edge of the blades is nearly 30 miles an hour, it is important, in order to diminish friction, that they should be as accurately shaped as possible, and should present no irregularities of surface. In this instance, the object was attained, by mounting the screw on a face plate and planing the surface, by means

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