slack part of the chain to fall into a curve, and the descending discs to overlap the trunks so much as to render them useless.”* In the "Algiers," nozzles were fitted on the metal dales, on the orlop decks, and thus when water was required in the lower part of the ship for the extinction of fire, the hoses were connected without interference with any of the gun-deck arrangements, or injury to the hoses themselves.

Massie's Pump is a double action lift and forcing pump, and has, in discharging as much as eleven tons in nine minutes, claimed a superiority to others.

Steam ships are fitted with Bilge Pumps, which are worked by the engine in extraordinary cases of leakage : and also with hand pumps, for the purpose of feeding the boilers, when the engines are not at work. These latter are turned when desirable to such purposes as washing decks, &c, and are capable of connection with the engine.

The pumps and main mast are all enclosed from the bottom of the ship as far as the orlop deck.

The quantity of water discharged by a hand pump in a given time, is determined by considering that at each stroke of the piston, a quantity is discharged equal to a cylinder whose base is the area of a cross section of the body of the pump, and height the play of the piston; thus, if the diameter be 4 inches, and the play 3 feet, we have to find the content of a cylinder 4 inches in diameter and 3 feet high. Now 4 inches is the of a foot, or 333, hence, 333 × 7854·110999 x 785408796 the area in square feet; hence, 08796 x 32639 the content of the cylinder in cubic feet = the quantity of water discharged by one stroke. A cubic foot of water weighs about 63.5 pounds; therefore, 2639 × 63.5=16·756 avoirdupois; and an imperial gallon is equal to ten pounds of water; whence dividing 16.756 by 10, we get the number of ale gallons=1.6756.

=

=

The resistance is equal to the weight of a column, whose base is the area of the piston, and whose height is that of the surface of the water in the body of the pump above the surface of the water in the well, together with friction.

Let the body of the pump be 6 inches in diameter, and the height to which the water is raised be 30 feet, the weight of the

*Treatise on Surveying.

piston and rod 10 pounds, and the friction of the whole weight of water.

Then

62=36=the pounds avoirdupois of 3 feet of the column of water; but the column is 30 feet, therefore 3: 30::36: 360 pounds, the weight of the whole column. Adding the friction we have 360=72, and this added to the weight of the column gives 360+72=432: and to this add the weight of the piston and pump rod, 432 +10=442=the whole resistance, which anything greater will overcome.

If the pump handle be in the proportion of 10 to 1, the pump may be wrought with a force of 45 pounds.*

The Body Post and After Deadwood are bored through in a fore and aft line, and the passage thus formed is bushed with two metal pipes, through the innermost of which the screw shaft projects. The fore part of this pipe is made water-tight round the shaft, by a collar and packing called the gland box, which is put on inside the ship; the after end is lined with strips of lignum vitæ. The projecting end of the shaft is fitted, on its appearance outside, with a metal head-piece into the mortise or "slot," of which the foremost end of the boss of the screw drops; and this connection is made permanent by the downward pressure of rods called Spanners, which reaching from the deck are screwed down upon the frame when the boss has entered. The mortise must of course be turned upright either to receive the screw or permit of its disconnection; the screw being kept upright during these operations by means of a moveable trigger, which is so arranged as to catch the upper blade when necessary. The shaft is made in pieces of about 24 feet in length, which are connected by fastenings called couplings.

The after part of the body and fore part of the stern-post, are fitted with metal bearings on which the bosses of the screw rest when it is in place. They are also faced by groovings of metal, which serve to guide the screw during its ascent or descent. These groovings are fitted with metal racks; the screw is suspended in a framing commonly called the banjo, from which corresponding pauls project; and which, being permitted to act upon the racks whilst hoisting the screw up, prevent danger in the event of the ropes being carried away. There is no such pro

* Grier's Mechanic's Calculator.

vision made for lowering, and therefore great care is taken to attend the falls during that operation. The diagrams connected with the article on steam may explain this fitment more clearly.

MEASUREMENT FOR TONNAGE.

The Common Rule for finding the burthen of ships, or what is called the builder's tonnage, is to multiply the length by the extreme breadth, and that product by half the extreme breadth, and divide the product by 94; thus: length x extreme breadth x half the extreme breadth = the builder's tonnage.

The following is called the Parliamentary, or new rule : divide the length of the upper deck between the after part of the stem and the foremost part of the stern-post into six equal parts. Depths: at the foremost, middle, and aftermost of these points of division, measure in feet and decimal parts of a foot the depths from the under side of the upper deck to the ceiling at the limber strake. In the case of a break in the upper deck, the depths are to be measured from a line stretched in a continuation of the deck. Breadths: divide each of those three depths into five equal parts, and measure the inside breadths at the following points, viz.: at th and at ths from the upper deck of the foremost and aftermost depths, and at ths and ths from the upper deck of the midship depth. Length: at half the midship depth, measure the length of the vessel from the after part of the stem to the foremost part of the stern post; then to twice the midship depth, add the foremost and aftermost depths for the sum of the depths: add together the upper and lower breadths at the foremost division, three times the upper breadths, and the lower breadth at the midship division, and the upper and twice the lower breadth at the after division, for the sum of breadths; then multiply the sum of the breadths by the sum of the depths, and this product by the length, and divide the final product by 3500, which will give the number of tons for register. If the vessel have a poop or half-deck, or a break in the upper deck, measure the inside mean length, breadth, and height of such part thereof as may be included within the bulk head: multiply these three measurements together, and dividing the product by 92·4, the quotient will be the number of tons to be added to the result as above found. In order to ascertain the tonnage of open vessels the depths are to be measured from the upper edge of the upper strake.

AMOUNT OF MATERIALS.

Although ships have increased in size considerably since the publication of Mr. Edye's calculations, a selection from that work will serve to exemplify the amount of materials employed in the construction and equipment, as well as the value of a 120-gun ship. Timber, 2197; iron, 136; copper bolts, 47; copper sheets, 18; metal nails, 3; pintles and braces, 2; lead, 9; oakum, 16; pitch and tar, 16; whiting and white lead, 9; oil, 1; three coats of paint, 9; spars, 106; rigging, 60; sails, 11; cables and anchors, 90; ballast, water, fuel, guns, ammunition, provisions, stores, 1860 tons; rope, 30,250 fathoms; blocks, 950; and trenails 64,458 in number.

Weight of the hull when launched, 2466 tons. Total weights received on board, 2143 tons; expense of labour, 15,643l.; of materials, 77,8781.; of hull, 93,5217.; of spars, 38794.; of rigging and blocks, 2994l.; of furniture and sea stores, 16,8051.

CHAP. V.

FLOATING.

BESIDES the construction of the ship, and the formation of those places whereon she may be built, there are frequently many difficulties to be surmounted before the fruit of all this labour is freely clear of the ground. For example:- A river too shallow for even the lightest draught of the vessel, has to be navigated; a shifting sand, or newly formed bank, to be crossed; and even in some cases (as in Russia), ships must be borne bodily on

"camels," which, being first filled with water, are secured to the ship's sides, and then pumped out. (See figs. 25, 26.)

Again, it may be only necessary to get the ship on an even keel; for all vessels sit deeper by the stern when empty. In launching the "Bombay," e. g., her heel was lifted over a bank

that had formed outside the dock gates, by securing several large lighters under the quarters on the same principle.

These considerations naturally suggest a notice of those purposes, to which a knowledge of the laws of fluid pressure we have already noticed may be applied. * Often in danger of foundering or grounding, and frequently called upon to convey weights into shoaler water than the ship's draught will admit of, we must be prepared to assign to every boat, spar, and other buoyant body its due amount of duty. The "Sulphur " grounded, and falling over in a tidal harbour, became filled with water at each flow of tide. Rafts of spars were constructed, and at low water lashed to the leeside. Larger spars were secured athwart the upper deck, whose ends projecting beyond the sides, rested on those rafts; up and down spars were also placed on the leeside, their heavier ends resting on the ground, and tackles brought from their heads to that side of the ship. Between the action of these tackles, and that of the rising tide on the raft, the vessel was finally uprighted. (See fig. 27.)

In 1801, the Dutch frigate " Ambuscade," carrying a press of sail with the wind right-aft, imperceptibly shipped so much water through the hawse holes (which in those times were very large and low) as to go down suddenly when near the Great Nore.

* Chap. I.