of a certain quantity of water, determined as explained. But to measure the specific gravity of a body, we always take, for our unit of weight, the weight of a quantity of water of the same volume with one unit of the volume of the body, whatever that unit may be. Thus, if the volume is measured in cubic inches, the unit of weight used in fixing its specific gravity is the weight of one cubical inch of water.". . . ." The specific gravity of the body is, in point of fact, no other than the number of cubical inches of water equal in weight to one of its cubical inches. So if the body be measured in cubical feet its specific gravity is the number of cubical feet of water whose weight shall equal one of its cubical feet. Thus, in the table of specific gravity the number 8.900 stated as the specific gravity of copper, means that each cubical inch, or cubical foot of copper weighs the same with 8.900 cubical inches or cubical feet of water. Thus, knowing the number of cubical feet in a body, and knowing its specific gravity, we can tell how much water it is equal in weight to, by multiplying their specific gravity by the number of cubical feet this specific gravity being in fact, the number of cubical feet of water equal in weight to each cubical foot." *

Rule. To determine the magnitude of any body from its weight:

As the specific gravity to its weight in ounces 1 cubic foot : its contents in cubic feet or inches.

To determine the weight from the magnitude:

As 1 cubic foot, or 1728 cubic inches: contents its specific gravity weight.

*Moseley's Mechanics.

CHAP. VI.

STOWAGE.

THE Stowage of a ship has reference to stability, speed, easiness of motion, economy of space, and convenience of access, and is therefore a subject of the greatest importance. In the distribution of the disposable weights, the first thing to be considered is the Ballast. Its quantity, which is proportionate to the weight of guns, masts, and other top weights, is determined by the builder when designing the ship, whose form and size are so arranged that she will carry the proposed quantity without being too much immersed.

The purpose of ballast is to increase that quality in a ship called Stability, which enables her as much as possible to resist inclination, and to recover her upright position. And as it seems impossible to accomplish this without the existence of some weight which will counterbalance the disturbing forces of wind and water and the top hamper of the armament, the supply of ballast is, in the first instance, stowed on the bottom of the ship. For, the weights above water are generally fixtures whereas those below, such as water, provisions, fuel, &c., are necessarily diminished by consumption. If, therefore, there were no ballast, and the weights below water-level were removed or reduced, the equilibrium would be dangerously affected, and the ship could neither fight her guns, nor carry a sufficiency of sail to be weatherly or speedy. It is true that a partial remedy would be found in filling the tanks with salt water, but it must be well remembered that mere immersion does not give stability. With her crew sitting on the thwarts, or standing up, or seated on the bottom, a sailing boat will have the same immersion, but very different degrees of stability.

The Weight of ballast in modern times is much less than formerly, on the idea that the increased solidity of the several appliances required in a ship added much to her stability. Whether this were a correct supposition or not, other circumstances defeated the supposed advantage. Thus, Mr. Fincham observes:-"In the year 1783, the larger class of three-decked

ships had 340 tons of shingle, and 140 tons of iron ballast. In 1812, they had 60 tons of shingle and 340 tons of iron ballast; and when, some years later, iron tanks were supplied for the ground tier, the use of shingle was discontinued. The extent of change that has taken place may be illustrated by the Caledonia,' of 120 guns. In 1814 she had 340 tons of iron ballast and 60 tons of shingle; and on being fitted out in 1835 she had no shingle, and only 177 tons of iron ballast; and at the present time, the ballast of three-decked ships varies from 100 to 230 tons."

"Fixed rules have been given at different times to determine what quantity of ballast should be used, and at present it is much less than it was formerly, as the greater dimensions which are now given to ships have increased their stability, whilst the solidity of the timbers of the frame, chain cables, iron tanks, and other heavy weights, which are now put on board ships, increasing the weight below, and lowering the centre of gravity, have still further added to the stability. The advantages in point of stability which were thus gained, have been partially lost by the recent increase in the weight of the armament which is now borne by ships in the British navy, an increase which is not wholly balanced by the greater quantity of shot and shell that is stowed in the hold."

Thus, the experiences of former times appear to be in favour of ships carrying large proportions of ballast. The days of St. Vincent supply instances of lengthened cruises made by ships in company in large numbers. The fleet of Nelson chased across the Atlantic twice, each ship holding her place.

Whether these remarkable facts were the consequences of powerful stability resulting from ballast, or merely those of a very high order of seamanship, they are deserving of deep consideration.

As, in general, stability will be increased by increasing the depth of the centre of gravity below the centre of buoyancy or displacement, it may be well to define these terms.

The Centre of Gravity of any body is usually defined as that point upon which the body would balance itself in any position, when acted upon solely by the force of gravity. Its tendency is always to assume the lowest position that it can have com

*Fincham's Outlines.

patibly with the conditions in which the body is placed; consequently, when it is in such lowest position, the body will be in stable equilibrium.

The same definition is applicable to the Centre of gravity of any system or combination of bodies. Suppose a rod to have two equal weights, as at A and B. The point c, upon which it would balance, is the common centre of gravity of those weights. But if any increase or diminution of weight were to take place on either side of c, the balance would be destroyed, and c would no longer be the centre of gravity; and to restore the balance, the prop must be moved towards that side where there was a preponderance of weight; from which it appears that the relation

Fig. 32.

C

B

between the centre of gravity and the weights on either side of it, is the same as that between the fulcrum of a lever, or the point of suspension of a balance and the weights which keep it in equilibrium.

In scientific investigations, the centre of gravity of any body or system of bodies, is used to represent the body or system itself, all the mass and weight being considered as concentrated at that point, and all measurements which concern their weight or its effects being referred to that point. Thus, reverting to the example above, the pressure weighing down upon the prop at c is equal to the sum of the two weights A and B, and, in any calculation involving the joint effect of those weights, that pressure and the point c at which it acts would be substituted for the action of the individual weights.

A ship with all her weights in is, in fact, a system of bodies connected together, and composed of the hull, stores, &c., each article having its centre of gravity, and the whole combined having one common centre of gravity, upon which, when floating at rest, the ship may be said to balance herself.

But the water exerts a pressure upwards equal to the whole

weight of the ship. Hence, in all questions affecting the flotation or the motions of a ship, we have to consider the effects of two antagonist forces, the weight of the ship and the pressure of the water, each of which must be treated as acting through its own centre of gravity; that of the former being called the centre of gravity of system, that of the latter, the centre of gravity of displacement, or immersion, or buoyancy.

A ship floating in the water, displaces a quantity of water equal in weight to the whole ship and her contents, and equal in bulk to that portion of her which is immersed. The result is that the water, in supporting the ship, presses upon all the part which is immersed with an upward pressure equal to her weight. As the surface of the part pressed upon is of an irregular shape, and as all fluids exert their pressure at right angles to the surface upon which they press, it follows that, in sustaining the ship, the water presses in an infinite variety of directions, the whole of which combined, may be represented by a vertical line passing through the centre of gravity of a body of uniform density corresponding in shape and dimensions to the immersed part of the hull of the ship. This line is called the Resultant of the pressure of the water, and the centre of gravity through which it passes, the centre of gravity of displacement.

When the ship is upright and floating at rest, these two centres are in the same vertical line, the ship herself, by the action of her own weight, adapting her trim so as to bring them into that position. But in a seaway, this state of things is disturbed. The accumulation of water in the form of a wave, changes the direction of the different pressures on the hull, and with them the resultant of the pressure; and the motion of the ship which follows, is simply a series of efforts on her part to recover her state of equilibrium or balance. For instance, a sea pressing on the bow draws the resultant of the pressure further forward than the centre of gravity of system. That point being no longer adequately supported, sinks until a sufficient pressure on her after body has been regained to bring the resultant back into its old position. As the body of the sea, carrying its resultant with it, passes under the centre of gravity of system, the ship is balanced for a moment at her ordinary trim; and as it passes aft the balance is again disturbed, and the bow sinks