site end of the lever to that in which the moments of inertia were greatest.

And if the weights are comparatively centralized, this case would be aggravated. For, as the power in the rudder is small, (small comparatively at first, and smaller because of the ship's having lost her way) when the after sails have ceased to act in that way, her turning will depend upon the momentum of the extremities and sides, which would not be great if the weights were centralized.

This may be the Pique's case, and the remedy would be to rake her masts or shift them further aft, bring her more by the stern by a small weight very far aft, and reduce the angle of pitch and scend by balancing the moments of inertia forward and aft.

If a ship have great proportionate length and her weights centralized very much, and if her centre of gravity of displacement is considerably before the middle of the length, she being on an even keel, though the moments of inertia forward be not in excess yet she will stay badly, for the power of the rudder in this case will be less (supposing the rudder to be of equal width each trim) from its being less immersed, than in the former case, and the resistance offered by the longer after-body will be greater,-consequently, though she may come to the wind quick, because of a small moment of inertia in her sides and extremities, yet from want of that inertia afterwards to overcome the great resistance of her long after-body, she will not continue her motion in turning, and she will, soon after coming head to wind, gather stern-way, even under favourable circumstances.

This is illustrated in many ships when they are light; the bread-room being in the after extremity and being empty, the moment of inertia of the after-body is much

reduced, while the resistance of the water is very little reduced. Should she be less by the stern, the action of the rudder will be reduced, the centre of gravity will be carried forward, and the length of the after-body increased, and the case would be worse.

With an exception, all the ships in our service would sail better and work better, if brought more by the stern than they are; but it must be done by shifting small weights very great distances aft, and not by greater weights less distances.

The effect of weights being as their distance when at rest, and as the square of their distance when in motion, the stowage of ships should be regulated with reference to this law,—that is, that every thing put on board should be referred to the centre of gravity of displacement, so that the moments of inertia, both before and abaft, should be equal, or only such irregularity as experiment should determine to be expedient. Thus, for instance, having stowed a ship with the moments of inertia as nearly equal as it could be practically arrived at, if it should be found that she pitches, a small weight should bemoved a great distance aft, and (to preserve the line of floatation) a large weight a small distance (a distance that shall make the moments of the weights equal) forward; should she still pitch, but in a less degree, it may be shifted further aft, or a greater weight the same distance aft, balancing its moment as before,-while, if she is found to scend on the first occasion (which is more likely) the operation must be simply the reverse.

The weights generally, in men-of-war, cannot be too much centralized if the above rule be attended to.

The provision and water should be so stowed that their consumption will effect an equal reduction of the

moments and of the moments of inertia before and abaft the centre of gravity.*

In the construction of the ship the weights of the fore extremity might be vastly reduced in weight, by shortening the upper decks,† by reducing the scantling of the beams where they are shorter than the beams generally; the timbering of the bow, as it would be comparatively straight, might be lessened, and the area of the bow might be lessened with advantage, and with it the shocks which the bow would receive; so also might the strength of the bow be reduced. It is even possible that the effective strength would be increased, for nothing is so destructive to a ship's general durability (where she has to be driven by a great power against a head sea) as a full bow.

Also I have no doubt but that it will be found that such enormous bowsprits as are now in use may be dişused with considerable advantage, for, by their action now in making a ship pitch they carry the resultant of the water further forward than the resultant of the sail is carried by their extension forward.

* For this reason the stowage of the bread further forward, as in the Trafalgar, would be better, were its removal forward compensated for by ballast in the bread-room.

+ The upper decks of the Leander, by Mr. Blake, and the Termagant by Mr. White, are advantageously shortened, yet not nearly enough.

LECTURE IV.

The importance of science when designing steam vessels. In no department of marine architecture is the aid of science more required than in the designing of steam vessels for war; certainly in no other branch is the departure from scientific principles attended with such serious consequences to the interest of the country.

For great speed must be attained at any cost; and if the form of the vessel be bad, it can be attained only by great power, great first cost, and an increase of cost when in work, therefore every fault in form is magnified. Velocity varies as the of the power, so if the speed is to be doubled the power must be increased (all other things being equal) 8 times; then, as this description of motive power is of immense weight, (so great that a difference in the description of boilers alone may make a difference of weight equal nearly to the weight of the whole motive power of a sailing vessel equally large,*) so, if this power be not increased or applied with judgment, it will immerse the vessel too deep, and prevent, in many ways, that which it was intended to produce, speed!+

The principles which should guide the same for steam as for sailing vessels.—The principles which have been' already stated as those which should guide in the construction of sailing vessels are equally applicable to the

*Greater power to some vessels would only serve to make a larger hole in the water.

†The difference between the weight of the Terrible's boilers and those of the Retribution is said to be 80 tons, about the weight of the masts of a large frigate.

construction of steam vessels; the difference consisting only in the difference of degree in which it seems expedient the several properties should be given; so entirely is this the case that we may reasonably expect that the improvement, demanded by economy, in the form of steam vessels, will lead to a material change for the better in the form of our sailing vessels also.

The points of difference will be better seen by a statement of the required properties.

The requisite properties in a steam vessel of war.The requisites in a steam vessel of war are a long ranging and proportionably heavy armament, sufficient tability to insure the ease and power to use this armament to advantage, great speed, coals equal to the consumption of a long distance,* together with sufficient provisions and water for the crew: and, if a large vessel, accommodation for troops with their baggage: and these qualities should be obtained at a moderate draught of water. The property of sailing fast with comparatively little sail will be a consequence of attaining the above properties.

How these properties may be obtained.— To effect all these, the proportionate length to breadth must be great, (much greater than in a sailing vessel), the mean breadth must be great, as compared with the extreme breadth, the difference between the draught of water when without coals in, and when with coals in, should be small; the form of bow should be that which would offer the least resistance, and the after-body that which would occasion least minus pressure.

Reasons why length may be greater in a steam vessel.— The proportionate length to breadth may be greater in

* A vessel may stow "a great many days' coals," and yet not be able to effect a long distance.