Thirdly, on the form of the after body, as causing a greater or less diminution of pressure forward, on ac count of the motion of the ship alone.
Fourthly, on the shape of the whole body, as afford ing a more or less easy and rapid transit of the displaced fluid to the stern; that is, to the void space, which other wise would be left behind for an instant.
Fifthly, on the form of the whole body, with respect to direction and the quantity of superficies, as causing more or less friction, and more or less adhesion of the fluid." in the construction of a ship, the displacement, Dr. Inman continues, is supposed to be a given quantity. The area of the midship section may be varied to a cer tain extent, and still the same displacement retained. The less this is, the less will be the resistance, since the quantity of fluid displaced in a given time will thereby be diminished; and this section will be the least pos sible (supposing the length given,) when the fore and after bodies are full, and every transverse section equal. But such a Form, on many accounts, and even from the consideration of the resistance alone, could not be adopt ed. The impact of the fore body against the fluid would he too direct, the motion of the after hody from the fluid would also be too direct, and the fluid displaced could not flow easily to the after parts of the ship.
Supposing the length of the ship to be undetermined, in that case. by increasing the length, the midship sec tion might be diminished without limit. The body might at the same time be properly formed for cleaving the fluid, and also transmitting it to the stern. But then (without entering into any other consideration except the resistance,) the friction would be so far increased by the extension of the body, as to retard the ship more than if it were shorter, and the midship section greater. It appears,. therefore, that the midship section cannot be too far diminished either by tilling the fore and after bodies, or by extending the whole or either of these bo dies without an increase of the resistance.
lithe proper area of the midship section be supposed to be determined, it becomes a question in a general view, how the fore and after bodies must be formed so that the ship may meet with the least resistance.
The fore body, Dr. Inman continues, must be formed not only so as to cleave the fluid with the greatest facili ty, but also so as to disperse it to the right and left, and thereby facilitate its transit to the stern; at the same time it must diminish the resistance in one point of view, to form the after body, so that the two streams, which may be conceived to flow on the sides of the ship, may at the stern take as much as possible the same di rection, namely, the one opposite to the direction in which the ship is moving. With these two views, there
the half of the ship before the middle must be fill ed a little more than the after part. Now this may be done two ways; either by carrying the greater trans verse section before the middle, or by filling the whole fore body of the ship, and keeping the greatest section in the middle. But the superiority of the former me thod appears from the consideration, that by this means the proper effect is produced on the fore body, whilst at the same time a finer run may be given to the after body. Whence is seen the propriety of placing the greatest transverse section of a ship before the middle.
Upon the whole, therefore, in constructing a ship from a given displacement for fast sailing, we must give a proper area to the midship section, and also carry that section something before the middle. Care must be taken to shape the fore and after bodies,—the former, so that the fluid may be separated with facility, and at the same time the displaced fluid dispersed, and trans mitted towards the centre ; and the latter, so that the fluid displaced may flow with as great facility as possible to the stern. At the same time, the after body must not be elongated so as to increase the friction.
These general remarks have been made without any reference to the amount of the acting power, that is, the quantity of sail, which, however, it is very important to attend to in a construction. Supposing a ship to be formed with a given displacement, so as with a certain motive power (not producing inclination,) to still the fastest, still it does not follow that its form is the best for moving through the water by means of sails, the power of which is exerted in inclining the ship, as well as in forcing it ahead. If this form wet e modified a little, so that a greater quantity of sail could be carried, without inclining the ship too far upon a wind, the in crease of which sail would store than counterbalance the addition thereby caused in the resistance, the ship would be improved by this alteration In its quality of sailing. This consideration, therefore, must be added to those already adverted to, in constructing a ship for fast sailing.