ON CALCULATING TILE TONNAGE OF SHIPS.
The method of constructing a scale of solidity, na turally leads to the consideration of the mode by which the tonnage of a ship should be calculated; an inter esting disquisition on which, we add from the pen df Mr. Morgan.
The propriety of having some scale by which the magnitude or capacity of ships may be compared with one another under one point of view, is universally ztc ktrowledged. Different causes, according to the ser vices to which ships are applied, conduce to the pro priety of a true method of comparison. If the num ber of guns a ship ot' war carries_be correctly stated, and subject to no variation, and the number of guns should bear a constant relation to the magnitude of the body, the designation of a ship according to the number of guns might be sufficient for general pur poses; as ships of war require chiefly to be compared with respect to their force.
The tonnage is generally used as a measure, by which ships built by contract are paid fur. In order that this method of estimating the value of a ship may be correct, the relative alteration of any dimension should alter the tonnage in the sante proportion, as it would effect the expense of building the ship. By the present method of measuring the tonnage, the in crease of the breadth of a ship increases the tonnage in a much greater proportion than the expense of building. Though other rules may be given for cal culating the tonnage of ships, which may be less in correct as a scale of payment of building than the one at present in use, yet, as no correct analogy can he established between the tonnage and the expense of' building, it appears desirable that some other scale of payment should be adopted, founded on more cor rect principles. If any part of the displacement of a ship be taken as a measure of the expense of building, it appears more reasonable that the weight of the hull, which is determined by the light displacement, should be taken, than the part of the displacement which is brought into the water by the lading. A
better scale of the expense of building ships even than this may probably be determined by attention to this particular object, there being certainly no ne cessity that the same scale of measurement should he used for the lading and expense of The disadvantage of paying for ships in proportion to their tonnage, is, Wowever, no doubt, in a great de gree corrected by a full examination of the design, previously to the settlement of the contract. In mer chant ships, however this mode of payment is in nu merous instances found to be injurious, by being the means of too little breadth being given to them in or der to reduce the tonnage, and thereby lessen the ex pense of building.
A correct method of calculating the tonnage, al though desirable for all ships for the sake of uniformi ty, is particularly necessary for merchant ships, which should always be compared by the true quan tity of lading they can carry, in correct proportion to which their dues should be paid. The tonnage should be the correct measure of the number of tons of the lading of a ship. This is the weight that will bring a ship clown in the water from the light water line, at which it swims when properly equipped with every thing on board, except the lading, to the load water line, at which it swims when laden.
This may be correctly found by determining the so lid content of the body between the light and load wa terlines, the weight of which, considered as sea water, is the true lading; this solid divided by 35, the num ber of cubic feet of sea water in a ton, gives the true tonnage or weight of the lading.
The rule commonly used in England does not•even approximate, on correct principles, to the true ton nage; the elements of the calculation being erroneous ly taken, the half breadth of the ship being substitut ed for the mean depth from the load to the light draught of water, and the divisor 94 substituted for 33, as a correction, though a very inadequate one, to this error.