The shipwright's vade-mecum [by D. Steel].

EXAMPLE. To the constant logarithm .......

0.4893557*

Add the logarithm of 480 (ounces)... 2.6812412

Produces the logarithm of 1481.2 nearly; ... 3.1705969

Again, the model is to be loaded until the surface of the water coincides with the load-water line. Now, the model being weighed, the weight of the ship is to be found by the preceding rule: then, the difference between the weights of the ship when light and loaded is the tonnage required.

A MECHANICAL METHOD OF FINDING THE CENTRE OF GRAVITY IN A SHIP.

THIS method of finding the centre of gravity is exactly similar to the experimental method described above for finding the tonnage, by the construction of a model to a scale of one quarter of an inch to a foot of the corresponding parts on the ship; and care must be taken to provide the wood as light as possible. The same model will, of course, answer both purposes.

The model being accurately constructed, may be suspended by a thin line or silk, in different positions, until it points out the centre of gravity; which will be found when the block hangs in a state of equilibrium: this practice is, doubtless, very simple, but it will be found very convenient.

Many useful discoveries may be made by models or blocks, and with as great certainty as by the nicest calculations; for it must be allowed, that, in calculating from a draught by a quarter scale, it will be liable to some inaccuracies which cannot be obviated in practice, by reason of various little alterations which may be made in laying the ship down in the mould loft; consequently, the draught and the ship will in those points disagree. And likewise, upon strict examination, we shall be enabled to find, that there are very few ships that have both their sides exactly equal in every respect.

In order to prove our block, we may suspend it by a line fastened to a hook in any part of a straight line drawn from the middle line of the stem to that of the post; this hook may be moved forward and aft to different places in the middle line, and a weight may be suspended from the upper part of the middle line on the post; if the two sides be exactly of equal dimensions and homogene, they will then be of equal weight: a plane passing through these three lines, whatever part of the middle line the hook be in, will likewise pass through the middle line of the keel, stem, and post. Therefore, if our model stands this proof, it will be as true to work from as the nicest calculations.

The model being suspended by the hook, the lines hanging at the stem and post corresponding to their middle lines, and to that which suspends the block, we may hold a batten out of winding with the line that suspendis it; and, with a pencil, draw a line upon it; a plane passing through this pencil line, at right angles to the keel, and passing likewise through the line that suspends the block, will likewise pass through the centre of * The constant logarithm is found by subtracting the logarithm of 35840, the number of ounces in a ton, from the logarithm of the cube of 48, or 110592.

gravity, which, therefore, must be somewhere in this plane. Again, move the hook to some other part in the middle line, and let the block be suspended from that point; draw also another pencil line, out of winding with this last line of suspension, and the intersection of the two lines will give the height of the centre of gravity above the keel, and likewise its distance from the post and stem; and, if the hook be moved to any other parts of the middle line, and a pencil line be drawn as before, it will Îikewise intersect in the same point; or, let there be ever so many points assumed in the middle line, and the block suspended by each, and pencil lines drawn, they will all intersect in the same point; and, as the centre of gravity will always be in that plane which passes through the middle line of the keel, stem, and post, it may with certainty be marked on the draught.

This will certainly require the utmost nicety; but, if well executed, will be found correct, provided the dimensions be taken very exactly, and from a true scale of equal parts.