I shall say something concerning it:

The Shipwrights have a Custom of measuring at *London* thus; They
multiply the length of the Keel into the breadth of the Ship at the broadest
place, taken from outside to outside, and the Product of that by the half
Breadth; this 2d Product of the Multiplication they divide by 94, or sometimes
100, and according to that Division, the Quotient thereof is so many Tuns; as
suppose the Length 60 Foot, and being multiplied by 20, the Breadth, produce
1200; and 1200 again multiplied by 10, the half Breadth 12000; if you divide
by 100, you need do no more than cut off the last Figures towards the Right
hand, which shall be the Answer, which rendereth the Ship to be 120 Tuns; but
if you divide the Sum 12000 by 94, you will have 127 2/3 of a Tun very
near: But this cannot be the true Ability of the Ship to carry, because two
Ships by this Rule, of equal Breadth and Length, shall be of equal Burthen,
notwithstanding the fulnes of sharpness of those Vessels, which may differ
them very much, or the one Ship may have more Timber than the other in her
Building, and so carry less: but the true way of Measure, must be by measuring
of the Body and Bulk of the Ship under Water, for if one Ship be longer in the
Floor than another of the same Breadth and Length, she shall be more in
Burthen than the other; as a *Flemish* Ship shall carry more than a
*French* or *Italian* Vessel of the same Length or Breadth;
therefore, I say, the Measure of a Ship is known by measuring her, as a piece
of Timber may be measured of the Form, to the draught of the Water, assign'd
her, the weight of the same Body of the same Water that the Ship swimmeth in,
shall be the exact Weight of the Ship, and all things therein, Loading,
Rigging, Victuals included therein: then if the Ship be measured to her light
Mark, as she will swim at being launched, the Weight of so much Water being
taken or subtracted from the Weight of the Water when she is laden, the
Residue shall be the Weight that must load Ability of carrying, called her
Burthen. By this means you may know the Weight of the Ship light, and what she
will carry to every Foot of Water assigned to her, which can be done by no
general Rules in Arithmetick, because of their great Irregularity, according
to the differing Forms of Ship; you may, if you please, first measure the
Content of the Keel, Post, Stem and Rudder, all of it that is without the
Plank, and under the Water-line, and note it by it self; then measure the Body
of the Ship in the Mid-ships, by multiplying of the depth of the Water-line,
and the breadth; then you may find the Content of the Want by the circular
part of the Ship under Water, being narrowed downward, and subtract this from
the whole Content of the Body found, by the depth of the Water-line and
breadth of the Ship, and this shall be the solid Content of that part of the
Ship, I mean, of solid Foot Measure, of 1728 Inches to the Foot; then proceed
to the fore part or after part of a Ship, and to 3 or 4 Timbers more, find the
mean Breadth at the narrowing aloft at the Waterline, and allow at the Floor
and the mean Depth, and measure that piece of the Ship, as I told you of the
middle part of the Ship, and so measure the whole Ship by pieces, and add them
together; and so many Feet as it maketh, so many Feet of Water shall be the
Weight of the said Ship, and the Reason may be considered thus: There is a
Ponderosity in the Water, but there is a greater in the Air; and there is a
Ponderosity in the Water it self, but not so much as in other things more
solid, as in Iron: Suppose a Gun or an Anchor of Iron it sinketh in the Water,
but yet is not so heavy in the Water as in the Air, by the weight of so much
Water as shall make a Body equal to the Body of a Gun, or an Anchor in
Magnitude; which Weight substracted [*sic*] from the Weight of the Iron
Body weighed in the Air, and so much must be the Weight of it in the Water.

Again, if a Body be lighter in weight than Water of the same bigness, it hath an Ability of lifting the Water, and can lift or carry so much as is that difference: as a piece of Cork or Wood of Fir-Trees. being lighter than Water, it swimmeth off the face of the water, and refuseth to be depressed without more weight added to it.

Thus a Ship being a Concave Body, is made capable of lifting, according to the greatness or littleness of this Concavity, respect being had to the greatness of the Timber put into it, or the Nature of it, all which maketh a Ship swim deeper or lighter in the Water.

I have proved by the *Thames* Water, that fresh Water is lighter than
salt Water; so then salt Water being heavier than fresh, causeth that a Ship
swimmeth deeper in fresh Water than in salt.

I shall not need to say any thing more concerning measuring of Ships, for it will be understood by those that have Judgement in the measuring of solid Bodies, the Matter it self being but a Nicety, rather than useful. I only touched it, to shew those that are curious minded, which way they may accomplish their Desires. I shall forbear to give Examples, because it will much increase this Treatise, and augment the Price, which might prove more prejudical to young Men, than advantagious.

*The Sea-Man's
Vade Mecum*, London, 1707. pp 127-131.

Transcribed by Lars Bruzelius

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