TONNAGE.

Tonnage has long been an official term, intended originally to express the burthen that a ship would carry, in order that the various dues and customs which are levied upon shipping might be levied according to the size of the vessel, or rather in proportion to her capability of carrying a burthen. It has hence arisen that the term: “tonnage,” as applied to a ship, has become almost synonymous with that of size. It is indeed the only term that is used to give an idea of the magnitude of merchant-ships, which are invariably spoken of according to their tonnage, or as being ships of 50, of 500, or of 1000 tons. Not only are all dues and customs levied according to the tonnage, but ships are also built, and bought, and sold, for a price per ton of their admeasurement; and by the conditions of Lloyd’s classification-list of shipping, they must be timbered and fastened, and must have their anchors, cables, and boats, all in proportion to the same datum. Tonnage, therefore, in so far as these considerations are involved, is virtually assumed to be a correct representation of the size of a ship.

In order to levy the dues on shipping, it was necessary to establish some one method to be in general use for calculating this tonnage; and consequently there has long been a rule, enforced by law, for the measurement of the tonnage of ships or vessels, and much mischief has arisen to the mercantile shipping of Great Britain through the erroneous principles which have been adopted at various times in forming these rules.

It is evident that tonnage may express several results of measurements, and in each case may be a sufficiently fair criterion of the burthen of a merchant-vessel, by which to levy the dues; because all that is required thus far, either on the part of the government or of private interests, or on that of the owner, is, that the dues should be levied in equal proportions on all vessels. Thus tonnage may be the product of a series of measurements, intended to express either the exact size, or an approximation to the exact size, of the ship. It may be the actual displacement or weight of the ship, either with or without cargo; it may be the displacement of the cargo, or the dead-weight which she will carry to a fixed draught of water; or it may be the capacity of the space which she has for the stowage of cargo. Yet to each of these there are most important practical objections. the first, or that by which the measured bulk of the ship would be the tonnage, might be easily evaded, and would lead to injurious results; because, as there must necessarily be fixed measuring places, such a law would have the effect of restricting the form of vessels to that one shape which would carry the largest cargo under the least possible measurements at these places. If the actual displacement, either load or light, of a vessel, were to be her tonnage, it would be subject to the inconvenience that it could only be calculated correctly from the drawing of the ship, and therefore would be inapplicable to foreign ships arriving in our ports. An exact account of the draught of water, either load or light, would also be necessary; which, as being a variable quantity when the load draught is involved, and one of great difficulty to be determined when the light draught is required, would present almost insuperable objections; and, besides, the draught of waiter is a quantity easily concealed or falsified for fraudulent purposes. Above all objections, however, may be reckoned this, that if either the load or the light displacement were taken to represent the tonnage, it would be the interest of the ship-owner to build large vessels with slight scantling and inadequate fastening, that the weight of the vessel might be small in proportion to that of the cargo. If. the tonnage were to express the difference between the two displacements, that is, the excess of the load displacement over the light, or the dead-weight of the lading, although this would be by far the most correct in principle,—in fact, although it be even mathematically correct,—there is the objection against it, that it involves a knowledge of both light and load draughts of water, and is therefore on this account, practically at least, as objectionable as the whole displacement. Lastly, if the tonnage were to be the space or capacity for the stowage of cargo, numerous openings would be afforded for fraudulent evasion; because the manner of measuring the ship for this space must be defined, and it is easy to build space which will not come within the limits of the defined measurements; or it is easy to build a vessel of such a form that the measurements made at the given measuring places shall not give a correct account of her capacity.

Until January 1836, the rule for computing the tonnage of ships was as follows. the length was taken on a straight line along the rabbet of the keel of the ship, from the back of the main stern-post to a perpendicular line from the fore part of the main stem under the bowsprit. The breadth was taken from the outside of the outside plank in the broadest part of the ship, either above or below the main wales, exclusively of all manner of doubling planks that might be wrought upon the sides of the ship. If the ship to be measured was afloat, a plumb-line was dropped over the stern, and the distance between such line and the after part of the stern-post, at the load water-mark, was measured; then was taken the length from the top of this plumb-line, in a direction parallel with the water, to a perpendicular immediately over the load water-mark, at the fore part of the main stem. Subtracting from this length the before-mentioned distance between the plumb-line and the after part of the stern-post, the remainder was reckoned to be the ship’s extreme length, from which three inches were deducted for every foot of the load draught of water. With the dimensions thus obtained, the rule then was: “From the length taken in either of the ways above mentioned, subtract three fifths of the breadth taken as above; the remainder is esteemed the just length of the keel to find the tonnage; then multiply this length by the breadth, and that product by half the breadth, and dividing by 94, the quotient is deemed the true contents of the tonnage.”

This rule is still in force for ships that were registered previously to the passing of the new act. It is evident that the tonnage as determined by it was intended to express the size or bulk of the ship, the half breadth being an assumed equivalent for a mean depth. The evils which arose out of this assumption were very great. As the depth was not at ail involved, it might be increased to any extent without increasing the tonnage; while, on the contrary, as the square of the breadth was involved, an undue preponderance was given to this dimension, and it became necessary, on the part of ship-owners, to restrict it within the least possible limits. The effect of such a law was obvious. The British merchant-ships, in order to profit by its inconsistencies, were built exceedingly narrow and deep in proportion to the length, so that, according to parliamentary returns, we find, on an average, the mercantile navy would carry 1 / 3 d more weight than its legally registered tonnage. In fact, the ships became little more than oblong boxes, most dangerous as sea-boats, and, from their want of stability, not capable of carrying sufficient sail to insure their safety on lee shores. Hence, after every gale of wind, the leeward coasts were covered with their wrecks; and hence Lloyd’s books registered annually the average loss of six ships in four days.
[7:21:327]

Out of this uncertainty of navigation arose a most heartless system of gambling in maritime-insurance, the temptations of which were as destructive to the mercantile honour and the morality of the nation, as the inefficiency of the ships was destructive to its seamen. Luckily, this cause of reproach has ceased since the enactment for calculating the tonnage of vessels, which has been in force from January 1836; and numbers of ships are now built that very forcibly prove how injuriously the old law must have operated in checking maritime and commercial adventure. Not that we mean to say the present law is by any means perfect, or that the rule for calculating the tonnage cannot be evaded, because this is not the fact; it can be and is evaded, but the evasions are not so destructive to the good qualities of ships as those which were commonly practised during the continuance of the old law. It is doubtful whether, in a nation whose existence depends upon her maritime superiority, ships should not be exempted from all dues. These pages are not the place to inquire how it might be possible to accomplish so desirable an exemption; or how compensating revenues could by any means be awarded to the harbours, the docks, or the lights, which are now maintained by a taxation levied upon the tonnage of shipping. It does not, however, appear difficult to conceive that the same revenue might be drawn from the freights which the ships carry, as the merchant would receive it back from the ship-owner in the shape of a diminished freightage.

It is exceedingly difficult, probably even impossible, to frame a rule for computing the tonnage which shall be at once of practical application, and yet not have in some degree the effect of restricting improvement in the qualities of merchant-ships. It is difficult to induce a man to forego a constant and positive gain for one that is only prospective and uncertain. We have seen that the obstacles which oppose themselves to correctly and satisfactorily determining either the light or the load draughts of water, are sufficient to prevent the difference between the light and load displacements from being taken to represent the tonnage. This is, however, the only correct measure of a ship’s power to carry cargo; the difference being, of course, exactly equal in weight to the cargo which either has or may cause it. All other quantities which can be taken as measures of that power are little more than assumptions, and whether they represent the external dimensions of a ship, or her internal capacity, they scarcely give an approximation even to the power which she may possess of carrying burthen; while, in either of the above cases, the fact that these quantities must be determined by measurements at fixed measuring places, affords opportunity for evasion, and indeed invites it. For if, by any arrangement of the dimensions, or by any peculiarity of the shape, a ship can be enabled to carry a greater burthen than her registered tonnage, the freight of that greater burthen is a premium which is offered to that one proportion between her dimensions, or that one peculiar form for her body, and a restriction is, to a certain extent, placed upon improvement; because the ship-owner will content himself with the best ships that he can obtain, possessing the advantages of those dimensions or of that form.

The present rule for computing the tonnage assumes it to be the space for stowage, and the internal capacity of the vessel is calculated in order to determine it. As there are necessarily fixed measuring places, the rule may, as we have said, be evaded by a certain build. Its phraseology may also be most easily evaded by building accommodations on deck, which will not come within the meaning of the terms that are used in it—“poop,” “half-deck,” or “break in the deck.” Under its operation vessels may also be advantageously built of very small register tonnage to carry cargoes of heavy goods; for which purpose they should be of the lightest materials, but with very large scantlings, that the internal capacity may bear but a small proportion to the load dis placement of the vessel. We have already however stated, that in its effects the present rule is far less injurious to the mercantile navy of England, than that which it is intended eventually wholly to supersede.

Rule for calculating the Tonnage of Ships or Vessels, as prescribed by Act of Parliament.

The tonnage of every ship or vessel required by law to be registered shall, previously to her being registered, be measured and ascertained while her hold is clear, and according to the following rule: (that is to say), divide the length of the upper deck between the after part of the stem and the fore part of the stern-post into six equal parts. Depths—at the foremost, the middle, and the aftermost of those points of division, measure in feet and decimal parts of a foot the depths from the under side of the upper deck to the ceiling at the limber strake. In the case of a break in the upper deck, the depths are to be measured from a line stretched in a continuation of the deck. Breadths—divide each of those three depths into five equal parts, and measure the inside breadths at the following points: videlicet, at one fifth and at four fifths from the upper deck of the foremost and aftermost depths, and at two fifths and four fifths from the upper deck of the midship depth. Length—at half the midship depth measure the length of the vessel from the after part of the stem to the fore part of the stern-post; then to twice the midship depth add the foremost and the aftermost depths for the sum of the depths; add together the upper and lower breadths at the foremost division, three times the upper breadth, and the lower breadth at the midship division, and the upper and twice the lower breadth at the after division, for the sum of the breadths; then multiply the sum of the depths by the sum of the breadths, and this product by the length, and divide the final product by three thousand five hundred, which will give the number of tons for register. If the vessel have a poop or half deck, or a break in the upper deck, measure the inside mean length, breadth, and height, of such part thereof as may be included within the bulkhead; multiply these three measurements together, and dividing the product by 92∙4, the quotient will be the number of tons to be added to the result as above found. In order to ascertain the tonnage of open vessels, the depths are to be measured from the upper edge of the upper strake.

Mode of ascertaining the Tonnage of Steam-Vessels.

In each of the several rules herein before prescribed, when applied for the purpose of ascertaining the tonnage of any ship or vessel propelled by steam, the tonnage due to the cubical contents of the engine-room shall be deducted from the total tonnage of the vessel, as determined by either of the rules aforesaid, and the remainder shall be deemed the true register tonnage of the said ship or vessel. The tonnage due to the cubical contents of the engine-room shall be determined in the following manner: that is to say, measure the inside length of the engine-room in feet and decimal parts of a foot, from the foremost to the aftermost bulkhead; then multiply the said length by the depth of the ship or vessel at the midship division, as aforesaid, and the product by the inside breadth at the same division, at two fifths of the depth from the deck, taken as aforesaid, and divide the last product by 92∙4, and the quotient shall be deemed the tonnage due to the cubical contents of the engine-room.

For ascertaining the Tonnage of Vessels when laden.

And be it further enacted, that for the purpose of ascertaining the tonnage of all such ships, whether belonging to the United Kingdom or otherwise, as there shall be occasion [7:21:328]to measure wnile their cargoes are on board, the following rule shall be observed and is hereby established: that is to say, measure, first, the length on the upper deck, between the after part of the stem and the fore part of the stern-post; secondly, the inside breadth on the under side of the upper deck, at the middle point of the length; and, thirdly, the depth from the under side of the upper deck, down the pump-well, to the skin; multiply these three dimensions together, and divide the product by 130, and the quotient will be the amount of the register tonnage of such ships.

To form a Scale of Tonnage, or a Seale of Displacements.

In order to form what is sometimes called a scale of tonnage, and sometimes a scale of displacement, on the plan of a ship’s body draw a horizontal line to represent her load water-section, and beneath this line draw other horizontal lines equidistant from each other, representing equidistant horizontal sections; then calculate the solid contents of the part of the body intercepted between each two successive sections. The sum of the contents of all the solids will be the load displacement of the ship, and the sum of the contents of all the solids below any one of the horizontal sections will be the displacement of the ship up to that horizontal section. Find in this manner the load displacement, and also the displacement up to every horizontal section; then draw a vertical line AB, and on A B, graduated to some scale of feet, set off the distance AB equal to the mean load draught of water of the ship, and on the same line AB let the points C, D, E, be taken respectively at the heights of the other equidistant horizontal sections; then from A, C, D, E, draw A a , Cc, Dd, Ee, so that E e , D d , Cc, and A a , shall respectively be in the same ratio to each other in length as are the solid contents, or the displacements of the parts of the body below the respective horizontal sections, to each other, in cubical contents. Thus, if the solid content of the whole displacement be double the solid content of the displacement up to the height C, then draw A a double the length of C c ; or if it be only one third more, then let A a be one third longer than C c ; and on the same principle with the other lines. Then trace a curve through all the points a , c, d, e, which curve will be the curve of displacements. Then, if A a be graduated by projecting all the points c, d, e, &c. on to it, the line A a will become a scale of displacement, or of tonnage.

The use of the scale is obvious, as, when the ship from which it was formed is at any draught of water within the limits of the height BA, her displacement at that draught may be immediately known by squaring a line out from its height, set off from B on the scale BA, to intersect the curve B edca, and then squaring up this point of intersection to the scale of tonnage An, which will be intersected at the number of tons in the displacement required. The effect to be produced in the draught of water by lightening or loading the ship to any extent embraced within the scale may also be easily ascertained; and, in the same manner, the quantity of lading which a ship will require to be put into or taken out of her to bring her to any certain draught of water, may be immediately known. In fact, so useful are these scales of tonnage that they ought to be calculated for every merchant-ship, and a scale drawn and placed in the hands of the commander. It would enable him to answer at once innumerable questions which are completely beyond his knowledge without it. Mr Parsons, a member of the late School of Naval Architecture, proposed to the committee from which the law for tonnage that is at present in force has ema nated, that, for all new vessels built or launched after a certain date, such scales of tonnage should be formed, which, when properly authenticated, should be attached to the register of the vessel, in order that from these scales the true tonnage or actual weight of the cargo might be ascertained, and either the whole or any portion of it which the committee might have determined, be taken as the register tonnage. There would have been many difficulties opposed to the practical application of this plan, although we can by no means consider them as insurmountable. Among these difficulties would have been the question as to what is or what ought to be considered as the light draught of water; and also the circumstance that many vessels are built from eye alone, of which, therefore, there would have been no drawings to form the scales of tonnage. Mr Parsons has traced the curves of tonnage for many classes of merchant-vessels, and for the several rates of men-of-war, and has published them in a work called Scales of Displacement. A somewhat similar work, as respects the men-of-war, and under the same title, has also been published by Mr Edye, the assistant surveyor of her majesty’s navy.

A very useful body of information on merchant-vessels resulted from the labours of the committee on tonnage, that has been already so frequently mentioned in this article. It is a valuable and extensive collection of particulars as to the form, proportions, dimensions, and tonnage of the mercantile navy of England. The measurements were all made, and the information was collected and arranged, by Mr Cradock, a member of the late School of Naval Architecture, who was, by the permission of the Lords Commissioners of the Admiralty, employed under the directions of the committee. In order that such a body of information may be more generally useful than it can possibly be while confined to the pages of a parliamentary Report, we shall enrich this article with it. The following accounts of the various methods of computing the tonnage, which are in use among the maritime nations of Europe, were obtained by her majesty’s government officially from the several powers, and are included in the report of the committee.

Methods at present in and among Foreign Nations for computing the Tonnage of Ships. France.— The three measures of length, breadth, and depth, arc multiplied together, and the product is divided by 94 for the tonnage.

In single-decked vessels the length is taken from the after part of the stem on deck to the stern-post; the extreme breadth is taken, being measured inside from ceiling to ceiling, and the depth from the ceiling to the under surface of the deck.

In vessels of tico decks, at Bordeaux, the length of the upper deck and that of the keelson is measured, and the mean taken for the length. But at Brest, Marseille, and Boulogne, the mean of the length on the two decks from the stem to the stern-post is taken as the length. The depth of the hold from the ceiling to the under surface of the lower deck is added to that of the height between decks, and considered as the depth. The extreme inside breadth is taken in the same way as in single-decked vessels.

At Bordeaux an allowance is sometimes made for the rake of the stem and stern of the vessel.

At Boulogne, in measuring steam-boats, the length of the coal and engine chambers is deducted from the length of the vessel, and her breadth is taken at the fore and aft extremities of the same, the mean of which is considered as the breadth. The depth is taken inside the pumps, from the lower surface of the deck, between the timbers.

At Brest, measures are frequently taken with a string, although contrary to law, and an error of seven tons in the tonnage of a cutter has been the result.
[7:21:329]

S pain. —Three breadths are measured at the following places: 1st, at the mizen-mast; 2d, a few feet abaft the foremast; 3d, at a point half way between the two former. The heights at which the three breadths are taken at the above places are, 1st, on a level with the deck; 2d, on a level with the upper surface of the keelson; 3d, at a level half way between the two former positions.

To find the area at each section, the half of the sum of the upper and lower measurements is added to the middle measurement, and this sura is multiplied by the height of one above the other. Then half the areas of the fore and after section is added to that of the middle section, and this sum is multiplied by the length which the sections are apart from each other. The result will express in Burgos cubic feet the capacity of the part of the hold between the fore and after sections, and it still remains to add the spaces between these and the stem and stern·post. The former may be found, without any considerable error, by multiplying the area of the foremost section by half its distance from the stem; and the latter in the same manner, by multiplying the area of the after section by half its distance from the stern-post. It is evident that the room occupied by the pumps must be deducted from the foregoing result, in order to obtain the fair quantity of space filled by the cargo.

Having thus found the capacity of the hold of any vessel in the above manner in Burgos cubic feet, it is to be divided by 41 61 / 779 and the result will be the amount of displacement of such vessel in tons of Burgos measure, because each ton is reckoned equal to 41 61 / 779 feet of Burgos.

Portugal.— For single-decked vessels, the length is measured from the cabin bulkheads to the forecastle bulkheads. The depth is measured from the upper surface of the keelson to the under surface of the beams. The extreme breadth of the deck is considered the breadth. the continued product of these three dimensions will give the contents in cubic feet, which, divided by 57 726 / 1000 , gives the tonnage.

Vessels of two decks. In these vessels two distinct operations are made; one for the hold, the other for the middle deck. For the hold :—The length is measured from the heel of the bowsprit to the stern-post. The breadth is the extreme breadth of the upper deck, deducting two feet. The depth is from the upper surface of the keelson to the under surface of the beams. For the middle deck :—The length is considered as half of that for the hold, the other half being allowed for cabins. &c. The breadth as before; and for the depth, the height of the middle deck to the under surface of the beams.

The foregoing is the method adopted at Lisbon, but at Oporto the length of the vessel is taken from the second timber at the bows to the stern-post; the breadth, at the widest part from the inside of each bulwark on the upper deck; and the depth, from the upper surface of the keelson to the lower surface of the beams of the upper deck at the main hatchway.

If the keelson be more than ordinarily thick, allowance is made accordingly; and where there arc two decks, the thickness of the lower deck is also deducted from the depth. The length is then multiplied by the breadth, and the product by the depth. This product is then divided by 96, the number of Portuguese cubic feet contained in a ton, and the result is the tonnage of the vessel.

Naples.— For vessels with two decks, the length is measured from one end of the vessel to the other, over all. The length is also measured from the after part of the stem to the rudder hatch, under the poop. the mean between these two lengths is multiplied by the extreme breadth of the vessel.

The depth is then taken from the bottom of the well to the lower surface of the upper or poop deck; and the above product being multiplied by this depth, and divided by 94, gives the tonnage.

For single-decked vessels, the tonnage is found by multiplying the extreme length by the extreme breadth, and the product by the extreme depth, and dividing by 94, as above.

Netherlands —The length is measured on deck from the stem to the stern-post. For the breadth, the hold is divided into four portions, and two measurements taken at each of the three divisions: 1. Across the keelson, on a level with its upper surface, from ceiling to ceiling: 2. The greatest breadth of the hold at each division. The mean of these six measurements is considered the breadth.

The depths are taken at each of the foregoing points of division, from the upper surface of the keelson to the lower surface of the upper deck between the beams, and the mean of these three is assumed.

The length, breadth, and depth are then multiplied together, and two thirds of the product is considered as the tonnage. Rut an allowance for provisions and water, cabins, and ship’s stores, varying from 30 / 100 to 45 / 100 , is deducted from the depth before it is multiplied by the length and breadth.

Norway.— From the after part of the stem, the length of the ship is taken to the inner port of the stern-post. Dividing the length of the vessel into four equal parts, the breadth is measured at each of these divisions. The depth of the vessel, from the under surface of the upper deck to the keelson, is taken at the above three points of division.

Then multiply the length by the mean of the three breadths, and this product by the mean of the three depths.

The result of the foregoing is divided by 242½, if there be no fractional parts of feet; but if there are, the calculation is made in inches, and the divisor becomes 322·767, the result thus obtained being the burthen of the vessel in wood lasts of 4000 Neva pounds each. To reduce these into commerce lasts, one of which is equal to 5200 Neva pounds, it is multiplied by 10, and divided by 13.

Russia. —Length of the keel in feet, multiplied by the extreme breadth over the sheathing, and the product multiplied again by half the breadth, and divided by 94, gives the number of English tons.

United States.— If the vessel be double-decked, the length is taken from the fore part of the main stem to the after part of the stern-post, above the upper deck; the breadth, at the broadest part above the main wales, half of which breadth is accounted the depth. From the length three fifths of the breadth is deducted; the remainder is multiplied by the breadth, and the product by the depth. The last product is divided by 95, and the quotient is deemed the true tonnage of such ship or vessel.

If the ship or vessel be single-decked, the length and breadth are taken as above for a double-decked vessel, and three fifths of the breadth are deducted from the length. The depth of the hold is taken from the under side of the deck-plank to the ceiling in the hold. These are multiplied and divided as aforesaid, and the quotient is the tonnage.

At Philadelphia a system of measuring, called carpenter’s tonnage, appears to be adopted. The rule is as follows.

For vessels with one deck, multiply the length by the breadth of the main beam, and the product by the depth. Divide this second product by 95.

For double-decked vessels, take half the breadth of the main beam for the depth, and work as for a single-decked vessel.

At New Orleans the mode at present in use is as follows.

Take the length from the stem to the after part of the stern-post, on the deck. Take the greatest breadth over the main hatch, and the depth from the ceiling of the hold to the lower surface of the deck at the main hatch.

From the length deduct 3 / 5 ths of the breadth, multiply the remainder by the actual breadth and depth, and divide by 95, for a vessel with a single deck; but if the vessel have a double deck, half the breadth of the beam is considered as equivalent to the depth, and is multiplied accordingly. (b z .)
[7:21:330]
[table]
[7:21:331]
[table]

(a) At forepart of main hatchway. (b) At fore part of main hatchway. (c) At fore part of main hatchway.
[7:21:332]
[table]

(a) At upper part of the deck below the upper deck.

(b) At 4 feet below the upper deck.

(c) At 8 feet below the same deck.

(d) At lower part of platform

(e) At 4 feet below the under part of the deck.

(f) At 8 feet below the deck

(g) At 12 feet below the deck.

(h) At under part of lower deck.

(i) At 5 feet below the lower deck.

In this article we have described the method of constructing a scale of Tonnage, or of Displacements. The manner of performing the calculations to ascertain the displacement of a ship has been explained in the article Ship-building ; it is therefore unnecessary to recapitulate it here. Many persons, merely for want of sufficient attention to such explanations are unable to perform these arithmetical calculations and hence the frequent great errors in the displacements of ships. The following experimental method of ascertaining, very nearly, the actual tonnage of a ship, may not therefore be altogether useless. In fact, we suggest it with the impression that it may be advantageously followed by merchant-builders.

Suppose a model of a vessel to he made on a scale of half an inch to a foot, then each cubic inch will represent eight cubic feet A cubic inch of sea-water weighs ·594 of an ounce, that is, eight cubic feet of sea-water on a half inch scale weigh ·594 of an ounce. Therefore each ton, reckoning 35 cubic feet of sea-water to the ton, will weigh 2·5987 ounces, or 5 tons will very nearly weigh 13 ounces, and will measure in bulk 21 7 / 8 cubic inches.

If a box be made to a half-inch scale, 175 feet long and 40 feet wide, interior dimensions, rectangular every way, of wood varnished, and with the end grain of the wood in the direction of the given dimensions, each 1 / 8 of an inch in depth of such a box will be 218¾ cubic inches, which, according to the foregoing scale, will be 50 tons in weight.

To use this box, fill it partly with sea-water, then take the model of the vessel, which is of course supposed to be of dimensions less than those of the box. It is presumed that the launching weight is known. Suppose this, for example, to be 45 tons. Let the model float on the water, having previously observed the depth of water in the box. Load the model until the water has risen 9 / 89 of an inch above this observed depth, the model then evidently floats at its mean launching draught of water.

The load draught of water at which the vessel should swim is known. Load the model until it is brought to this water-line, and observe
[7:21:333]
[table]

(k) At upper part of the deck next below the upper deck.

(D At under part of half deck.

(m) At 3 feet below half deck.

(n) At 6 feet below the half deck.

(o) At under part of half deck.

(p) At 3 feet below the half deck.

(q) At 6 feet below the half deck.

(r) At 3 feet below the upper deck.

(s) At lower aide of half deck.

the rise of water in the box remembering that every 1 / 8 of an inch rise in the box indicates 50 tons increase to the displacement of the vessel Thus suppose, when the model swims at the draught of water wished for the vessel, that the water has risen an additional 1 / 4 of an inch more than at the launching draught of water; as it is known that the vessel will carry 50 tons for every 1 / 8 of an inch rise in the box, she will evidently carry 100 tons of cargo at the draught of water to which she now swims.

An additional advantage may be gained by observing the weights used according to the scale already mentioned, that is, 13 ounces to re-

present 5 tons. It is, that the vessel’s lading may be so apportioned as to bring her to the exact trim forward and aft that may be wished Suppose, for instance, a steam-boat to carry a 35 horse power engine; place 91 ounces at the centre of weight given by the engineers in the space allotted for the engine-room; then apportion the other weights, as crew and effects, ship’s stores, cargo, &c. over the respective spaces alloted for their reception. It will be seen whether the vessel, as formed, will bear them there, and swim at the trim intended; if not, either the vessel, the weights, or the trim, must be altered.

In consequence of the minuteness of the scale, and of the difficulty of observing it on account of the attraction of cohesion of the water it would be better, and very easy, to have an index or rather two, one forward and the other abaft, moving on a fulcrum over the ed e e of the box, and pointing to a graduated arc, which might, by lengthening the index-arm of the lever, indicate to any degree of exactness or minuteness required. Of course the inner arm of the lever would rise and fall with the model.

The foregoing remarks will only apply to a box of the exact dimensions given, that is, one of which every 1 / 8 of an inch in depth will give a solid content of sea-water, weighing, acorrding to the scale, 50 tons, under the supposition that a cubic inch 1 / 8 water weighs ·594 of an ounce. If other than sea-water is used, a different calculation must be made. The water used should be that in which the vessel is to be [7:21:334]