SHIP-:BUILDING. 



c«vity in the immerled body with more or lefs power, in pro- 

 portion to its didance without the centre of gravity, to bring 

 the fliip upright, where the afting force or power ceafes 

 which occafioiied tlie veflel to heel. 



The line of fupport is the vertical or perpendicular line 

 fiippofed to pafs through the centre of gravity, and inter- 

 feding a line perpendicular to the keel of the velfel through 

 the point, called the meta-centre. For if a floating body is 

 inclined by any power which does not change the pofition of 

 its centre of gravity, the line of fupport mult neceflarily pafs 

 between that power and the centre of gravity ; and the force 

 or moment of that power is equal to the weight of the float- 

 ing body, multiphed into the diltance of its centre of gravity 

 from the line of fupport. As the line of fupport muft pafs 

 between the centre of gravity and the power applied to heel 

 the veflel, the moment of that power, or its force, multiplied 

 into its diftance above the meta-centre, is equal to the mo- 

 ment of its gravity, or the weight of the floating body mul- 

 tiplied into the diftance of the centre of gravity below the 

 meta-centre. 



Suppofe the veifel inclined, or heeling by the power of the 

 wind on the fails, if the line of fupport pafles on the wind- 

 ward-fide the veflel would upfet, as the power and gravity 

 are at the fame lide operating to incline it ; but if it pafles 

 to the lee-fide of that power, the veflel will be redrefled, as 

 the power and weight operate to that effeft ; and if the 

 moments of the power and gravity be not equal, the body 

 will not remain at refl:, but will incline more or lefs, as the 

 power or the weight prevail. 



Hence it is plain, that the diftance from the centre of 



gravity to the line of fupport, multiplied into the weight of 



j the veflel, is the meafure of the ftabihty of the veflel, or its 



1 effort to redrefs itfelf when inclined, and that its ftability is 



as that diftance. 



The meta-centre ufually fignifies a point to which, if the 

 centre of gravity of a floating body be raifed, the fmalleft 

 lateral effort will make it incline. It is plain, that in an ho- 

 mogeneous cylinder, or fpliere, the meta-centre, and centre of 

 gravity, being always in the fame point or centre of the 

 fphere, however thefe bodies are inclined, have no ftability. 

 The centre of gravity muft, by no means, be placed above 

 the meta-centre, becaufc if it were the velTel would overfet. 

 This centre, which has likewife been called the Jhif ting centre, 

 \ depends upon the fituation of the centre of cavity, for it is 

 that point where a vertical line drawn from the centre of 

 cavity interfe£ls a line palling through the centre of gravity, 

 and being perpendicular to the keel. 



The centre of gravity of a fliip, is that point by which 

 it may be fufpended, and the parts remain in perfeft equili- 

 brium. It is alfo the centre of all the forces, or momenta, 

 which prefs it vertically, or direftly downwards towards the 

 centre of the earth. 



The lower the centre of gravity is placed, the farther is it 

 from the line of fupport, and coirfequently the greater ftability. 



In fliips of war, the centre of gravity can never be far 

 removed from the load-water-line ; for if the centre of gra- 

 vity could be placed nearer the keel, it is not to be defired, 

 as the farther it is removed from the load-water-line, the 

 rolling of the fliip becomes more uneafy. 



The centre of motion is that point upon which a vefTcl 

 ofcillates or rolls when put in motion. This centre is always 

 in a line with the water's edge, when the centre of gravity is 

 even with, or below the furface of the water ; but whenever 

 the centre of gravity is above the water's furface, the centre 

 of gravity is then the centre of motion. 



The longitudinal axis of a fhip is an imaginary line, which 

 pafles horixontally from head to ftern through the centre of 

 gravity. 



The tranfverfe axis is an imaginary horizontal line, palling 

 athwartfhips through the centre of gravity. 



The "vertical axis is an imagintry perpendicular line, 

 drawn though the centre of gravity when the veffel is ill 

 equilibrium. 



It is about thefe axes that every fhip or veflel in motion 

 may be fuppofed to turn. In rolling, fhe may be fuppofed 

 to ofcillate on the longitudinal axis ; in pitchiiig, on the 

 tranfverfe axis ; and in working, &c. to turn on her ver- 

 tical axis. 



From conftantly obferving that the performance of fhip* 

 at fea depends materially on their ftability, both naval archi- 

 tefts and navigators muft, at all times, be defirous of dif- 

 covering in what particular ciicumftances of conftruftion 

 this property confifts, and according to what laws the fta- 

 bility is affefted by any varieties that may be given to their 

 forms, dimenfions, and difpofition of contents ; which are 

 determined, partly according to the fkill and judgment of 

 the conftructor, and partly, in fome veflels, as we fliall fhew, 

 by adjuftments after the veffel is afloat. 



The form of the immerfed body, and the weight of the 

 fhip, are the chief terms in the compofition of liability, and 

 they are only to be attained, in the requifite degree, by full 

 dimenfions near the load-water-line, with fufficient capacity. 



At firft fight, it is certain that all the weight above the 

 load-water-line helps to make the fliip crank, and, of confe- 

 quencc, the lighter the upper works the ftiffer the fhip. 



Conftruftors may vary the form of a fliip chiefly m three 

 dimenfions, that is, in the length, breadth, or depth : let us 

 examine how far enlarging of fhips, in any of thefe particu- 

 lars, will contribute toward making them carry fail, or, in 

 other words, gain ftability ; for although the wind may, in 

 one fenfe, be faid to conilitute the power by which fhips are 

 moved forward in the fea, yet if it a£ts on a veflel deficient 

 of ftability, the effeft will be to heel the ftiip rather than to 

 propel it forward ; ftability is, therefore, not lefs necelFary, 

 than the impulfes of the wind are to the progrelTive motion 

 of veffels. 



If the length only, without altering the other dimenfions, 

 be enlarged, the centre of gravity and the meta-centre will 

 continue the fame height, and her ftability in refpcdl of incli- 

 nation to one fide will increafe in proportion to the weight of 

 the fhip ; and as the weight generally iucreafes ordiminifhei 

 in proportion to the length, we may fay that in ftiips that 

 differ only in length, their ftability will be in proportion to 

 their length. 



Yet although an increafe of length would enable a fliip to 

 carry the molt fail, confequently fail faiter, it muft not be 

 carried to an extreme ; becaufe if to conitrudted, a fhip 

 would neither tack nor veer fo quickly ; neither would fhe lift 

 or rife in a fea like one fliorter ; flie vould ftrain more, and be 

 very liable to have the fea break over her. The influence of 

 the rudder may be weakened, and may even be totally loft. 

 The greatell judgment is therefore required in proportioning 

 the length, which may be proportionally greater in thofe 

 fliips that generally navigate in the fmoothcr feas, or are not 

 intended to be deeply laden. 



By altering the breadth, the ftability is materially afledted ; 

 for by enlarging it we gain, and by diminifliing the breadth 

 we lofe a great deal of the ftability. M. Bouguer has proved, 

 that the ftability increafes in proportion to the cubes of the 

 breadths : for, fuppofing the bottom homogeneous, then, 

 ift, the increafe of weight, and of confequeiicc ftability, will 

 be double the increafe of the breadth t and 2dly, the addi- 

 tional weight will a£t with fo much the greater force, as the 

 length of the lever is increafed, or as the meta-centre i» 

 railed, and the height of that poinr is augmented in pro- 

 portion to tlic fquare of the breadth : hence the liability 



will 



