snip BUILDING. 35 



of which is the point where the keel touches the foundation of the dock, 

 while the power and weight, that is, the weight of the immersed part and 

 of the ship's body act in the perpendiculars which pass through the centre 

 of gravity. All the magnitudes, save the distance of the perpendiculars through 

 the centre of gravity, are known or may be readily calculated. If we now 

 take AN {pL 7, Jig. 9) as the natural water-line, and KL the temporary water- 

 line, where the keel first touches the foundation, we draw QH through the 

 centre of gravity of the volume KFML, perpendicular to KL, and FG parallel 

 to QH. If, then, D be the usual pressure, d that of KFML, and GH = b, 

 draw SEO parallel to QH at the distance GE from G = -jf, it will pass as 

 well as PBO through O, the centre of gravity of the ship, when we have the 

 necessary points for determining the distance, PBO being perpendicular to AN. 



5. The Masts and Sails. Theory has hitherto accomplished little in 

 determining the length and proportions of masts. We must, then, take 

 experience as our guide. The position of the masts exercises an important 

 influence on the qualities of a ship, a difference in them often improving 

 the action of the whole vessel. Not less important in the art of ship-building 

 is the form of the sails, for however perfect may be the construction of the 

 ship's body, without a correct position of the masts and the right number 

 of well-shaped and well-fitted sails, the desired object will never be attained. 

 The wind drives the vessel forward while it fills the sails ; they should, 

 therefore, be as large as possible, though there are limits which cannot be 

 exceeded without danger. We shall presently consider the dimensions, 

 positions, and different kinds of masts and sails. The centre of gravity is 

 a matter of importance also in sails. Fig. 18 represents the centre of 

 gravity and the form of the various sails in a ship. The centre of gravity 

 is marked by the sign ©. C is the centre of force of the whole system, 

 and D the line of draught. 



6. Stowage, Rolling, Pitching, and Falling of a Ship. An important 

 point in the construction of a ship is the stowage, or the distribution of the 

 burden in the hold. We have many examples showing that a ship built on 

 the best model sails much worse than an inferior vessel, because it is not 

 well stowed. The main point in stowage is to bring the centre of gravity 

 as low as possible, so that the ship may resist the action of the wind on the 

 sails with the greatest possible uniformity. 



All the calculations of equilibrium which we have thus far presented are 

 disturbed by the action of the winds and sea, and hence new mechanical 

 conditions must come into play. These produce certain motions of the 

 vessel which may exert a very unfavorable influence not only on its sail- 

 ing, but on its firmness in general. Among these motions is the rollings 

 when the ship constantly inclines from one side to the other. This is pro- 

 duced either by the shock of a wave against the side of the ship, when it 

 takes place above the centre of gravity, or by the motion of the waves 

 among each other. PL 7, Jig. 19 : let ADB be the transverse section of 

 a ship, AB the water-line, E the centre of gravity of the whole ship, and G 

 the point where the surface of the water would intersect the perpendiculars 

 through the centre of the laden ship, and BH the direction of the force 



687 



