82 NAVAL SCIENCES. 



draw the perpendiculars 1.1, 2.2, 3.3, to 28.28, and the ship will be divided 

 into a certain number of equal vertical parts. Now, let OPO (jig. 2) be a 

 section of the ship, in which the lines 1.1, 2.2, 3.3, 4.4, represent transverse 

 sections to the outside of the ship, at the different heights 1, 2, 3, 4, of the 

 sketch {fig. 1), observing that at the right of our drawing the sections are 

 forward of the centre of the ship's profile, and at the left are abaft the 

 same. Divide the height under the water-line, WW (fig. 1), into feet, 

 draw horizontal lines through the points of division, so that the ship's body 

 will be divided into a number of equal horizontal parts, corresponding to 

 the division in the section (fig. 2). Measure half the breadth on the differ- 

 ent horizontal lines, according to the scale of the ship, and it will give the 

 value of the numbers required in the calculation. This half breadth may 

 also be found by the plan of the water-line (fig. 3) ; double the results thus 

 obtained, and it will give the displacement for the portion of the ship's body 

 between E and F (fig. 1). For the portions forward of Ffand abaft Ee the 

 calculation can be easily made, and the results added to those obtained before. 

 In determining the displacement, some inches must always be added when the 

 ship is at anchor in rough water or at flood tide, or under a press of sail at 

 sea. This is on the principle that a particle of water which is in motion, 

 and reaches the surface of a body, no longer exercises its pressure on all 

 sides, but strives to escape in the direction of its motion, and hence its 

 vertical pressure against the body is diminished, which must accordingly 

 sink deeper than when the water is quiet. The pressure of a particle of 

 water in motion is in proportion to its depth below the surface, less the 

 depth proceeding from the velocity in the direction of the motion. This is 

 shown by an experiment of Romme. He took two tubes (fig. 4), one 

 straight, ah, the other bent, cde ; both were open, and so wide that they 

 could admit the float gf, the lower end of which was cork and the upper a 

 graduated rod. These tubes were first immersed in standing water, the 

 float was inserted, and the degree of immersion noted on the scale ; they 

 were then placed in running water flowing in the direction hi, the bend of 

 the tube, cde, lying with the stream, when it appeared that the float was im- 

 mersed one inch deeper. When the bend of the tube was held against the 

 stream, the float rose an inch higher than in standing water. Upon mea- 

 suring the velocity of the water, it was found to be seventy feet in thirty 

 seconds ; and according to the velocity, the water must have risen or fallen 

 in the tube about 1 inch 1 line. 



As salt water has a greater specific gravity than fresh, a ship sinks deeper 

 in the latter, making a difference of about six inches in a ship of the line of 

 120 guns. 



3. Centre of Gravity. It is important to ascertain the centre of gravity, not 

 only of the part of the ship displacing the water, but also of the whole body of 

 the ship, since the sailing of the ship depends on the right position of this. 

 The method of determining the gravity of each is explained in Statics, and we 

 need add nothing to what has been said above. In like manner, when we 

 wish to determine the centre of gravity of the immersed portion, we must 

 find also that of the part above the water, it being necessary that they both 

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