The two points above named whose relative positions are vitally concerned with this subject—_i.e._, centre of buoyancy and centre of gravity—are determined by shipbuilders for many of their vessels, although the stability may not be calculated to its full extent. The position of the centre of buoyancy is easily ascertained from, and in fact usually forms part of, the displacement calculation. While the position of centre of gravity may be found by means of calculation alone, _i.e._—by the process of estimating the position of the centre of gravity of each of the component parts, and from this deducing the common centre of gravity of the whole ship—the work is so laborious, complex, and so liable to error, that it is scarcely ever adopted at the present day by mercantile shipbuilders. The position can be ascertained with comparative ease and greater accuracy by means of “inclining” experiments with the finished vessel, or closely estimated before-hand by means of data obtained in the manner alluded to from previous vessels of similar type.[6] Another point concerned with stability is that termed the “metacentre,” which is found by calculation from the lines of the vessel. Referring to Fig. 14, a vertical line drawn through the centre of buoyancy =B_{1}= cuts the original vertical line at =M=. The intersection =M=, when the vessel is inclined to an indefinitely small angle, is the “metacentre.” It is approximately the same in all ordinary vessels for inclinations less than say 10°, but varies with greater inclinations. The corresponding intersections of the consecutive vertical lines for all degrees of inclination are embraced in the term “metacentrique.” These features in stability investigations were originated by Bouguer, to whom reference has already been made. The manner in which they are concerned with stability will be indicated further on. (See also footnote on preceding page.) RESISTANCE POWER AND SPEED. A ship, in moving through the water, experiences resistance due to a combination of causes, which combination, according to modern accepted theory, is made up of three principal elements. 1st—“Frictional” or “skin friction” resistance, due to the particles of water rubbing against the ship’s hull; 2nd—“Eddy-making” resistance, due to local disturbances or eddies amongst the particles of water—almost wholly at stern of ship; 3rd—Surface disturbance of the water by the passage of the ship, resulting in the creation and maintenance of waves: known as “wave-making” resistance. The conditions which govern each of these elements, and their relative importance, may be generally indicated.