696 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q6. 



Whatever may be the means by which naval arcliitecture receives progressive 

 improvement, it seems to be generally allowed, that the art of constructing 

 vessels has, at the present period, attained to a degree of perfection far sur- 

 passing any that has been known to former limes, either ancient or modern : 

 yet it is equally certain, that some principles, by which the construction of 

 vessels is materially influenced, still remain to be developed and explained. It 

 is frequently remarked by navigators, as well as by naval architects, that altera- 

 tions apparently the most trivial, in the form of a vessel, in the distribution of 

 the ballast, or in the position and extent of the masts and sails, will wholly 

 change the qualities of a ship from bad to good, or the reverse. As these 

 changes cannot be attributed to fortuitous causes, it is necessary to allow that 

 they are consequences of principles certain and definite, though in many cases 

 unknown, or imperfectly estimated by conjecture. The proportions and dispo- 

 sition of [)arts, which operate to produce good or bad eiiects on the sailing of 

 ships, are probably in these instances so intricately combined, as to make it 

 scarcely possible from mere observation, however extended and diversified, to ac- 

 count satisfactorily for changes so remarkable: it must also be acknowledged, 

 that some of the data on which the theory of naval architecture is founded, 

 being imperfectly known, particularly the laws of the different resistances to the 

 ship's motion,* it would be unsafe to rely entirely on deductions a priori for ex- 

 plaining this subject. 



* The laws of resistances, opposed to bodies which move in fluids, and varying in a duplicate ratio 

 of the body's velocities, are demonstrated by Sir Isaac Newton, in the 2d book of the Principia, on 

 conditions restrained to the particular case in which the motion of the resisted body is extremely slow, 

 and tiie fluid perfectly compressed. On these conditions, tlie pressure which resists the motion of the 

 body is exactly balanced by the pressure on the posterior part, and consequently the only force op- 

 posed to the body's motion, is tlie inertia of the fluid, which is displaced while the body moves 

 through it : for the resistance of friction depending on the body's velocity must be, in a physical 

 sense, evanescent, when the motion is very slow. It is evident, that the theory of resistances 

 founded on these principles ought not to be applied to the solution of cases in which the velocity is 

 much increased, without great care and circumspection ; for by the increase of velocity, 3 ditferent 

 forces begin to have operation, of which the Newtonian theory takes no account; i. e. the pressure 

 on die anterior part of the body, the pressure on tlie posterior part, and the resistance of friction. 

 The pressure on the anterior part will evidently be a constant or invariable quantity as long as the 

 moving body continues at tlie same depth. The pressure on the posterior part will depend on the 

 velocity of the body's motion, and when that velocity is =: 0, this pressure will be precisely equal, 

 and contrary to tliat which acts on the anterior part. Moreover, when the body's velocity is equal to 

 that with which tlie fluid rushes into empty space, the pressure on the posterior part will be =: 0, 

 and of consequence all the pressures on the posterior surface, corresponding to the intermediate velo- 

 cities, must be found between these limits. When the surfaces of the moving body are smooth, it 

 has been supposed that the effects of friction are not very considerable. This opinion is however dis- 

 proved, to the satisfaction of any one who consults the account of the very accurate and well devised 

 experiments on the motion of bodies through the water, made under the direction of the committee 



