488 EXPLANATION OF THE PLATES. 



could possibly have been obtained in the direct resistance, by this alteration of the 

 form of the body. 



In giving the general form to the body, she was not so sharp forward as to be 

 according to the opinion of the day ; but the form and adjustment of the body were 

 such as to ensure her being easy against a heavy head sea, and with it as little 

 direct resistance as could be obtained without endangering other essential qualities 

 on which the general efficiency of the vessel depends. 



To determine the elements that relate to the form, and are essential to the 

 resistance the body will meet in passing through the water, considerable difficulty is 

 involved, and it yet remains for the progress of knowledge to develope ; and in a 

 great degree this subject must rest on mere opinion, as the theories laid down do 

 not notice clearly the different forces that materially affect the motion of bodies ; as 

 the pressure on the anterior part of the body, the minus pressure on the posterior 

 part, with the resistance that arises from friction, as well as the adhesion of the 

 fluid. The writings of several authors on this subject, as Bouguer, Euler, &c., we 

 find the theory is laid down by Newton, and applied by them to the motion of ships 

 is of little practical utility. Euler has, however, given his theory in a very general 

 manner, without enforcing the correctness of his conclusions in practice. M. Romme, 

 in his Art de la Marine, gives some interesting experiments, as made by himself at 

 Rochfort ; and with a general expression for the resistance of ships drawn from 

 these experiments, and from the Newtonian theory ; but we find the Academy of 

 Science did not consider his experiments sufficiently numerous, or on a scale suf- 

 ficiently large to be conclusive. And the experiments made by the British Society 

 for the Encouragement of Naval Architecture have, in a degree, set aside his 

 conclusions ; especially on one material point, viz., that while the midship section 

 remained unaltered, the form forward and abaft would not greatly affect the velocity, 

 which appeared not to be the case ; only that this element materially influenced the 

 resistance of the body. 



In the theory, as laid down by Chapman, the Swedish ship-builder, an expression 

 has been investigated on small parts of the surface, partly by the Newtonian theory, 

 and partly frorn^experiments of his own ; and which he applied in calculating the 

 resistance applied to a vessel ; which method, although the results may not be 

 strictly true, is still very useful, as giving the relative plane of direct resistance very 

 nearly, and was used in constructing the Nile ; and is, in most cases, as the form of 

 steam boats varies in terms of the area of the midships, applicable in comparing their 

 direct resistance, instead of the area of midship section, which is now often taken by 

 constructors of steam boats to determine the velocity likely to be obtained by any 

 power given. 



