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 from 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.