employment of Oblique Riders, 30^ 
unaltered, while the keel is shortened, in the same manner as 
any soft and thick substance, pressed at one edge between the 
fingers, will become concave at the part compressed, (Lect. 
Nat. Phil. I. PI. 9. F, 117); and this strain, upon the most 
probable supposition respecting the comparative strength of 
the upper and lower parts of the ship, must amount to more 
than one third as much as tlie mean value of the former, being 
equivalent to the effect of a weight of about 1000 tons, acting 
on a lever of one foot in length, while the strain, arising from 
the unequal distribution of the weight and the displacement, 
amounts, where it is greatest, that is, about 37 feet from the 
head, to 5260, in a 74 gun ship of the usual dimensions ; and 
although tlie strain is considerably less than this exactly in 
the middle, and throughout the aftermost half of the length, 
it is no where converted into a tendency to “ sag,'' or to 
become concave. It must however be remembered, that when 
arching actually takes place from the operation of these forces, 
it depends upon the comparative strength of the different parts 
of the ship and their fastenings, whether the curvature shall 
vary more or less from the form, which results from the sup- 
position of a uniform resistance throughout the length. An 
apparent deviation may also arise from the unequal distribution 
of the weight through the breadth of the ship : thus the keel 
may actually sag, under the step of the mainmast, even when 
the strain, as here calculated, indicates a contrary tendency 
% 
with respect to the curvature of the whole ship. 
Foree of the Waves, 
The magnitude of the strain on the different parts of a ship 
is subjected to very material alterations, when she is exposed 
MDCCCXIV. S s 
