Dr. Young's Remarks on the 
330 
omitted ; while the cohesive strength of the external planking, 
considered as a tie, is still probably more than sufficient for 
resisting the smaller force, which occasionally operates in a 
contrary direction: although the strength of the ship, for 
resisting such a force, is certainly much diminished by the 
change. From the manner in which these wedges are driven 
by Mr. Seppings, it may easily be understood, that they may 
tend to produce a convexity below, without raising any part 
of the keel from the blocks, merely causing it to press more 
strongly on them at the midships; so that if this difference 
becomes equal to that of the weight and pressure after 
launching or floating, there may be no tendency to any further 
change whatever; and hence it may happen, that without any 
other superiority of stiffness, or even of workmanship, a ship 
may appear wholly exempt from arching, as the Tremendous 
did, and some other ships are said to have done. Without 
the operation of some such cause, even a hollow cylinder of 
compact oak, 180 feet long, 50 feet in diameter, and six inches 
in thickness, if such a mass could be supposed to exist, would 
exhibit, when immersed to the depth of its axis, a degree of 
arching just perceptible, from the longitudinal pressure of the 
water only, amounting to about of an inch besides a 
* “ The stiffness of a cylinder is to that of the circumscribing prism, as three 
times the bulk of the cylinder is to four times that of the prism.” (Lect. Nat. 
Phil. II. 83. Art. 339. B.J : but the radius of curvature of a prismatic beam is 
i 2 af 
(P. 46, Art. 321.) b being the depth, m the weight of the modulus,/ the force, and 
a the distance of its application ; and taking m for the weight of the modulus of the 
cylinder, its radius of curvature will be JBut since the stiffness is as the 4th 
power of the diameter, (P. 49, Art. 333.) that of the hollow cylinder in question will 
