2g6 Mr. Seppings on a new principle of constructmg 
is to the arch ; for no weight or pressure on the fabric can 
alter its position in a longitudinal direction, till compression 
takes place at the abutments, and extension of the various 
ties. 
This arch-like property of the diagonal frame, not only 
opposes an alteration of position in a longitudinal direction, 
but also resists external pressure on the bottom, either from 
grounding or any other cause, because no impression can be 
made in its figure in these directions, without forcing the 
several parts of which it is composed into a shorter space. 
The connection which is kept up by means of this trussed 
frame firmly attached to the timbers of the ship by circular 
coaks and bolts, together with the shelf pieces united to the 
sides and to the several beams by means of the same sort of 
fastenings, gives such unity to the whole as to bear no com- 
parison with that heterogeneous and badly connected mass of 
materials for which it is substituted. 
It has hitherto been a generally received opinion, that stiff- 
ness or inflexibility in a ship is not strength, but that a yield- 
ing or bending of the fabric is an essential quality to preserve 
it from being destroyed by the shocks which it is destined to 
sustain. 
This misconception must have arisen from an equally in- 
correct idea, which is, that a ship is an elastic body, because 
there is a considerable degree of elasticity in the materials of 
which it is composed. But it should be remembered, that this 
elasticity of the materials must be very inconsiderable, inas- 
much as the minute degree of elasticity in each piece must 
necessarily be neutralised in the fabric, by the various direc- 
tions and tendencies of the numerous parts of which it is 
