case of grounding. The slight projection of
the keel itself then gives too little purchase for a dangerous amount
of leverage on the frame. A long keel is made up of several pieces of
square timber, with their ends shaped into scarfs, an overlapping and
interlocking arrangement of great strength. The foremost keel piece is
scarfed into the stem, which is the fore-end of the vessel's bow. The
aftermost keel piece joins the stern-post, on which the rudder hangs.
Elm makes a good keel, especially with oak for stem and stern-post.

The frame, to pursue our simile, is to the ship what ribs are to our
bodies. In the same way the planking is the skin. The frame, or ribs,
determines the vessel's form. There were, and still are, many
varieties of frame. In a very small vessel there are very few timbers.
The keel is probably all in one piece, and the planks may possibly run
from stem {84} to stern without a break. In this case the unity of
each piece supplies enough longitudinal resistance to strains. But
when a vessel is large, and more especially when she is long, the
strains known as hogging and sagging are apt to rack her timbers apart.

A ship is not built for mere passive resistance, like a house, or even
for resistance only to pressures and vibrations, like a bridge. She is
built to resist every imaginable strain of pitching and rolling, and so
requires architectural skill of a far higher kind than is required (in
the constructional, not the aesthetic, sense) for any structure on the
land. When a ship is on the top of a single wave she tends to hog,
because there is much less support for her ends than for her centre,
and so her ends dip down, racking her upper and compressing her lower
parts amidships. When the seas are shorter she often has her ends much
more waterborne than her centre, and this in spite of the fact that the
extreme ends are not naturally waterborne themselves. Then she sags,
and the strains of racking and compressing are reversed, because her
centre tends to sink and her ends to rise. Now, a series of hogging
and sagging strains alternately compresses and opens every resisting
join in every {85} timber, with the inevitable result of loosening the
whole. To meet these strains longitudinal strength must be supplied.
The keel supplies much of it, so does the planking (or skin) to a
lesser degree; but not enough; and the ribs, by themselves, are for
transverse stiffening only. Four means are