478

NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

extinguished the atmosphere is in a dangerous condition. On no
account should a naked light ever be used for trying the atmosphere of any
suspected place.

ISHERWOOD SHIPS.

The vessels described so far have been designed on the transverse
frame system, that is, the frames have been closely spaced with widely
spaced heavy longitudinals laid across them. But longitudinal framing,
invented by Sir Joseph W. Isherwood, Bt., and now popularly known
as the Isherwood system, is extensively employed especially in the
construction of oil tankers. The frames 'in this system are also closely
spaced but arranged longitudinally, the transverse girders being massive
and spaced widely apart, as in Figure 78, which represents a section of
a tanker.

Fig. 78.—Isherwood Framing for Tanker.
. A> are closely spaced longitudinal frames at the bottom, the sides
and the decks. (
B, middle line longitudinal bulkhead extending from the bottom of
the tank to the top of the expansion trunk.
C, a strong transverse girder consisting of a deep floor plate, a deep
vertical web frame in the lower hold united to a heavy lower deck
beam; a smaller web frame in the 'tween deck is associated with an
upper deck beam of proportional dimensions. Z>, brackets to secure
floor plates and beanas to the midship bulkhead; note the longitudinal
stiffenexs just showing on the left side of the bulkhead.