BLOCKS &t

The strop may be of rope or wire fitted into the score round the shell
and spliced as shown in the illustrations of tackles. Tiie length of a
rope strop is about one and one-third the round of the block.

Sheaves are either of lignum vitae or metal Lignum vitse is an
exceedingly hard wood dark in colour and has self-lubricating properties
The bush of the sheave may be plain, that is just a hole drilled in gun-
metal (Fig. 73), or a roller bush (Fig 74), which runs with less friction.
Metal sheaves (Fig. 75) are used for heavy work. The size of a block
is the length of its shell; the size of a sheave is its diameter.

An External bound block (Fig 76) is one stropped with a heavy iron
band, an eye being welded on it for a hook or shackle.

An Internal bound block is one having an iron strop inside the shell,
one lug of which is sometimes extended outside the shell in the form
of an eye to take the standing part of the purchase The strop can be
withdrawn from the shell for cleaning and painting; pins of blocks are
scraped and rubbed with blacklead, so also are the sheaves and bush.

Fig. 76.

External Sound
Block

Fig. 77

Internal Bound
Block.

Fig. 78.

Iron Snatch
Block.

Fig. 79.

Wood Snatch

Block,

A Snatch block is a loose block having a hinged clamp at the side
(Figs. 78 and 79), so that the bight of the rope may be slipped over the
sheave and the clamp closed again. They are handy as portable lead
blocks. Malleable iron pulley blocks (Fig. 80), are now universally used
for cargo work, and Fig. 81 illustrates another of Messrs. Davey & Co.'s
cargo blocks fitted with self-lubricating sheaves specially designed
for iujavy work, the gunmetal centre, or bush, of the sheave having