Blocks. A block is a pulley, or system of pulleys, mounted in a frame. A block consists of the shell or frame; the sheave or wheel on which the rope runs; the pin or axle on which the sheave turns; and the strap or part by which the block is made fast to any particular station. This last is of either rope or iron, the other parts may be of either iron or wood. A single block contains but one sheave; a double block has two sheaves, one above and one below. Blocks are of many sizes and varieties, and wooden ones with iron fittings have since 1804 been very generally made by machinery, which was originally designed in 1802 by Mark Isambard Brunel, and which was first erected at Portsmouth, where it has ever since been in use.