Speaking of the ropes used in blocks, while taking down the old barn timbers, Mr. Gregg suggested that it would not be a bad idea if the boys were taught a few general items concerning hempen ropes; so he asked them to memorize the following: A rope 1⁄4 inch in diameter will carry 450 pounds, and 50 feet of it will weigh one pound. If 5⁄8 inch in diameter, it will carry 3,000 pounds and 7 feet will weigh one pound. When a rope is 3⁄4 inch in diameter, it will carry 3,900 pounds, and 6 feet will weigh 1 pound. A rope one inch in diameter, the same as we have in our blocks, will carry 7,000 pounds, and 3 feet 6 inches will weigh one pound. "It is not likely that sizes greater than these will ever be used by you. If they are, you can obtain a fair knowledge of their strength by finding their areas, and comparing them with the areas of the ropes given, taking the rope having one inch in diameter, as a constant example."
Wire ropes are much stronger than hempen ones, whether made of steel, brass, or bronze. The care and preservation of ropes is deserving of consideration, particularly in localities where the atmosphere is destructive to hemp fibre. Such ropes should be dipped when dry into a bath containing 20 grains of sulphate of copper per gallon of water, and kept soaking in this solution some four days, before they are dried. The ropes will thus have absorbed a certain quantity of sulphate of copper, which will preserve them for some time, both from the attacks of animal parasites and from rot. The copper salt may be fixed in the fibres by a coating of tar or by soapy water. In order to do this the rope is passed through a hot bath of boiled tar, drawn through a ring to press back the excess of tar, and suspended afterwards on a staging to dry and harden.
The figures given are intended for new manila ropes, and do not hold good for ropes made of inferior hemp. It is always safer never to load a rope to more than 60 per cent. of its capacity, and not even this much when it is old and weathered.
Jessie reminded her father of his promise to give them some information regarding the power of blocks and tackle and the qualities of the inclined plane. Accordingly, Fred, George, and Jessie joined their father in his den after supper, and George placed his blackboard in a convenient place with chalk, rule, and other requisites.
When all were seated, the father said: "Some time ago I tried to explain to you the uses of the lever in quite a number of different situations; to-night I'm going to show you how the various ropes and pulley blocks are made to do service for mankind. These devices are used very generally, especially in building operations, where heavy beams, girders, or blocks of stone have to be raised. On board ship, it is the favourite mechanical power by which rigging is raised, cords and ropes tightened, and goods lifted from or lowered into the hold.
Fig. 11. Blocks and tackle
"The pulley, the main feature of the third mechanical power, may be explained almost on the same principle as the lever, as you will see upon examining the sketch ([Fig. 11]) I now make on the blackboard.
"The pulleys seen in the blocks around which the rope runs may be considered so many levers whose arms are equal, and whose centres are fulcrums.