"In describing this power, it will perhaps be better to begin with the first and simplest form of the combination. The pulley, weight, and rope I show now ([Fig. 12]) is the simplest form of making use of this power. It is called a snatch-block and often employed for drawing water from wells, or for hoisting light weights. It is very handy, but we do not get any additional power from it, though we get a change of direction and quick movement. From its portable form, its low cost, and the handiness with which it can be applied, this arrangement is one of the most useful of our mechanical contrivances.
Fig. 12. Theory of block and tackle
"When pulleys are adjusted, as I show you in this sketch ([Fig. 13]), the block which carries the weight is called a movable pulley, and the whole, as shown, a system of pulleys.
Fig. 13. Double block and tackle
"In this illustration, suppose the weight is 20 pounds. It is supported by two cords, A and B; that is, the two sections of the cord support 10 pounds each. Now, the cord being continuous, the power must be 10 pounds.
"We leave out of consideration the weight of pulley and the friction of the various parts.