"We have seen that the weight is sustained by two cords; if, therefore, it has been raised two feet, each cord must be shortened two feet. To do this, the power P must run down four feet. To get the full value of this machine the cords must be parallel.
"If we increase the number of movable pulleys, as sketched at [Fig. 14], to three, the relation of P to W will be as 1 to 8 and the distance through which P will travel will be eight times that through which W is raised.
Fig. 14. Multiple blocks and tackle
"If we apply this principle to the sketch ([Fig. 11]), which illustrates the blocks you used to-day in lifting the large timbers, and which is the usual form of pulley employed to lift heavy weights, you will notice that there is a four-sheave block at the top, and a three-sheave block at the bottom, with the end of the rope fixed from the top block. The three-sheave block is movable. A power of 10 pounds will, with this form of pulley, balance a weight of 60 pounds.
"Suppose a block of stone weighing 8,000 lbs. is to be raised to the top of a wall and we use a system of pulleys where each of the two blocks has four pulleys; we shall find that it will require a power of 1,000 pounds to raise it.
"Now, as to the inclined plane: this is called the fourth mechanical power, and it is not in any way related to the lever, but is a distinct principle. Some writers on the subject reduce the number of mechanical powers to two, namely, the lever and the inclined plane. The advantages gained by this are many for just so much as the length of the plane exceeds its perpendicular height is an advantage gained. Suppose A B C ([Fig. 15]), I make in the sketch, is a plane standing on the table. If length A B is three times greater than the perpendicular height C B then a cylinder at R P may be supported upon the plane A B by a power equal to a third of its own weight. That is, a block of that weight would prevent the roller or cylinder from going farther. From this we gather that one third of the force required to lift any given weight in a perpendicular direction will be quite sufficient to raise it the same height on the plane; allowance, of course, must be made for overcoming the friction, but then, you see, you will have three times the space to pass over, so that what you gain in power, you will lose in time. We see the use of the inclined plane every day we pass a building under construction, where the workmen wheel bricks, mortar, and other materials from the street to the floors above, using long planks for the plane or tramway. Merchants, too, often make use of an inclined plane when rolling heavy boxes and packages from the street to the floors of their warehouses.
