"It takes ten pounds to balance the same weight by a line passed over a single fixed pulley, or through a block," continued the principal. "You gain nothing except at the expense of time. If you pull one rope down a foot, the other is raised only a foot. With a movable pulley, you have to pull down two feet to raise the weight one foot. With one pound of power, you raise two pounds of weight. Now, if there are two pounds of weight, and you exert only one pound of power, what becomes of the other pound?"
"It is supported by the fixed end of the line."
"In the purchase-block attached to the mast, we must exert a power equal to one-half of the weight of the mast, the other half being supported by the shears. Bates has made fast the snatch-block in the deck, but we gain nothing in power by its use. What is it for then?"
A dozen hands were raised, but most of the boys were studying the problem. The principal waited until one of these appeared to have made up his mind.
"Without the snatch-block we could only pull on the up-and-down rope, and not more than three or four of us could get hold of it for the want of room to stand near it," replied the student indicated.
"That's the idea exactly," replied the principal.
"Forty of us could get hold of the rope while it is run out from the snatch-block, parallel with the deck," added the thoughtful boy.
"Precisely so; well answered. Then the snatch-block only enables us to change the direction in which the power may be applied. In unloading vessels, they often use a horse to hoist the cargo. The animal could not pull straight down on the rope, but the snatch-block enables him to draw the rope parallel with the top of the wharf. I think you will remember what a snatch-block is, and what it is for. Now man the line."
The students took hold of the line, and walked away with it. The mast rose in the air, and hands were then placed at the guy-lines to keep it in place. When the lower end of the mast was above the deck, Bates took the girt-line attached to the shear-head, and drew the mast into position. Two of the students were sent into the hold to direct the tenon into its step, which is the mortice above the keelson.
The mast was lowered slowly into its place, the square tenon adjusting itself in its place as it belonged, so that there was nothing more to be done, except to wedge it in at the mast-hold in the deck.