"We closed our lesson on the Forces of Nature," said Mr. Wilson, "by glancing at the windmill and the water- mill-two contrivances by means of which man is enabled to utilize for his own benefit the natural forces, wind and running water.
"Without attempting to inquire into the precise manner in which these mills accomplish their work,wewere content to learn through them a new name-machine. We learned that any contrivance for transmitting a force from one point to another, or for alteringthe direction of mo vement, is a machine. Let us now take another example.
"A workman wants to raise a basket of bricks or a pail of mortar from the ground to the scaffold at the top of the building where he is working. He does not wish to descend and remount the ladder, so he lowers a rope. One end of this is hooked on to the basket, and he raises the weight by his own muscularforce. This force is actually applied at the upper end of the rope, but the rope transmits the force to the basket. Hence the rope is a machine. It changes the point of application from the upper end of the rope to the basket at the bottom.
"It is important to remember that the man raises the basket in this way by exerting a direct upward force, greater than the force of gravity caused by its weight, which is tending to pull it downwards.
"We will now proceed with a little experiment. I have here a simple contrivance, consisting of a small wheel with a hollow, grooved circumference, the wheel itself being fixed to a bracket. I will pass this cord round the wheel, and to each end of the cord attach a pound weight. The weights, when left to themselves, balance each other;there is a downward force of one pound on either side of the wheel.
"Now I will hang a small weight (this half-ounce weight will do) on one side. That, and the pound weight together, immediately overcome the weight on the other side; they fall and it rises. We will now return to the workman"s scaffold, and imagine such a wheel set up there. Twobaskets of bricks, both pulling downwards by the force of gravity, would balance each other; but one brick more ineither basket would cause that one to sink and the other to rise.
"Suppose the man were to unhook one of the baskets. How could he manage to keep the other in its original position?""To do this he would have to pull downwards, just as the basket did, and with the same force, sir," replied Fred. "Exactly, and if he pulled a little harder, what wouldhappen then?"
"I suppose the basket on the other side would rise, sir." "Yes, Fred, it would; that is to say, the man by pulling the cord downwards over the wheel, exerts an upward force on the basket at its opposite end, and causes it to rise. In other words, the wheel and the cord constitute a machine, for they change the point of application, andthey alter the direction of the force.
"Workmen usually employ this means of raising their materials to the scaffold, because it is easier to pull downwards than it is to lift the entire weight upwards.
"It frequently happens, however, that the men have to raise heavy masses of material (such as great sheets of lead, or coils of leaden pipe)which it is impossible for them to lift from the ground with only their own muscular force. We will see how theydo it. Let us return to our wheel and cord experiment. I will take one part of the cord, as it hangs over the fixed wheel, pass it round and under a similar loose wheel, and then fasten the end of it to the beam above. Here we have a contrivance consisting of a fixed and a movable wheel, with the same cord passing round both. When I pull at the other extremity of the cord, the second wheel actually moves up as the cord is pulled down.
"Now, we will hook a 2 lbs. weight to the movable wheel, and a 1 lb. weight to the free end of the cord. When left to themselves the two weights balance each other. It would be just the same if we used 4 lbs. and 2 lbs., 100 lbs. and 50 lbs. They would balance, and the slightest additional weight hung on the end of the cord would cause the movable wheel with its heavier load to rise.
"This is the kind of contrivance used for raising heavy weights. With such a contrivance fixed to the scaffold, men are enabled, by pulling downwards at the end of a cord, to raise a heavy mass of material, which they could not possibly lift without help. Such a contrivance is a machine. The men apply the force at one end of the rope; it is transmitted to the other. The force is applied downwards ; the heavy mass is raised upwards. The small force exerted by the men is increased in magnitude or intensity, so that it is enabled to raise a much heavier weight than it could without such a machine.
"We employ a vast number of machines to do an almost endless variety of work. Some of them are very complicated. But however complicated they may appear to be, they are all found on examination to be made up of a few simple contrivances, which have been invented for the purpose of altering force in one or more of the ways we have mentioned."Lesson 06
