helps to keep it in its place; but if, as in a pumphandle, the fulcrum is a pivot, there is considerable friction upon this pivot.

In the Pulley, the friction between the cord and the pulley does not hinder the motion, because the cord does not slide along the groove; but the pulley turns round with it. In this case the friction which hinders the motion is upon the spindle or pin round which the pulley moves.

In the Wheel and Axle, too, the friction of the cords round the wheel and the axle does not hinder the motion. That which does hinder it is upon the pivots, round which both wheel and axle move.

In the Screw, there is great friction between the thread and the groove.

In Toothed Wheels, there is friction between the teeth every time they come together, and much pains is taken to make the teeth of such a form that they shall move most smoothly and with least wear from rubbing.

The commonest machine for lessening the difficulty of dragging weights along a road is (as we all know) a waggon or cart. The wheels turn round, and so the friction upon the road does not hinder the motion; indeed, in slippery weather there is sometimes difficulty, because there is not friction enough.

In railroads, you may sometimes see the guard putting gravel upon the rails because they are too slippery. The friction that does hinder motion in a waggon or cart is at the axles, and these are made of iron, that they may not wear out, and rendered as smooth as possible by means of cart-grease. The friction at the axles of a carriage is sometimes so great as to set the axletrees on fire-because constant rubbing produces heat.

In machines where there are many parts, it requires great care in fitting the parts together, because friction not only hinders the motion, but also may make it irregular. If there is a great pressure at one part, the

friction will wear it away, and the machine will get out of order.

In a watch, where there are many wheels, the chief friction is upon the pivots round which the wheels turn; and, since it is of great consequence that the motion should be regular, rubies and diamonds are often used, holes being made in them for the pivots to turn in.

Friction is made use of for stopping carriages when they are going too fast. When coaches go down a steep hill, an iron slipper is put under one of the wheels: this prevents the wheel turning round, and the slipper rubs upon the road. When a railway train is about to stop, there is generally a very disagreeable smell, something like that of burning wood and oil mixed. This is from what is called "the break." The break is a piece of wood pressed down upon one of the wheels to prevent its moving round. The guard can screw down the break; but the friction is so great that it burns the wood, on which oil is put to prevent it from catching fire: this is the cause of the smell.

CHAPTER LXIV.

MACHINERY.-PART I.

A MACHINE is a contrivance to enable us to employ force in the most effective way. Sometimes we want to obtain increase of force, and at others greater speed. Sometimes it is desirable to produce one kind of motion by means of another kind-as when, by turning round a wheel, we move the sucker of a pump up and down.

These, and many more results, are produced by Machinery. By combinations of the different mechanical powers, machines have been constructed, capable of executing the most complicated works of every kind.

A machine may be moved by hand. A Pump is a

simple instance of such a machine. There is much less labour required to raise water from a well in this way than if it were drawn up in a pail.

But when machines began to be used, it was soon found that other forces besides that of our hands could be employed. A Waggon is a machine for drawing weights along a road, and the strength of a horse is used in this machine. Horses are constantly employed in different machines: sometimes, by turning round a wheel, they move the sucker of a pump, and so draw up water from below the earth; sometimes corn is threshed by a machine, which is turned by wheels moved round by horses in the same manner.

In Scripture we read of two women grinding at a Mill. This was done by means of two Millstones, the lower one being fixed, and the upper one capable of being moved round upon it, by means of handles. This was a Hand-machine for grinding corn.

In the course of time, some person thought of finding out other means than the use of the hands for moving the upper millstone. The first plan was to make use of the wind. Four great arms with sails were fixed on to an axletree, in such a manner that when the wind blew against them they turned the axletree round. When this had been done, all that was necessary was to fix a toothed-wheel to the axletree, which worked upon another toothed-wheel, and thus several toothed-wheels were set in motion, the last of which turned round the upper millstone. This is called a Windmill. There are contrivances in the mill for winnowing the flour, and doing other things, by means of different wheels, all moved by the great axletree.

Wind is a very powerful force, but it is irregular. Sometimes there may not be wind enough to move the sails, and at other times it may be so violent that a mill cannot be worked with safety. Except in great storms, it is easy to guard against the sails being moved too

quickly, but there is no way of making up for the absence of wind.

The employment of Water-power is not liable to these objections. A stream does not usually fail except in very dry seasons, and the water may be kept from running away by means of dams or embankments. Water exerts a force which does not appear so violent as that of the wind, but is steady, and really very great. So when the idea of a mill was once started, men soon began to use water, instead of wind, whenever there was a convenient stream. The water is dammed up to a height above the level of the stream, and a large wheel is placed at the opening through which the water falls into the stream below. This is the principal wheel of the mill, and is turned round by the force of the falling water. The great axletree of this wheel answers to the great axletree of the windmill, by means of which all the smaller wheels are set in motion.

But a stream of water is not to be found in every place, and a far more powerful force has been discovered than either water or wind: this is Steam. Steam is the vapour produced by boiling water. We know when the water boils in a kettle by the steam issuing from the spout. If the steam were not permitted to escape through the spout it would soon force open the lid. Steam will burst open strong iron vessels, if enclosed in them; and a gun has been constructed, in which, if a quantity of steam be shut up, it at last expands with such violence as to send out a volley of bullets with as much force as a large cannon. This force of steam, which can so easily be produced, and may be so destructive, has, by the ingenuity of man, been brought under complete management, so that we may say of steam, what is commonly said of fire, "It is a bad master, but a most excellent servant."

Without describing the Steam-engine, it will be sufficient to say that the force of steam is employed to move

an iron rod up and down, and by this means, with the help of a crank, to turn a wheel, as has been before described. The axle of this wheel answers to the great axle of the windmill or watermill, and sets in motion a system of wheels and rollers arranged to suit the purpose of the particular machine.

The force of steam is much greater, and more easily produced and regulated, than that of either water or wind, and is capable of performing what neither wind nor water could effect. But since a steam-engine is expensive, and requires very careful management, both wind and water are still very commonly used in the more simple machines.

CHAPTER LXV.
MACHINERY.-PART II.

THE PAPER-MAKING MACHINE.

IT has been said that every Tool may be properly called a Machine, and man uses tools in almost every work he undertakes. More contrivance is necessary to produce linen out of the fibres of a plant than is required to dig the ground or saw up wood into planks; but the spade, the saw, and the common weaver's loom, are all Machines -all are moved by hand, and they differ only in one being more simple than the other, and it is usual to call spades and saws Tools, and a loom a Machine.

But we can scarcely speak of the use of Machinery without including the application of other forces than that of the hand. It is, indeed, the ability to apply such forces that makes machinery so valuable.

In order to understand the application of machinery, we must first consider the nature of the work as it is performed by hand. The Paper-making Machine will afford us a beautiful illustration.

The mode of paper-making by hand has been de