rod backwards and forwards.

A knife-grinder's

wheel is turned by a crank, which is worked by the foot moving up and down.

The crank is of great use in machines, because by its help we can obtain one kind of motion from another.

In the steam-engine, the steam moves an iron rod up and down. The rod being connected with a wheel by means of a crank, turns it round, and thus from an up-and-down motion we obtain a circular motion.

If, on the other hand, we have a wheel turning round, to which we attach by a crank the rod of a sucker of a pump, the sucker will be moved up and down as the wheel turns round. Thus, from a circular motion we obtain an up-and-down motion.

We must observe that though we can move weights by the mechanical powers, we can only do so slowly. A great block of stone raised by a lever is moved much more slowly than the hand which moves it. I can by means of a movable pulley raise twice the weight which I can raise without it, but then I raise it twice as slowly. And so of the rest. It is commonly said, what you gain in power you lose in speed. But then in many works the speed is of no consequence compared with the power. For instance, we often see a great package, which ten men could not lift, drawn, slowly to an upper warehouse-room by means of a crane. One man does this easily, and if we look at him we see him turning a handle round very quickly. This moves a small toothed-wheel, and that a much greater one, so the package is raised; and although it moves slowly, that is of no conse

quence.

LESSON 82.

THE VILLAGE BLACKSMITH.

UNDER a spreading chestnut-tree
The village smithy stands;
The smith, a mighty man is he,
With large and sinewy hands;

And the muscles of his brawny arms

As strong as iron bands.

His hair is crisp, and black, and long,
His face is like the tan;

His brow is wet with honest sweat,
He earns whate'er he can,

And looks the whole world in the face,
For he owes not any man.'

Week in, week out, from morn till night,
You can hear his bellows blow;
You can hear him swing his heavy sledge,
With measured beat and slow,

Like a sexton ringing the village bell
When the evening sun is low.

And children coming home from school
Look in at the open door;

They love to see the flaming forge,
And hear the bellows roar,

And catch the burning sparks that fly
Like chaff from a threshing-floor.

He goes on Sunday to the church,
And sits among his boys;

He hears the parson pray and preach,
He hears his daughter's voice

Singing in the village choir,

And it makes his heart rejoice:

* He owes not any man.] He owes nothing to any man.

It sounds to him like her mother's voice,
Singing in Paradise!

He needs must think of her once more,
How in the grave she lies;

And with his hard, rough hand he wipes
A tear out of his eyes.

Toiling, rejoicing, sorrowing,
Onward through life he goes;
Each morning sees some task begin,
Each evening sees it close;
Something attempted, something done,
Has earned a night's repose.

Thanks, thanks to thee, my worthy friend,
For the lesson thou hast taught!

Thus at the flaming forge of life

Our fortunes must be wrought!

Thus on its sounding anvil shaped
Each burning deed and thought!

LONGFELLOW.

LESSON 83.

GRAVITY.

ONE great use of machines is to help us to raise weights. Perhaps you have never thought much about the reason why things are heavy. Newton, a great philosopher, once observed an apple fall from a tree, and he set about asking himself why an apple falls to the ground. This was not the only apple which Newton himself had seen fall, and apples had been falling ever since the creation. It was one of the commonest sights in the world. But when a very

clever man began to think of the reason why it fell, he found out more and more about it; and by starting from this point, he at last arrived at the knowledge of the way in which the planets move round the sun.

I do not intend to say anything now about the motion of the heavenly bodies, but will only point out a few things which occur every day before our eyes. If you stand over a well and let a stone fall from your hand, it will move quickly to the bottom. If you throw a stone upwards, it will ascend every instant more slowly, until at last it ceases to ascend, and begins to descend. It will then come down through the air just as the stone went down to the bottom of the well. If you throw a stone at some mark at a distance, it will always at last come to the ground. If you try to lift a large stone lying on the earth, it will be difficult to do so; it will be just as if somebody were pulling against you.

We discover then that there is some power which is constantly drawing bodies to the ground. This is called the power of gravity or of attraction, that is, the power of drawing to.

If we fasten a weight to a string, and hold the string in our hand, the string will hang down in a straight line; such a line is called vertical.

The plane to which this line is perpendicular is called horizontal. The surface of standing water is horizontal.

Gravity always pulls bodies down to the ground in a vertical direction.

The reason why arrows or bullets fall in different directions is, that the bow or gun sends them in one direction, and gravity pulls them in another, so they

fly in some direction between the two, till they at last are pulled down to the earth.

All bodies of the same size are not of the same weight. This is because the particles are not so close in one body as in another. A ball of lead is much heavier than the same-sized ball of cork. But if we take two balls of lead, one of which is twice as large as the other, it will require twice as much force to hold up the larger as it does to hold up the less ball.

If we find a ball of wood of such a kind that it will just balance two balls of cork of the same size, then we shall find it will take exactly twice the force to hold up the wooden ball as it does to hold up the one cork ball.

So the weight depends upon two things, (1) the size of the body, (2) the closeness with which the particles are packed. This last is called its density.

The mass of a body depends upon its size and density together. The mass of the wooden ball just mentioned is twice as great as the mass of the cork ball: the size is the same, but the density is twice as great.

The mass of the larger of the two leaden balls is twice as great as the mass of the less. The density is the same, but the size is twice as great.

It may at first sight appear that this force of attraction, or gravity, causes a great deal of trouble. We are constantly employed in raising weights, and find it often laborious work.

us.

But we shall soon see that gravity is very useful to Indeed we could not remain upon the ground but for this, for it is our weight that prevents our being blown about like feathers.