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What is it that enables us to build walls that will stand firm? We lay one course of bricks or stones upon another, always taking care that each course is quite horizontal. This we discover by the use of the plumb-line. Then the force of gravity draws down the bricks upon each other, and the more bricks there are the more strongly they press. So the wall cannot easily be stirred, and the strong winds do it no hurt. The mortar would not hold the bricks together if it were not for their own weight; but cottages are sometimes built of blocks of stone without any mortar or cement at all, and the walls are kept quite firm by the weight of the stones.
A simple experiment will show you how gravity acts in this respect. Lay five or six books on each other upon a table. You will not easily draw a lower one out, because the upper ones press upon it, and you may give a good push to the pile without disturbing it; but if you set the books up side by side, you can draw any one out with perfect ease.
Gravity, too, is of use even in moving weights; for if we press against the rope by which we draw them, we make our own weight help us. This is why we can pull down with more force than we can push upwards; and large stout horses are chosen for drays, not because these are always the strongest, but because they are the heaviest, and so by pressing their weight forwards they can move very heavy waggons.
If we try to pull a heavy body along a level road we shall not find it very easy to do so. And the rougher the road the harder our task will be. The same body which we could scarcely move upon the ground, can often be pushed along upon a frozen poħd with the greatest ease. This is because the ice is so much smoother than the ground; and we soon discover that our difficulty in dragging a body upon level ground arises from the rubbing together of the two surfaces. This is called friction, and is sometimes so great, that it is almost as troublesome to draw a body along the ground as to carry it in our arms.
In every machine there is a loss of power by friction, in consequence of the parts of the machine rubbing together.
The simplest case is that of the inclined plane. Travellers have often reason to complain of their trunks being rubbed by the use of such a board as has been described in a former lesson. But if a cask is rolled by cords up the plane, as in fig. p. 247, the friction is in this case avoided.
When a lever rests upon a fulcrum, as in fig. p. 242, the friction helps to keep it in its place; but if, as in a pump-handle, the fulcrum is a pivot, there is a 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, as 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 cartgrease. The friction at the axles of a carriage is sometimes so great as to set the axle-trees on firebecause 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.
In France, the usual way of preventing a carriage from going down hill, is by the use of a break of this kind, which is pressed upon the wheel by a handle in the driver's seat. The horses commonly used in that country are well trained, and not so spirited as our English carriage horses, so they can safely be left more to themselves, and do not require to be so carefully held back by the reins as our horses do. It is very amusing to see the driver when he comes to the top of a steep hill, hang his reins on a hook by his side, and seize the break handle with both his hands, using all his force to check the motion of the carriage in this way, and leaving his horses to regulate their own pace for themselves.
LINES ON A COMMON FIELD DAISY,
WHICH CAME UP IN THE GARDEN OF THE REV. DR. CAREY, AT
THRICE welcome, little English flower!
Never to me such beauty spread :
Thrice welcome, little English flower!
Thrice welcome, little English flower!
* Embryo.] Germ. The first beginning of a bud.
Resplendent hemisphere.] Brilliant country. The word hemisphere," which means properly "half the globe," is here inaccurately used.
§ Flora's giant offspring.] Large flowers. The flowers and plants in tropical regions are very large and brilliant.