observed, passes four times over them, and the resistance capable of being overcome is as 4 to 1; that is, if a power of 100 lbs. was applied, it would be equal to a weight of 400 lbs. to be raised. Each fold of the rope bears a fourth of the weight, the last being the power applied; that over the top fixed pulley is of no more use than to aid the person pulling the rope to give it a proper direction.

Thus, then, an increase of pulleys decreases the weight, and allows a smaller power to overcome a larger; still it is always at a loss of time and space.

The ropes used about a pulley are called tackle, and the pulleys blocks; therefore when a sailor or workman collects together all necessary for the application of this machine he speaks of the block and tackle.

Fig. 16.

"Mechanics and Mechanism," by R. S. BURN.

1. GROOVE, a furrow; that which is graven or hollowed out.

THE INCLINED PLANE.

THIS is another of the primary mechanical powers, and is of use to man in many of his daily occupations of raising or lowering weights short distances, as it gives to a small power facility in overcoming a larger.

If a cask be on a flat surface or plane, it will be at rest

on any part of it where it may be laid; but had a man

to lift it on to a cart

(fig. 17), he would have

the

to apply a power equal to its weight to prevent its falling upon ground. Were he, however, to place a plank up to the bottom of the cart, he then makes an inclined plane, and he would only have a part of the weight of the cask then to support (fig. 18). Or had the man to load the cart with casks, he might have to lift them from the ground perhaps 4 feet; but by placing a plank 8 feet long, and forming an inclined plane, he can roll them

Fig. 17.

up

Fig. 18.

with one-half the power he would have to exert when lifting them; yet he would be double the time, as the space would be twice that of the height. An inclined plane, then, is seen to be a slope, and according to its

height will the time be of a body in rolling down it. Thus, if it be 16 feet high at one end, and its length be 32 feet, a cannon-ball or cylinder will, by the force of gravity, fall through the 16 feet in 1 second, but to roll down the incline it would take 2 seconds; if it were 64 feet high, and the inclined plane were twice 64, or 128 feet long, then a ball would fall through the space or height in 2 seconds, but would take twice two, or 4 seconds, to roll down the incline.

Thus this mechanical power is in proportion as the length of the plane exceeds its height; and if a cask weighing 3 cwt. had to be rolled into a cart or part of a warehouse 4 feet high, and a plank 12 feet long was used, then a power of 1 cwt. would balance it, because the inclined plane is three times the perpendi cular height. A slight power over the hundredweight would move the cask onward.

If a loaded cart, omnibus, or coach, on a plane at the bottom of a hill, had a plummet-line hung from the top, it would fall straight to the ground; but as the vehicle moved up the hill, the steeper it became the more the plummet would fall towards the back of the conveyance, and the heavier the load would become to the horse, increasing the difficulty to the animal in dragging it up

the hill.

If the rise be 1 foot in 20 on a road, the horse has to lift the 1-20th of the load, as well as to overcome the friction and gravity; because in 20 feet the load has to be raised up a height of 1 foot, and the weight to be overcome at any part of the 20 feet is the twentieth of what it would be if raised that height at once, being gradually lifted, as it were, the twentieth part at a time over the 20 feet. The greater or less the slope, the greater or less power is required to overcome the resistance. It is this

reason that causes drivers, on ascending steep hills, to wind from side to side, by which the incline is made less.

On railways, a locomotive engine can draw a train and 700 persons 22 miles an hour, up an incline of 3 inches in every 8 feet; but were the incline 1 foot in 12 feet of length, then the engine could not move forward.

In the coal districts the inclined plane is of common occurrence on the railways. It is a curious sight to see twenty loaded waggons, set off from the top of a hill, rushing down towards a river, without anything but a rope attached to the last one; this rope is attached to a small wheel or drum, and while the loaded waggons descend by their weight and velocity, empty ones on a parallel line are drawn upwards.

In building houses an inclined plane is often used for the easy transit of wheel-barrows; and it is believed that the ancients, in erecting their immense works of art, used inclines formed of mounds of earth.

The inclined plane is beautifully illustrated in that exciting and pleasing sight, the launch of a ship. Whether the destiny of the splendid triumph of man's handicraft be war or commerce, still it strikes with awe, wonder, and gratification to see it move majestically down the sloped ways, breast all opposition, and then settle buoyantly and calmly on the surface of the

waters.

To the drayman, in unloading ponderous hogsheads, the inclined plane is of great use; and again, when he drags the empty butts from the cellar, he places down a plank or two, puts a hook into the bung-hole at the end, fixes a rope to one of his horses, which he drives on, and up pops the barrel.

The stairs of a dwelling-house are in principle an

inclined plane, having steps to allow of a footing. This forcibly struck us once on seeing a Highlandman who had never been in any other habitation than a cabin. He mounted the stairs well enough; but when about to return, after looking at them for a moment, he sat down, and descended as we would a steep declivity having footholds cut in it.

When roads are made to the tops of high hills, they are either wound round and round, or made so broad as to allow of tacking from side to side.

Chisels, adzes, and other tools which are sloped only on one side, are in principle inclined planes.

"Mechanics and Mechanism," by R. S. BURN.

THE WEDGE.

THE wedge (fig. 19) is in the form of two inclined

с

Fig. 19.

a

planes, abc and dbc, joined at their bases. It is used to rend wood, rocks, &c., also to raise heavy weights short distances, and compress substances closer together. More power is gained by striking the head of the wedge with a hammer, either small or large, than by pressure, as the momentum of the blow seems to shake the particles of matter, and cause them to separate. A thin wedge requires less power to move it forward than a thick one, less resistance being offered, as in the case of an inclined plane. The power of the wedge cannot be correctly estimated, as the force, number of blows, and incline have all to be taken into account.