springs are moved, in correspondence with the irregularities, while all above, by the inertia of the matter, have a soft and steady advance.—Arnott.

Again, springs of carriages convert all percussion into mere increase of pressure: that is to say, the collision of two hard bodies is changed by the interposition of one that is elastic, into a mere accession of weight. It is probable, that under certain modifications, springs may be applied with great advantage to the heaviest waggons.

In surmounting obstacles, a carriage with its load being lifted over, the springs allow the wheels to rise, while the weights suspended on them are scarcely moved from their horizontal level.

Why are "under-springs" so advantageous in very modern carriages?

Because they insulate from the effects of shocks, all the parts, excepting the wheels and axletrees themselves. When only the body of the carriage is on springs, the horses have still to rattle the heavy frame-work below it, over all irregularities.

Why, in descending a hilly road, is it common to lock or fix one of the wheels of a carriage?

Because the friction is then increased, and there is less chance of a rapid descent; the horses having then to pull nearly as much as on a level road, with the wheel free.

We have noticed a very effectual mode of “locking " the hind wheels of carriages, on the continent, by screwing a bar transversely, against the outer rim of the wheels; by this means, the wheels may be either partially or wholly locked, according to a powerful screw, in the centre of the bar. This mode is adopted by the Paris diligences; we first noticed it in a Swiss calêche, of great strength. The bar is rather unsightly, but our excellence in the construction of wheelcarriages should not lead us always to look for ele

gance, where convenience is a main point, as in a vehicle for travelling.

Why should a road up a very steep hill, be made to wind or zig-zag all the way?

Because, to reach a given height, the ease of the pull is greater, exactly as the road is made longer.

Why is it important to make roads as level as possible?

Because, a horse drawing on a road where there is a rise of one foot in twenty, is really lifting one twentieth of the load, as well as overcoming the friction, and other resistance of the carriage.—Arnott.

THE WEDGE.

Why are cutting instruments, knives, razors, the axe, &c. examples of the wedge.

Because at the same time that we pull them lengthwise, we press them directly forward, against the object. a saw, too, is a series of wedges.

Why does a razor, (if drawn lightly over the hand) dart into the flesh; whereas, if pressed against the hand with considerable force, it will not enter?

Because of the vibration of particles produced by the drawing action, which enables the razor to insinuate itself more easily. We witnessed an example, only a few days since, when a bon vivant, in a fit of mischievous extacy, seized a pointless table knife, and passed it very lightly down the back of his friend's coat. The injury was not immediately seen, but the cloth proved cut, from the collar to the waist; whereas, had the knife been heavily pressed against the cloth, the coat would have escaped injury, and the gay fellow the expense of his folly.

Why is the wedge so important an agent in the arts and manufactures?

Because it exerts enormous force through a very small space. Thus, it is resorted to for splitting masses

of timber, or stones. Ships are raised in docks, by wedges driven under their keels. The wedge is the principal agent in the oil-mill. The seeds, from which the oil is to be extracted, are introduced into hairbags, and placed between planes of hard-wood. Wedges inserted between the bags, are driven, by allowing heavy beams to fall on them. The pressure thus excited is so intense, that the seeds in the bags are formed into a mass nearly as solid as wood.-Lardner.

The details of an extensive oil mill near Garrat are as follow :-A magnificent water-wheel, of 30 feet, turns a main shaft, which gives motion to a pair of vertical stones, raises the driving-beams, and turns a band, which carries the seed in small buckets from the floor to the hopper. The shock on the entire nervous system, produced by the noise of the driving-beams as they fall on the wedges, is not to be described. The sense of hearing for the time is wholly destroyed, and the powers of voice and articulation are vainly exerted. The noise is oppressive, though a rebound, comparatively tuneful, takes place, till the wedge is driven home; but afterwards the blows fall dead, and produce a painful jar on the nerves, affecting the auditor for some hours with a sense of general lassitude.

THE SCREW.

Why does a screw enable a small force to produce such prodigious effects?

Because every turn of the screw carries it forward in a fixed nut, or draws a moveable nut along upon it, by exactly the distance between two turns of its thread this distance, therefore, is the space described by the resistance, while the force moves in the circumference of the circle described by the handle of the screw; and the disparity between these lengths or spaces is often as a hundred or more to one.—Ärnott.

Why may the screw be called a winding wedge?

Because it has the same relation to a straight