half of the entire length of the rope, by continually yielding rope on the one side, and depressing the bucket or chain by his weight on the other. In this case the pulley must be attached to some part of the building, or it is recommended that each chamberfloor of a dwelling-house should have a staple fixed near the exterior of a window, to which staple the pulley may be attached by a hook. This is, perhaps, the simplest fire-escape yet proposed, and we need scarcely add, the simpler the means the more likely is it to succeed in extreme danger.

FRICTION.

Why is the friction greater between pieces of the same substance, than between pieces of different substances, with dissimilar grains?

Because, it is supposed, of the roughnesses, or little projections in the former, mutually fitting each other, as the teeth of similar saws would.

"But for friction," observes Dr. Arnott, "men walking on the ground or pavement would always be as if walking on ice; and our rivers, that now flow so calmly, would all be frightful torrents."

Why does the friction of various woods against each other vary?

Because of their different degrees of hardness; the soft woods in general giving more resistance than the hard woods; thus, yellow deal affords the greatest, and red teak the least friction. Soft metals also produce greater friction, under similar circumstances, than those which are hard.-G. Rennie.

Why is the friction of surfaces, when first brought into contact, often greater than after their attrition has been continued a certain time?

Because the smoother the surfaces are the less will be the friction, and that process has a tendency to remove those minute asperities and projections on which the friction depends. But this has a limit, and after

a certain degree of attrition the friction ceases to decrease.

Why does smearing the surfaces with unctuous matter diminish the friction?

Because it fills up the cavities between the minute projections which produce the friction.

Why has plumbago, or black lead, been substituted for oil in clocks and chronometers?

Because, when mixed with spirit, it readily adheres to the surface of a steel pivot, as well as to the inside of the hole in which it runs, so that the rubbing surfaces are no longer one metal upon another, but plumbago upon plumbago. These surfaces, by their mutual action, speedily acquire a polish inferior only to that of the diamond, and then the retardation of the machine from friction is reduced almost to nothing, and wear and tear from this cause is totally prevented.

Why are jewelled holes injurious to the pivots of watches and chronometers?

Because, sooner or later, however perfect the polishing may be, the hard substance of the jewel grinds and cuts the steel pivot, and the metallic particles clog the oil.

Why is a peculiar metal requisite for pivot-holes?

Because it must preserve the oil in a fluid state, have little friction with the steel pivot, and be in a degree softer than the pivot, for it is of less consequence that the hole be worn than the pivot. Brass is objectionable, on account of its liability to rust, and gold is too soft for the purpose. Now, an alloy possessing the above requisites has lately been discovered by Mr. Bennett, watchmaker, of Holborn. It consists of pure gold, silver, copper, and palladium, and its small expense, compared with that of jewels, is not its least recommendation.

STRENGTH OF MATERIALS.

Why is a hollow tube of metal stronger than the same quantity of metal as a solid rod?

Because its substance, standing further from the centre, resists with a larger lever. Hence, pillars of cast-iron are generally made hollow, that they may have strength, with as little metal as possible. Masts and yards for ships have been made hollow, in accordance with the same principle.

Why does a plank bend and break_more_readily than a beam, and a beam resting on its edges, bear a greater weight than if resting on its side?

Because the resisting lever is smaller in proportion as the beam is thinner. Where a single beam cannot be found deep enough to have the strength required in any particular case, as for supporting the roof of a house, several beams are joined together, and in a great variety of ways, as is seen in house-rafters, &c. which, although consisting of three or more pieces, may be considered as one very broad beam, with those parts cut out which do not contribute much to the strength.-Arnott.

Why is a beam, when bent by its weight in the middle, very liable to break?

Because the destroying force acts by the long lever, reaching from the end of the beam to the centre, and the resisting force or strength acts only by the short lever, from the side to the centre; while only a little of the substance of the beam on the under side is allowed to resist at all. This last circumstance is so remarkable, that the scratch of a pin on the under side of a plank, resting as here supposed, will sometimes suffice to begin the fracture.-Arnott.

Why is a suspension bridge more economical than an ordinary, or insistent bridge?

Because a suspension-bridge varies its curve so as to adapt it to any variation or partial excess in its

load, in consequence of which, the strength of the chains may, with great precision, be adjusted to any required strain, and no more: while, in insistent bridges, the liability of the arch to a fatal derangement of its form by partial or excessive pressure, requires an enormous increase of weight and strength, beyond what is requisite for the mere support of its load, supposing it to be uniformly distributed.-Ainger.

Why is iron admirably adapted for the construction of suspension bridges?

Because the greater part of the weight of these bridges arises from the chains themselves, wherefore, the best material for the purpose, is that which has great tenacity with small weight; and iron is at the same time the most tenacious, and, excepting tin, the lightest of the common metals. A square inch of good iron requires about 25 tons to separate it, and it will not be stretched or otherwise affected, with less than half that weight. Rope bridges, have, however, been introduced, with the advantages of economy and portability, into British India, where a rope-bridge, 160 feet in length, is so light and portable, that it has been several times set up and removed in a few hours.—Ainger.

We may here mention, that Mr. Bevan has found the strength or cohesion of cast-iron, to be upwards of 30,000 pounds to the square-inch, though much depends upon the mode of applying the force.

Why is iron best cemented by cast-iron?

Because pure iron, when surrounded by and in contact with cast-iron turnings, and heated, is carbonized very rapidly, so as to exhibit all the properties of steel.

Why is heated air now used in smelting iron?

Because it requires but three-fourths of the quantity of coal requisite, when cold air, that is, air not artificially heated, is employed for that purpose; while the produce of the furnace in iron, is at the same time

greatly increased. It is supposed, that this improvement will accomplish a saving in the cost of the iron, in Great Britain, to the amount of at least 200,000l. a-year. Jameson.

Why are piles for bridge-building, driven by great weights being suddenly let fall on them?

Because the body of the workman being too weak, to give a forcible downward push directly, he employs a certain time in carrying a weight up to such an elevation above his work, that when let fall, its momentum may do what is required. Here the continued efforts of the man in lifting the weight, to a height of perhaps thirty feet, may be just sufficient to sink into the earth one inch; and the continuance has, therefore, balanced forces, which are to each other in` intensity, as thirty feet to an inch.-Arnott.

Why does an ill-built bridge generally flatten in the arch? Because the builder has not sufficiently attended to the effect of the horizontal thrust of the arch on its piers. Each arch is an engine of oblique force, pushing the pier away from it. In some instances, one arch of a bridge falling, has allowed the adjoining piers to be pushed down towards it, by the thrust, no longer balanced, of the arches beyond; and the whole structure has given way at once, like a child's bridge, built of cards.-Arnott.

The principle of bridge-building is beautifully illustrated by the small toy-models; the stones being represented by separate pieces of wood, which the juvenile architect is required to form into an arch, or arches. It could be wished that the above and such scientific toys were better appreciated in England. They seem only to suit the caprice of the moment. Thus, the Chinese, Indian, and other puzzles, were but the favourites of a year, and Dr. Brewster's splendid kaleidoscope was less understood, and more abused, than any modern discovery.