wedge, that a road winding up a hill or tower has to a straight road of the same length and acclivity. Why is the screw, in some respects, a disadvantageous contrivance? Because it produces so much friction, as to consume a considerable part of the force used in working it. Why do mathematical instrument makers mark divisions on their work with the screw? Because it can easily be made with a hundred turns of its thread in the space of an inch, and at perfectly equal distances from each other. If we suppose such a screw to be pulling forward a plate of metal, or the edge of a circle, over which a sharp-pointed steel marker is placed, which moves up and down perpendicularly, the marker, if let down once for every turn of the screw, will make just as many lines on the plate; but, if made to mark at every hundredth or thousandth of a turn of the screw, which it will do with equal accuracy, it may draw a hundred thousand distinct lines in one inch. Why may a printing press be said to do the work of fifty men Because a solitary workman, with his screw or other engine, can press a sheet of paper against types, so as to take off a clear impression; to do which without the press, the direct push of fifty men would be insufficient; and these fifty men would be idle and superfluous, except just at the instant of pressing, which recurs only now and then. This, and the two preceding illustrations, are almost literally from Dr. Arnott's works, in which the importance of having correct notions on the subject of the simple machines, or mechanical powers, is illustrated by many other familiar examples. THE PULLEY. Why is the pulley an advantageous machine? the weight (let it be supposed ten pounds) is equally supported by each end of the rope, and that a man holding up one end, only bears half of it, or five pounds; but to raise the weight one foot, he must draw up the two feet of rope; therefore, with the pulley, he lifts five pounds two feet, where he would have to lift ten pounds one foot without the pulley. Why have fixed pullies no mechanical advantage? Because the weight just moves as fast as the power; yet such pullies are of great use in changing the direction of forces. A sailor, without moving from the deck of his ship, by means of such a pulley, may hoist the sail or the signal flag to the top of the loftiest mast. Why is the pulley on ship-board called a block? Because of the block or wooden mass which surrounds the wheel or wheels of the pulley. Hence the machinery for making these pullies is called blockmachinery. Of that at Portsmouth, invented by Brunel, there is a set of magnificent models in the possession of the Navy Board. They consist of eight separate machines, which work in succession, so as to begin and finish off a two-sheaved block four inches in length. Mr. Faraday, in a lecture at the Royal Institution in 1829, stated generally, that the block-machinery of Portsmouth, by adjustments, could manufacture blocks of one hundred different sizes; could, with thirty men, make one hundred per hour; and, from the time of its completion in 1804-5, to that day, had required no repairs from Maudslay, the original manufacturer. The total cost was £46,000, and the saving per annum, in time of war, was £25,000. after allowing interest for capital, and paying the expense of all repairs. Why is a chair or bucket, attached to one end of a rope which is carried over a fixed pulley, used as a fireescape? Because a person, by laying hold of the rope on the other side, may, at will, descend to a depth equal to 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 P 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, 66 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 betweenthe 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 |