Why is steaming indispensable for ship-building? Because the planks cannot be otherwise curved or twisted, as in the bends of the hull of the vessel.

To give an idea of the enormous quantity of timber necessary to construct a ship of war, we may observe, that 2,000 tons, or 3,000 loads, are computed to be required for a seventy-four. Now, reckoning fifty oaks to the acre, of 100 years standing, and the quantity in each tree at a load and a half, it would require forty acres of oak-forest to build one seventy-four; and the quantity increases in a great ratio, for the largest class of line-of-battle-ships. A first-rate man-of-war requires about 60,000 cubic feet of timber, and uses 180,000 pounds of rough hemp, in the cordage and sails for it. The average duration of these vast machines, when employed, is computed to be fourteen years. It is supposed, that all the oaks now in Scotland, would not build two ships of the line. In Sweden, all the oak belongs to the king, or the proprietors of estates can only dispose of it to government; so that, when not wanted for the navy, it is often left to decay, and indeed, is generally much neglected.

Why is teak wood superior to‘oak?

Because it is stronger and more buoyant. Its durability is more decided; and, unlike the oak, it may be put in use almost green from the forest, without danger of wet or dry rot. The oak contains an acid which corrodes and destroys iron; the teak, on the contrary, possesses an essential oil which preserves iron.

Why are beech and elm good timber for the lower keels of ships, and the piles of bridges and harbours?

Because both, when under water, are extremely durable; though neither stand the effects of the atmosphere.

Why is fir preferable to oak for common building?

Because it is lighter, far more elastic, more easily worked, straighter, and of much greater length. The

best that comes in the form of deals, is from Christiana and Frederickstadt, chiefly on account of the vast superiority of the saw-mills there.

Why was chestnut used in ancient roofs ?

Because of its lightness and durability. The largest roof of the ancient construction is that of Westminster Hall, which is of chestnut. The support of every piece of timber is apparent; and the only strain which appears directly across the timber is on the boards and rafters between the great trusses; and it does not appear to be in the least decayed, although constructed four hundred and fifty years since.

COALS AND GUNPOWDER.

Why are coals so productive of grand mechanical effects?

Because of their great hidden powers, which we can at pleasure call into action. Thus, it is well known to modern engineers, that there is virtue in a bushel of coals, properly consumed, to raise seventy millions of pounds weight a foot high. This actually is the average effect of an engine at this moment working in Cornwall. The Menai Bridge, one of the most stupendous works of art that has been raised by man in modern ages, consists of a mass of iron not less than four millions of pounds in weight, suspended at a medium height of about 120 feet above the sea. The consumption of seven bushels of coal would suffice to raise it to the place where it hangs.

The great pyramid of Egypt is composed of granite. It is 700 feet in the side of its base, and 500 in perpendicular height, and stands on eleven acres of ground. Its weight is, therefore, 12,760 millions of pounds, at a medium height of 125 feet; consequently, it could be raised by the effort of about 630 chaldrons of coal, a quantity consumed in some foundries in a week.J. F. Herschel.

Why is gunpowder another important source of mechanical power?

Because of the tremendous force which it exercises in certain operations, as blasting rocks, &c. in the progress of mechanical works. Thus, in the progress of cutting the Delaware Canal, four kegs of gunpowder, containing about 100lb. were, in 1829, used for a single blast, and had the effect of rending in pieces more than 400 cubic yards of rock.*

Yet it is only when we endeavour to confine gunpowder, that we get a full conception of the immense energy of that astonishing agent. In Count Rumford's experiments, twenty-eight grains of powder in a small cylindrical space which was just filled, tore asunder a piece of iron which would have resisted a strain of 400,000 pounds, applied at no greater mechanical disadvantage.

BALANCES.

Why are we enabled to determine the relative weight of a body, compared with the weight of another body, assumed as a standard, by means of the balance?

Because the balance consists of an inflexible rod or lever, called the beam, furnished with three axes; one, the fulcrum, or centre of motion. situated in the middle, upon which the beam turns, and the other two near the extremities, and at equal distances from the middle. These last are called the points of support, and serve to sustain the pairs or scales. These points and the fulcrum are in the same right line, and the centre of gravity of the whole should be a little below the fulcrum, when the position of the beam is horizontal. The arms of the lever being equal, it follows,

* By way of parallel with this effect, though produced by different means, we may mention that in 1825 there was opened in CochinChina, a canal twenty-three miles long, eighty feet wide, and twelve feet deep. It was begun and finished in six weeks, although carried through large forests, and over extensive marshes. Twenty thousand men were at work upon it day and night; and it is said that seven thousand died of fatigue

that if equal weights be put into the scales, no effect will be produced on the position of the balance, and the beam will remain horizontal. If a small addition be made to the weight in one of the scales, the horizontality of the beams will be disturbed; and, after oscillating for some time, it will, on attaining a state of rest, form an angle with the horizon, the extent of which is a measure of the delicacy or sensibility of the balance.

Why should not the weights of a balance be touched by the hand?

Because that would not only oxydate the weight, (or cause it to rust) but by raising its temperature, it would appear lighter when placed in the scale-pan, than it should do, in consequence of the ascent of the heated air. For the large weights, a wooden fork or tongs should be employed; and for the smaller, a pair of forceps made of copper; this metal possessing sufficient elasticity to open the forceps on their being released from pressure, and yet not opposing a resistance sufficient to interfere with that delicacy of touch, which is desirable in such operations.-Kater.

Why does one weight alone serve to determine a great variety of others, by the steelyard?

Because the steelyard is a lever, having unequal arms, and by sliding the weight along the longer arm of the lever, we thus vary its distance from the fulcrum, taken in a reverse order; consequently, when a constant weight is used, and an equilibrium established, by sliding this weight on the longer arm of the lever, the relative weight of the substance weighed, to the constant weight, will be in the same proportion as the distance of the constant weight from the fulcrum is to the length of the shorter arm.

Why is the spring steelyard in very general use? Because of its portability; as a spring that will ascertain weights from one pound to fifty, is contained

in a cylinder only 4 inches long, and inch diameter. To use this instrument, the substance to be weighed is suspended by a hook, the instrument being held by a ring passing through the rod at the other end. The spring then suffers a compression, proportionate to the weight, and the number of pounds is indicated by the division on the rod, which is cut by the top of the cylindrical tube.—Kater.

The dial weighing machine is a modification of the same principle, connected with hands on a dial or clock-face to denote the weight.

WATER.

Why do water-wheels vary in their construction? Because of the different ways in which the mechanical force of the liquid is intended to be applied.

Why are certain of these wheels called overshot?

Because the water by which they are impelled descends from its level to a lower one; its weight during the descent (falling, as it were, over the wheel) causing the wheel to turn. That this may be possible, it is only necessary that there should be a sufficient supply of water at the superior level, and that there should be a means of carrying it off after its descent, so as to prevent by its accumulation, the equalisation of the two levels. Hence the necessity of flood-gates in a mill course. On the circumference of the wheel the weight of the water is made to act in its descent, in a direction as nearly as possible at the right angles to the spokes, or radii; this pressure, however, acting only at one side of the wheel; thus making the whee revolve, and communicate motion to its axis; and this motion being transmitted by wheel-work, and other contrivances, to the machinery which it is required to work.

Why are other wheels called undershot?

Because the flat or float boards placed at equal distances on the rim, and projecting from it, in direc