Egyptian rulers, whose plan to prevent the evils of over-populousness, was, to accustom the lower orders to a spare diet, and employ them in the construction of huge edifices, destined for tombs, or the temples of religion. Hence, the pyramids and excavated temples, which still excite the wonder of the world, and prove what may be effected by the aid of the simplest machinery, with time, numbers, and perseverance.— Belzoni.

Why do the more ancient Egyptian monuments exceed the later in design and execution?

Because, among the Egyptians, every thing advanced to a certain point of perfection;-there stopped, never to advance, but rather to recede.—Belzoni.

Why are light-houses built of a circular form?

Because, partaking of the properties of the arch, it best enables them to withstand the fury of tempests, from every quarter. The Eddystone light-house, built by Mr. Smeaton, the English engineer, is a splendid triumph of this principle.

Why were mirrors first used for reflecting lighthouses?

Because of the following trivial circumstance.

At a meeting of a society of mathematicians, at Liverpool, one of the members proposed to lay a wager, that he would read a paragraph of a newspaper, at ten yards distance, with the light of a farthing candle. The wager was laid, and the proposer covered the inside of a wooden dish with pieces of looking-glass, fastened in with glazier's putty,—placed his reflector behind the candle, and won his wager. One of the company marked this experiment with a philosophic eye. This was Capt. Hutchinson, the Dock-master, with whom originated the Reflecting Light-houses, erected at Liverpool, in 1763.

The revolving lights, as at Calais, are an improvement upon this invention. Lieutenant Drummond's

ingenious application of ignited lime to the illumination of light-houses, a brilliant discovery of the present day, has been already noticed.*

Why is hempen-rope preferable to iron-chain for the scale of a weighing beam?

Because the rope resists a greater weight falling into the scale than is resisted by the chain, and is altogether stronger than the chain: the hemp yields by its elasticity, and continues its resistance through a considerable space and time, and thus at last gradually overcomes the momentum; while the iron, by not yielding, either requires to be strong enough to stop the mass suddenly, or breaks.

Why are chain cables stronger than those of hemp or rope??

Because the chain, by its weight, hangs as a curve or inverted arch in the water, while the rope being nearly of the weight of water, is supported by it, and becomes almost a straight line from the anchor to the ship; and when a great wave dashes against the ship, the straight rope can only yield by the elasticity of its material, and, comparatively, therefore, a little way; but the bent chain will yield until it be drawn nearly straight, and by this greater latitude of yielding, and consequent length of resistance, it will stand a greater shock.-Arnott.

Why is British oak more durable than that of North America?

Because variable weather, as in Britain, conduces to firmness, whereas, the hot summers of North America impoverish its growth.

Why is steaming prejudicial to timber ?

Because the heat and moisture together, always weaken that constituent principle of the timber, upon which its strength and durability in a great measure depend.

* See POPULAR CHEMISTRY, page 50.

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 chesnut 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 chesnut. 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 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 1001b. 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.