The application of the tooth and pinion is exempli fied in the representation of a rice-mill moved by water, at page 37 of Barrow's Travels. They seem to understand, in practice at least, that power and velocity vary inversely in machinery; as, for instance, that power is gained, or time, according as the moving force is applied either to the circumference or the axis of a wheel.

It is remarkable that they should seem always to have possessed that particular application of the principle of the wheel and axle, by which the greatest

power is attained within the least space; and, at the same time, with the greatest simplicity, as well as strength, of machinery. The cylinder ab consists of two parts, of unequal diameter, with a rope coiled round both parts in the same direction, the weight to be moved being suspended by a pulley in the middle. Every turn of the cylinder raises a portion of the rope equal to the circumference of the thicker part, but at the same time lets down a portion equal to the circumference of the thinner; and, as the weight is suspended by a pulley, it rises at each turn through a space equal to only half the difference between the span of the thicker and thinner parts of the cylinder. The action of the machine, therefore, is very slow; but the mechanical advantage is great in proportion, or, in other words, power is gained at the expense of velocity," according to an invariable law of mechanics.

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The over-shot water-wheel is used commonly in corn-mills, wherever the nature of the country affords streams available for the purpose. In cottages, a domestic mill was frequently seen by our embassies, composed of two circular stones put in motion by a single man or boy, or sometimes an ass or mule, the power being applied at the end of a lever fixed in the uppermost stone.

The juice of the sugar-cane is expressed in mills similar to those used in India, according to the description of Dr. Buchanan. It consists of two upright cylinders, which are put in motion by a buffalo yoked to a beam passing from the top of one cylinder. The mill is fed by introducing the cane between the rollers, by which it is crushed and carried over to the other side. The expressed juice runs through a channel below into a large reservoir, whence it is transferred to boilers, and, being there sufficiently inspissated, is sent in tubs to the refiners. In the above instance the mechanism might be evidently economized and improved by causing the cylinder, which communicates motion, to turn two others instead of only one: this is said to be the practice in our West Indian colonies.

The Chinese excel in their contrivances for raising water in the irrigation of their lands, and it is probable that these inventions are nearly as old as their husbandry itself. One of them is an ingenious species of chain-pump, which we give here as it is well described and figured in Staunton's Embassy.* The pump consists, in the first place, of a hollow trough or trunk, of a square make." Flat and square pieces of wood, corresponding exactly to the dimensions of the cavity of the trunk, are fixed to a (jointed) chain, which turns over a roller or small wheel placed at each extremity of the trunk. The square pieces * Vol. ii. p. 480.

of wood fixed to the chain move with it round the rollers, and lift up a volume of water equal to the dimensions of the hollow trunk. The power used in working this machine is applicable in three different ways: if the machine be intended to lift a great quantity of water, several sets of wooden arms are made to project from various parts of the lengthened axis of the roller, over which the chain and lifters turn. These arms are shaped like the letter T, and made round and smooth for the foot to rest upon.