they may be so called, produced by the revolution of the wheel, without any motion of the buckets themselves, well

deserves to be noticed.

Two hard wood posts or uprights are firmly fixed in the bed of the river, in a line perpendicular to its banks; these posts support the pivots of an axis of about ten feet long: this is the axis of a large wheel, consisting of two rims of unequal diameter, the rim which is nearest to the river's bank being fifteen inches less than the other; but both rims dip into the stream, and both rise above the trough or spout which receives the water and conveys it to the land to be irrigated.

The only materials employed in the construction of this water-wheel, except the axle and the two posts on which it rests, are afforded by the bamboo. The rims, the spokes, the ladle-boards or floats, the tubes or buckets, are made of entire lengths, or large pieces, or thin slices, or single joints, of bamboo; neither nails, pins, nor screws, nor any kind of metal, are used; the parts are firmly bound together by cordage of split bamboo or cane.

The wheel is thus framed,-sixteen or eighteen spokes are inserted obliquely into the axis near each extremity, and cross each other at about two-thirds of their length; the wheel is strengthened where the arms cross, by a concentric circle, and the ends of the arms or spokes are secured to the two rims. The spokes proceeding from the inner end of the axis next to the river's bank support the larger or outer rim, and those from the outer or inner end of the axis support the smaller rim, which is next to the land.

Between the rims and the crossing of the spokes is a triangular space, which is woven with a kind of close basketwork to serve as ladle-boards or floats; these, successively entering the stream, are impelled by the current, and turn round the wheel.

The buckets are tubes of bamboo, closed at one end and attached to the two rims of the wheel, and so fixed that when they are on a level with the axle, they have an inclination of twenty-five degrees to the horizon and to the wheel's axis.

The closed end of the tube is of course the lowest, and it is fastened to the outer and larger rim; the open end is attached to the smaller rim nearest the land. By this position, the buckets which dip into the stream as the wheel revolves, fill with water and rise with their mouths uppermost; their inclination gradually alters as they rise, but not

so much as to let their contents flow out until they reach the top, when they pour the water into a wide trough, from which it is distributed to the plantations.

These wheels are from twenty to forty feet in diameter, according to the height of the land on the river's bank and the consequent elevation to which the water must be raised.

Fig. 4.

CHINESE WHEEL.

A wheel of thirty feet carries twenty tubes or buckets, about four feet long and two inches inside diameter, each of them holding six-tenths of a gallon, or twelve gallons in the whole. With a stream of moderate velocity, the wheel will

make four revolutions in a minute and lift forty-eight gallons of water, or 2,880 gallons in an hour, or more than 69,000 gallons per day.

The position of the bucket as it rises to the level of the axle is shown at A. See figure.

Thus, at a very trifling expense, a machine may be constructed, which, without labour or attendance, will furnish a large and constant supply of water for agricultural purposes at a considerable elevation. There are many places in England where such means of irrigation might be used with advantage.

CHAPTER II.

THE NATURE AND PROPERTIES OF WATER.

HAVING thus briefly noticed the early use of water as a motive power, it may perhaps be well, before proceeding farther, to consider the nature and properties of water itself, so far as concerns the present treatise.

It was long held to be one of the four elements, and a hundred years have not yet passed away since it was proved that water is not a simple substance, but that it is composed of oxygen and hydrogen. The honour of announcing this fact appears to belong to James Watt; the researches of Dr. Black, and the experiments of Mr. Warltire, Mr. Cavendish, and Dr. Priestley, led him to this conclusion, which is stated in a letter dated April 26th, 1783, addressed by Mr. Watt to Dr. Priestley, through whom it was communicated to the Royal Society, who printed it in their Transactions.

The idea of the four elements must now be treated only as a beautiful allegory, a subject on which sculptors and artists may exercise their imagination and their skill.

According to Dr. Dalton, water consists of 8 parts by weight of oxygen and 1 of hydrogen in 9 of water. It exists in four states: as a solid in the form of ice, liquid as water, in the state of vapour as steam, and it exists also in combination with other bodies. Although it is, strictly speaking, in the liquid form alone that we have now to consider it, yet it may be noticed, as important to the

present subject, that water expands in bulk and decreases in density from a temperature of 39 degrees of Fahrenheit's thermometer up to 212, when it boils and evaporates into steam; that below 39 degrees it again expands and decreases in density down to 32 degrees, when it crystallises into ice.

It is seldom that this difference of density and volume is of much consequence in mechanical practice, yet it is most important as it respects the great operations of nature; the expansion of water in assuming the form of ice should never be forgotten by those who have to construct hydraulic works; its bulk is then increased in the proportion of 9 to 8, and the force with which it expands is so great that scarcely anything will resist it. The strongest pipes and vessels of iron are split, the heaviest weights are lifted, masonry and even rocks are rent asunder, when water having found its way through some small chink or opening, has frozen within the mass; and experiments have rendered it probable that a single cubic inch of water confined and frozen, exerts within the range of expansion a force equal to 13 tons. It is therefore absolutely necessary that care be taken either to prevent water from lodging or to provide means for allowing it to expand when it freezes.

By thus becoming lighter than water, ice floats upon the surface, and by exposure to the sun's rays is quickly melted; besides the warmest water also comes to the top and assists in melting the ice.

The earth, however, is a good non-conductor of heat, and in the temperate climate of England, the frost seldom reaches more than 10 or 12 inches below the surface of the ground in the coldest winter, so that water-pipes laid at the usual depth of two feet underground are sufficiently protected from freezing. The combination of water with other bodies, although a chemical operation, is most valuable to those whose business it is to build dams and sluices. If water be thrown on quick lime, nothing but a red heat will again separate them; plaster of Paris in the state of powder becomes solid when mixed with water. Barrow lime and other limes of similar character from the lias beds, and Roman cement, become solid and remain hard under water.

A cubic foot of rain-water at a mean temperature, when the barometer stands at 29.5 inches, weighs 1000 ounces avoirdupois, or 62 lb.; consequently water is generally taken as the standard point in tables of specific gravity, in

which the weight of equal bulks of other substances are compared with that of water in ounces to the cubic foot.

A cubic foot contains 1728 cubic inches, and the imperial standard gallon 277 274 inches; a pound avoirdupois weight of water contains 27.64 cubic inches; so that an imperial gallon may be taken to weigh 10 lb., and a cubic foot to contain 6 gallons.

A pipe of one inch in diameter, and one yard in length, contains 28.26 cubic inches, or a pound of water very nearly. Hence the following practical rule is generally used by millwrights to find the quantity of water in a pipe of any given diameter:

Square the diameter of the pipe in inches, and you have the weight of water in pounds per yard of the pipe's length; shift the decimal point one place to the left and you have the quantity of water per yard in gallons.

Thus if a pipe be 12 inches in diameter, 144 will express the weight of water it contains in pounds per yard long, and 14.4 the quantity in gallons, with sufficient exactness for any practical purpose in mill-work.

The force required to compress water, even to render the diminution of its bulk appreciable, is so great, that practically it may be regarded as incompressible, and consequently nonelastic. Hence it is that when a strong iron vessel, as the cylinder of a hydraulic press, is burst by overloading it, no explosion takes place; the breaking of the metal at once relieves it from a force so limited in its range of expansion, that the fragments do not fly asunder, but fall as if broken by mere weight.

The author witnessed the compressure of water by the apparatus of Mr. Perkins; and Professors Colladon and Sturm, in a series of experiments carefully made on water completely deprived of air, reduced its bulk with a pressure of 24 atmospheres.

This resistance to compression renders water so useful a medium for transmitting and multiplying power by means of Bramah's press, and thus by the hydraulic ram, the impulse of a fall of water a few feet in height through an enclosed pipe, when suddenly checked, strikes as it were a blow which will force a portion of the stream much higher than the level of the dam from which it flows.

Water in falling is subject to the same laws of gravitation as other heavy bodies, but in the operation of those laws there is this difference; namely, that a detached body, as a