struck upon the arms bearing the letters of the compass, announcing every change of wind.* Such a contrivance may be considered a conceit, but it has the advantage of letting you know when the wind shifts much about; and when it does, there is as little chance of settled weather as in frequent changes of the barometer. A better contrivance would be to have a hammer suspended from the dart by a supple spring, and a bell of a different tone attached to each of the arms, indicating the cardinal points; and when different bells were struck, their tone would announce the direction in which the wind most prevailed. There is an ingenious contrivance for indicating the directions of the wind by an index on a vertical disc, like the dial-plate of a clock; a public example of which may be seen in the western tower of the Register-House in Edinburgh, and a private one in the entrance hall of Cassiobary House in Hertfordshire, belonging to the Earl of Essex. This latter method is a very convenient one of fitting up a weather-cock.

97. With regard to the origin of the name of weather-cock, Beckmann says, that vanes were originally cut out in the form of a cock, and placed on the tops of church spires, during the holy ages, as an emblem of clerical vigilance.+ The Germans use the same term as we do, wetterhahn; and the French have a somewhat analogous term in coq de clocher. As the vane turns round with 'every wind, so, in a moral sense, every man who is "unstable in his ways" is termed a weather-cock.

98. Anemometer.-The force of the wind is measured by an instrument called the anemometer, or measurer of the wind's intensity. Such an instrument is of little value to the farmer, who is more interested in knowing the direction than the intensity of the wind, as it has great effect on the weather. The intensity of the wind has, however, a material effect in modifying the climate of any locality, such as that of a farm elevated upon the gorge of a mountain pass. Still, even there its direction has more to do in fixing the character of the climate

than the intensity; besides, the anemometer indicates no approach of wind, but only measures its force when it blows, and its strength can be sufficiently well appreciated by the senses. The mean force of the wind for the whole year at 9 A.M. is 0.855, at 3 P.м. 1·107, and 9 P.M. 0·605; so that the wind is most active in the day, when the temperature is highest, an effect which might be anticipated on knowing the cause of the air being moved in currents.

99. The best instrument of this class is Lind's anemometer, which, although considered an imperfect one, is not so imperfect, according to the opinion of Sir William Snow Harris of Plymouth, who has paid more attention to the movements of the wind than any one else in this country, as is generally supposed; but as it is an instrument of no use to the farmer, I need not describe it.‡

100. Ventilation. -The principle of ventilation, whether natural or artificial, lies in a change of the density of the air. "We may be filled with admiration,” says Dr Arnott, " on discovering how perfectly the simple fact of a lighter fluid rising in a heavier, provides a constantly renewed supply of fresh air to our fires, which supply we should else have to furnish by the unremitted action of some expensive blowing apparatus; but the operation of the law is still more admirable as respects the supply of the same vital fluid to breathing creatures. The air which a man has once respired becomes poison to him; but because the temperature of his body is generally higher than that of the atmosphere around him, as soon as he has discharged any air from the lungs, it ascends away from him into the great purifying laboratory of the atmosphere, and new takes its place. No act or labour of his, as by using fans and punkas, could have done half so well what this simple law unceasingly and invisibly accomplishes, without effort or attention on his part, and in his sleeping as well as in his waking hours."§ This process of natural ventilation necessarily goes on in every stable and byre; and were the simple law allowed

109. The velocity of water issuing from an orifice is as the square root of its altitude. Thus, calling the velocity issuing 1 foot below the surface 1; that escaping from a similar orifice 4 feet below the level, will be 2; at 9 feet, 3; at 16 feet, 4; and so on. From this we learn, that of water issuing from two similar vessels, it will issue, from similar orifices, from the one kept constantly full, twice as fast as from the other. A short tube will assist the issue of water from an orifice to the extent of half as much more.

110. "The friction or resistance which fluids suffer in passing along pipes," says Dr Arnott, "is much greater than might be expected. It depends chiefly upon the particles near the outside being constantly driven from their true course by the irregularities in the surface of the pipe. An inch tube of 200 feet in length, placed horizontally, is found to discharge only a fourth part of the water which escapes by a simple aperture of an inch; and air passing along tubes is so much retarded, that a person who erected a great bellows at a waterfall, to blow a furnace two miles off, found that his apparatus was totally useless. Higher temperature in a liquid increases remarkably the quantity discharged by an orifice or pipe, apparently by diminishing that cohesion of the particles which exists in certain degrees in all liquids, and affects so much their internal movements." The simplest way of ascertaining the discharge of water from an orifice, such as a pipe or duct of a drain, is to measure the quantity discharged in a given time.

111. Water-ram.-It has been long observed that, when a cock at the end of a pipe is suddenly stopped when water is issuing out of it, that a shock and noise are produced. A leaden pipe, even of great length, is often widened or burst in this way. Lately, the forward pressure of an arrested stream has been used as a force for raising water, and the apparatus has been called a water-ram. The ram may be described as a sloping-pipe in which the stream runs, having a valve at its lower end, to be shut at intervals, and a small tube rising from near that end towards a

reservoir above, to receive a portion of the water at each interruption of the stream. Water allowed to run for one second in a pipe 10 yards long, 2 inches wide, and sloping 6 feet, acquires momentum enough to drive about half-a-pint, on the shutting of the cock, into a tube leading to a reservoir 40 feet high. Such an apparatus, therefore, with the valve shutting every second, raises about 60 half-pints or 4 gallons in a minute. The valve is ingeniously contrived so that the stream works it as desired. The action of the ram may be compared to the beating of an animal's pulse. The upright tube is usually made wider at the bottom, where it first receives the water, so as to constitute there an airvessel, which, by the air's elasticity, converts the interrupted jets first received into nearly a uniform current towards the reservoir. The supply of air to this vessel is maintained by the contrivance called a sniffing valve.

112. The effect produced by moving water depends on the quantity of water that strikes in one minute of time against the surface of the opposing body, and on the velocity with which the collision takes place. If the collision happens in a direction vertical to the surface of the body, its effect is equal to the pressure of a column of water, having for its base the surface impinged on, and an altitude equal to that of the column which generates the velocity of the stream. If the water impinges obliquely on the surface, the force may be resolved into two others-one parallel to the side of the body, and the other perpendicular to it. The latter alone is effective, and is proportional to the square of the sine of the angle of incidence. From this law we learn to calculate the amount of resistance required in an embankment against the force of a stream.

113. Water-wheels.-The motive power of water is usefully applied to drive machinery by means of water-wheels. When water-power can be obtained to drive the thrashing machine, or other fixed machinery of a farm, an immense advantage is gained over the employment of horses. It is found that water-power, in the thrashing of grain alone, saves the work of one pair of horses out of every five pairs. Any form

of water-wheel, therefore, is more economical than horses. When a wheel with float-boards merely dips its lower part into the stream of water, and is driven by its momentum-that is, both by the bulk and velocity of the water-it is called an undershot wheel. This wheel is employed in low falls with large quantities of water. When the water reaches the wheel near the middle of its height, and turns it by falling on the float-boards of one side as they sweep downwards in a curved trough fitting them, the modification is called a breast-wheel. This form is employed in moderate falls commanding a large supply of water. When the floatboards are shut in by flat sides, so as to become the bottoms of a circle of cavities or buckets surrounding the wheel, into which the water is allowed to fall at the top of the wheel, and to act by its weight instead of its momentum; the modification is called the overshot wheel. This form requires a high fall, but comparatively a small supply of water, and is most desired when circumstances will permit its adoption. To have a maximum of effect from undershot wheels, they are generally made to turn with a velocity about one-third as great as that of the water; and overshot-wheels usually have their circumference turning with a velocity of about 3 feet per second.

114. The resistance between a meeting solid and fluid is nearly proportioned to the extent of surface opposed by it to the fluid; hence large bodies, because containing more matter in proportion to their surface, are less resisted, in proportion to their weight, than small bodies of similar form. This law explains how, by means of air or water, bodies of different specific gravities, although mixed ever so intimately, may be easily separated. Thus, when a mixture of corn and chaff, as it comes from the thrashing-machine, is showered down from the sieves in a current of air, the chaff, in being longer of falling, is carried further by the wind, while the heavier corn falls almost perpendicularly. The farmer, therefore, by winnowing in either a natural or artificial current of air, readily separates the chaff from the grain, and even divides the grain itself into portions of different quality.

115. Friction of water.-Friction affects

the motion of streams of water very sensibly. The velocity of a stream is greater at the surface than at the bottom, in the middle than at the sides; and the water is higher along the middle than at the sides. But for the retarding power of friction, the water in open channels and ditches would acquire so great a momentum as to destroy their sides, and to overflow them at every bending. Rivers issuing from a high source, but for friction, and the effect of bending, would pour down their waters with irresistible velocity at the rate of many miles per hour. As it is, the ordinary flow of rivers is about 3 miles per hour, and their channels slope 3 or 4 feet per mile.

116. Velocity of streams.—To measure the velocity of a stream at the surface, hollow floating bodies are used, and the space they pass over in a given time-one minute-is observed by the watch. It is very difficult to ascertain the true velocity of an irregular stream. To learn what quantity of water flows in a stream, its breadth and depth are first measured at various places to obtain a mean of both; and the sum of these constituting the section of the stream is then multiplied by the velocity, and the product gives the number of cubic feet per minute.

117. Horse-power of water.-It may be useful to know the rule for calculating the number of horse-power any streami may exert if employed as a motive power. It is this:-multiply the specific gravity of a cubic foot of water, 624 lbs., by the number of cubic feet flowing in the stream per minute, as ascertained by the preceding process, and this product by the number of feet in the fall, and, cutting off the three figures on the right hand, divide by 44, and the product is the answer. Thus,-Multiply the number of cubic

feet flowing per minute in the
stream, suppose-

By the weight of a cubic foot of
water, 624 lbs.

And then multiply the product by the number of feet of fall, available, suppose,

Strike off the three figures on the right hand,

350

624

175

700

2100 21,875

12 262,500

500

horse

power.

And the quotient 6, gives the number of horse-power.

123. Electricity, in its natural and compound state of positive and negative combined, appears to be diffused equally throughout any given mass of matter; but when decomposed and separated into its component elements, each of the fluids is confined to the surface of the substance in which it has been set free, in the form of an exceedingly thin layer, not penetrating sensibly into the substance of the mass.t

124. The atmosphere is the part in which the electricity, liberated by various processes, accumulates; it constitutes, in fact, the great reservoir of sensible electricity, our solid earth being rather the field in which this mighty power is again collected and neutralised. Sensibly, electricity is found in the atmosphere at all times and in every state, but varies both in kind and intensity. It owes its origin