or any other specified time, is found by multiplying the revolutions made in that time by 2, and dividing by 1000. Fig. 47. The rate per hour at which the wind blows at any time is found by observing the revolutions made in, say, two minutes; multiply by 30 and 2 or at once by 60, and divide by 1000. Thus, suppose 800 revolutions were made in two minutes, the velocity of the wind would be at the rate of 48 miles an hour. 441. In this form the anemometer only gives the whole velocity between two observations; it does not register the velocity at any moment. To effect continuous registration an elaborate machinery is required-too complicated to be here described-by which the result is transferred to paper by a pencil, or by photography. 442. The force of the wind is also ascertained by noting the pressure which it exerts on a plane surface of metal perpendicular to the direction of the wind. The pressure is generally given in pounds avoirdupois on the square foot.* The instrument is of simple construction, consisting of a plate a foot square acting on a spiral spring, to which an index showing the degree of pressure is attached. The plate is kept perpendicular to the wind by a vane. This is the principle of Osler's anemometer, which, by means of machinery, leaves a pencilling of the pressure of the wind for every instant. 443. The pressure is also measured by Lind's wind-gauge, fig. 48, which consists of a tube half an inch in diameter, in the form of a siphon, one end of it being bent at right angles, so as to face the wind. It turns freely on a vertical axis, and a vane keeps the mouth of it directed to the wind. It is halffilled with water, and when the wind blows into the mouth of the instrument, it drives the water up the other leg, to which a scale showing the pressure is attached. The zero of the scale is the level at which the water stands when the air is calm. It may also be made to register maximum gusts of wind, by filling into the tube a chemical solution which colours bits of prepared paper, placed at different levels on the scale-limb of the instrument. Fig. 48. 444. Many observers who have no wind-gauge give the force of the wind by estimation. The scale generally adopted in this country is 0 to 6,-0 representing a calm, and 6 a hurricane, or the greatest known force of the wind. Observations by the scale 0 to 6 are converted into pressure in pounds on the square foot by simple squaring. Sailors use the scale of 0 to 12. Observations on this scale are, of course, reduced to the preceding scale by dividing by 2. In Sweden, In most European countries the force of the wind is given in kilogrammes on the square metre; and since a kilogramme equals 2.2046 lb. avoir., and a square metre 10.76 square feet, a pressure of one lb. on the square foot is equivalent to 4.88 kilogrammes on the square metre. Russia, and some other countries, the scale adopted is 0 to 4. In addition to this Wind Table, another (Table VIII.) is given at the end, which shows for the pressure in pounds on the square foot the corresponding velocity in miles per hour. 445. But a more important observation of the wind is the direction from which it comes. The direction of the wind is indicated by the point of the compass from which it blows. Those in common use are N., N.N.E., N.E., E.N.E.; E., E.S.E., S.E., S.S.E.; S., S.S.W., S.W., W.S.W.; W., W.N.W., N.W., N.N.W. When thirty-two points are given, an additional one is inserted between each of the above two. If greater accuracy be required, the exact point is indicated by degrees; thus, W. 41° N. means 41° from W. in the direction of N. 446. All winds are directly caused by differences of atmospheric pressure. The wind blows from a region of higher to a region of lower pressure, whether the differences of pressure be measurable by the barometer, as is generally the case, or be not measurable, as in the case of sea-breezes, and sudden squalls and gusts which are of short duration. In the latter case, it is probable that, though the difference of pressure cannot be measured by the ordinary barometer, it might be measured by one of great range, such as Howson's or King's. 447. Differences of atmospheric pressure, and consequently all winds, arise from changes occurring either in the temperature or in the humidity of the air. If two neighbouring regions, from any cause, come to be of very unequal temperature, the air of the warmer region being lighter than the other will ascend and be poured over it from above, while the heavier air of the colder region will flow in below to supply its place. Thus a difference in the temperature of the two districts gives rise to two currents of air-one blowing from the colder to the warmer along the surface of the earth, and the other from the warmer to the colder in the upper regions of the atmosphere; and these currents will continue to flow till the equilibrium is restored. Of this class of winds Land and Sea Breezes are the best examples, and they are at the same time the most general as well as the most readily comprehended of the winds. On the sea-coast a breeze sets in from the sea in the morning; at first a mere breathing on the land, it gradually rises to a stiff breeze in the heat of the day, and again sinks to a calm towards evening. Soon after, a breeze springs up from the land and blows strongly seaward during the night, and dies away in the morning, giving place to the sea-breeze as before. These breezes are caused by the land being heated to a much greater degree than the sea during the day, by which the incumbent air, being also heated, ascends, and the cooler air of the sea-breeze flows in to supply its place. But during night the temperature of the land and the air above it falls below that of the sea, and the air thus becoming heavier and denser flows over the sea as a landbreeze. 448. It is interesting to observe the effect of the rotation of the earth on these breezes when the sea-coast lies east and west. Thus on the coast of the Gulf of Lyons, the sea-breeze from the S. veers to the S.W., and dies away in the west; while the land-breeze from the N. gradually turns to the N.E., and dies away in the east. On the coast of Algeria opposite, the sea-breeze veers from N. to N.E., and dies away in the east; while the land-breeze veers from S. to S. W., and dies away in the west. Thus in each place the daily course of the winds may be regarded as rotatory, acquiring the maximum force when they blow from the N. and S., and falling to the minimum in the E. and W. 449. Again, if the atmosphere of one region be more highly charged with aqueous vapour than the atmosphere of surrounding regions, the air of the more humid atmosphere being on that account lighter will ascend, while the heavier air of the drier regions will flow in below and take its place. And since part of the vapour will be condensed as it ascends, and heat be thereby disengaged, the equilibrium will be still further disturbed. It is in this way that all the more violent commotions of the atmosphere-gales, storms, tempests, and hurricanes originate. 450. Winds are classified into CONSTANT, PERIODICAL, and PREVAILING WINDS. CONSTANT, PERIODICAL, AND PREVAILING WINDS. 451. The Trade-Winds.-When the portion of the earth's surface which is heated is a whole zone, or a large part of a zone, as in the case of the tropics, a surface wind will set in towards the equator from both sides; these having united, will ascend, and then separating, flow as upper currents in opposite |