WIND is the air in motion. The force of the wind is measured by anemometers (q.v.), of which some measure the velocity, and others the pressure. The following are a few velocities of wind, translated into popular language: 7 m. an hour is a gentle air; 14 m., a light breeze; 21 nt., a good steady breeze; 40 m.,' a gale; 60 na., a heavy storm; and 80 to 150 in., a hurricane sweeping everything before it. We also add a few com parisons of velocity and pressure: o m. an hour is a pressure of 2 oz. on the sq.ft. ; 10 m.,lh. •' 20m., 2 lbs. ; 30m., 4+ lbs.; 40 m., 8 lbs.; 51 in., 13 lbs.; 00 m., 18 lbs.; 70 m., 24 lb.s.; 80 Iii., 32 lbs.; and 100 m., 50 lbs. During the severe storm which passed over London, on Feb. 6, 1867, the pressure was 35 lbs. to the sq. ft., corresponding to a velocity of 83 in. an hour. A pressure of at least 70 lbs. to the sq.ft. has since been recorded at the Liverpool observatory. Wind is most frequently measured by estima tion.
Seamen require more than landsmen to pay attention to every variation in the strength of the wind, as well as its direction, anti to adopt such phrases as will render that strength generally intelligible. The anemometer (q.v.), which is used on land for this purpose, is unsuited to the wants of seamen. They have found it convenient te divide winds into 12 kinds, in relation to strength, designated thus: Faint air, light air, light gentle breeze, fresh breeze, gentle gale, moderate gale, brisk gale, fresh gale, strong gale, hard gale, and storm. This classification was determined in 1806 by Beaufort
according to the amount and kind of sail which one of her majesty's ships could safely carry at the moment. The estimate of the wind's force by the scale 0 to 12, means that 0 represents a calm, and 12 a hurricane. If such estimations be divided by 2, and the quotient squared, the result will be the pressure in pounds, approximately.
All wind is caused, directly or indirectly, by changes of temperature. Suppose the temperature of two adjacent regions to become, from any cause, unequal, the air of the warmer, being lighter, will ascend and flow over on the other, while the heavier air of the colder region will flow in below to supply its place. Thus, then, a difference in the temperature of the two regions 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 strata of the atmosphere; and these currents will continue to blow till the eguilibrium be restored.
Winds are classed into constant, periodical, and variable winds.