256 The Winds. 



On the average of twenty years at Greenwich, thirty-three 

 days in every year are calm, when no pressures greater than 

 \\h. are recorded. On thirty -two days the wind is above 

 moderate, and the pressures exceed 5 lb. The remaining three 

 hundred days of the year are occupied by moderate or gentle 

 winds. Of these gentle winds the largest share falls to the 

 summer months, in each of which the occurrence of a fresh 

 wind is an even chance ; but not one single January without 

 fresh wind has occurred for twenty years. Strong and mode- 

 rate winds alike are far most frequent from south-west, where 

 the wind lies for about a third part of the whole year (119 days) . 

 North-east is the next most frequent point, and from E., 

 S.E., and N.W., the winds are the rarest of the dial. 



A change of the wind from S.W. to N.E. produces nearly 

 as great an alteration of temperature and moisture in the air 

 as a change from day to night. The weight and density of 

 the air are at the same time greatly increased, and the baro- 

 meter (whose other oscillations are extremely small) becomes a 

 valuable index of the reversed conditions of the weather. 

 Professor Dove has shown that the change of wind from one 

 of these directions to the other is subject to a curious law arising 

 from the rotation of the earth. No law in meteorology is more 

 easily verified, or better stands the test of daily observations 

 than this, which is called Dove's "Law of gyration of the 

 Wind." 



Suppose a north wind to arise, by a body of air commencing 

 to move towards the equator ; in a short time it will become 

 north-easi in the same manner as if it formed a portion of the 

 constant trades. Should the indraught be replaced by an 

 impulse outwards from the equator, the northerly element of 

 motion is destroyed, and that from the east alone remains. The 

 wind therefore veers towards the east and south, and at length 

 becomes south-icesi in the same manner as if it formed a 

 portion of the constant anti-trade. Should an indraught 

 towards the equator once more return, the southerly element 

 of motion is destroyed, and that from the west alone remains. 

 The wind then veers towards the west and north to the point 

 from which it originally set out. At every alternate impulse 

 north and south, the wind veers half round the dial in the 

 same direction as tlie sun. In the southern hemisphere the 

 same mode of reasoning leads to the same conclusion, or, in 

 other words, in both hemispheres, the wind, in its ordinary 

 changes, veers round the dial in the same direction as the sun. 

 When wind shifts round the dial in an opposite or retrograde 

 direction, it is said to " back/' 



In this manner the vane of Osier's anemometer at Green- 

 wich, in twenty years, has performed two hundred and forty- 



