WIND. 17 



which induce an inflow of denser air from tho surrounding 

 regions. Thus in summer, when the sun is vertical over the 

 northern part of India, an upcast or cyclonic system prevails in 

 that region, causing an indraught, and thus giving rise to the 

 southerly monsoon ; whereas in winter, such region being at mid- 

 day about 47 north of the path of the sun, and its temperature 

 thus much reduced, a downcast or anti-cyclonic system with its 

 outflowing currents takes the place of the cyclone, and gives rise 

 to the northerly monsoon. 



Although from habit, and probably also from the fact of 

 barometers being usually marked " rain," " change," " fair," " very 

 dry," and so forth, many persons have accustomed themselves to 

 associate the relative heights at which the mercury stands with 

 wet or dry weather, the barometer, as a matter of fact, is 

 influenced only by the weight of the atmosphere, which, as has 

 already been shown, depends chiefly upon its temperature. If, 

 therefore, a barometer be placed under the influence of light air 

 which has traversed the ocean and become laden with moisture, 

 the mercury will stand at a low level, on account of the lightness 

 of the atmosphere, and at the same time will indicate rain, 

 simply because the wind from that quarter happens to have 

 acquired moisture, as just described. 



If, on the other hand, a barometer be placed, say, in the midst 

 of a vast tract of dry warm land or desert, the rarefied air from 

 this will cause it to fall, but a low barometer under such condi- 

 tions would not indicate rain. Therefore it is evident that the 

 movements of a barometer must be interpreted with due regard 

 to the physical conditions which surround it. 



It is, however, not always easy to determine these. Indeed, 

 it is almost impossible to do so, with any degree of certainty, 

 except by the help of synoptic charts. 



From what has just been said, the reader will doubtless be 

 prepared to find temperature rising as a storm approaches. 

 This is usually what happens ; indeed, the trace or diagram of 

 the thermometer's movements is often roughly that of the 

 barometer inverted. Still, this must not be taken as a universal 

 rule. In northern latitudes the barometer rises and the thermo- 

 meter falls for northerly gales, while the converse indicates 

 wind from the southward. 



The form assumed by the surface of the mercury in a 

 barometer tube shows which way the mercury is inclined to 



c 



