[ 90 ] 



apart they indicate a low gradient. High gradient is followed 

 by high winds and low gradient by light winds. Air does 

 not blow directly from regions of high pressure to those of 

 low pressure ; the atmospheric movement caused by the rota- 

 tions of the earth results in an alteration of the direction 

 of the current. In the northern hemisphere if you stand 

 with your back to the wind, the barometric pressure on 

 the regions to your left hand is lower than on those to 

 your right hand. In the southern hemisphere if you stand 

 with your back to the wind the regions of lower pressure will 

 be on your right hand. The same principle is expressed in 

 other words thus : if you stand with the high barometer to 

 your right and the low barometer to your left the wind will 

 blow on your back. In the Southern hemisphere the re- 

 verse will be the case. Thus the isobars indicate direction 

 of the wind, and the distances between the lines its strength. 



in. Actual barometric readings have to be reduced to 

 a common standard by the meteorological office, as the 

 elevation of the place of observation and the temperature at 

 the time of observation, make a difference in the height of the 

 column of mercury, apart from the difference of pressure 

 causing movements of wind. All the readings are therefore 

 corrected or reduced to sea-level and 32 F, for comparison. 

 A reduction has also to be made on account of difference of 

 gravity due to difference of length in the diameter of the 

 earth at different latitudes. All reduction is made for the 

 latitude of 45. 



112. Solar radiation is also recorded in meteorological 

 stations with the help of a radiation thermometer. This con- 

 sists of a delicate thermometer having a dull blackened bulb 

 and inclosed in a glass tube from which the air has been 

 removed. This instrument is freely exposed to the heat of 

 the sun and its maximum reading registered. The greatest 

 amount of solar radiation which occurs during the day is 

 indicated by the excess of this temperature over the maxi- 

 mum temperature of the air in the shade. 



