138 The Realm of Nature CHAP. 



put in this form : " Stand with the Low pressure on 

 your Left hand and the high pressure on your right ; then 

 in the Boreal (northern) hemisphere the wind is Blowing 

 on your Back ; but in the southern hemisphere in your 

 face." The student should impress these statements by 

 studying Plates V. and VI. 



193- Winds of the Southern Hemisphere in January. 

 The theoretical arrangement of atmospheric pressure 

 and winds ( 178-181) is changing from hour to hour in 

 response to the changes of day and night ( 1 84) and summer 

 and winter (185). The two maps (Plates V. and VI. which 

 should be referred to continually in reading what follows) 

 are reduced from Buchan's Challenger maps, and give 

 the average conditions of the atmosphere in January and 

 July for the fifteen years 1870 to 1884. In the map for 

 January the equatorial zone of low pressure, as limited by 

 the isobars of 29-90 inches, is narrow over the ocean but 

 widens greatly over the three southern continents, where the 

 heat of summer causes the air to ascend, flow away, and 

 reduce the pressure over the land. Another consequence 

 of high te'mperature over the continents is, that the south 

 tropical belt of high pressure is broken into three isolated 

 portions lying altogether south of the tropic in the three 

 oceans. The southern area of low pressure and of 

 steep gradient, as limited by the isobar of 29-90 inches, 

 occupies the whole surface south of 40 S., the isobars 

 running nearly straight east and west. From the three 

 south oceanic regions of high pressure, surface winds 

 blow outward, forming the south-east trades on the 

 northern margin toward the equatorial low pressure, and 

 the brave west winds on the southern toward the great 

 south polar low pressure. The portions of the equatorial 

 low-pressure zone extended to the south by the continents 

 produce monsoons, or an indraught of surface air toward 

 the land. On the north-west coast of Australia the Australian 

 low-pressure area draws the trade wind round to form a 

 north-west monsoon. On the west coast of Africa the 

 south-east trade is drawn in to form a light south-west 

 monsoon, and in the Gulf of Guinea it is drawn in strongly 



