40 
HINTS TO TRAVELLERS. 
there is a region of low pressure the wind blows in towards it from every 
side.. (3) The wind never blows perpendicularly to the isobars or directly 
from higher to lower pressure, but always in a curved or spiral path 
inclined to the isobars. (4) In the northern hemisphere the wind blow's 
out from a high pressure area in the same direction as the hands of a 
watch move, but in the southern hemisphere in the opposite direction. 
Also in the northern hemisphere the wind blows into a low-pressure area 
in the direction opposite to that of the hands of a watch and in the 
southern hemisphere in the same direction as the hands of a watch 
move. (5) Recognising that the wind blows nearly parallel to the 
direction of the isobars, the following statement (known as Buys Ballot’s 
Law) expresses its direction both for high-pressure and for low-pressure 
areas: If you stand with the lower pressure on your left hand, and the 
higher pressure on your right hand, in the northern hemisphere the 
wind will be blowing on your back, but in the southern hemisphere 
in your face. 
Raw fall Maps .—Rainfall is represented on maps by lines of equal 
precipitation termed Isohyets. These represent actual figures without 
reduction for elevation or other local conditions, and a rainfall map can 
consequently be studied as a direct record of observed facts. The map 
of mean annual rainfall brings out clearly the equatorial zone of heavy 
rains crossing the Amazon valley, the Congo valley, the south-eastern 
peninsulas of Asia and the Malay archipelago. North and south of this- 
belt are the nearly rainless regions of the tropical deserts, extended 
northward and southward over the continents, and merging nearer the 
poles into the fairly-watered temperate zones. The rainfall maps for 
separate months show the intimate relation between rainfall and the 
direction of the wind taken in conjunction with the configuration of the 
land. Even on the coast, when the prevailing wind is off shore, there 
may be scarcely any rain, as on the west coast of tropical South America. 
In the very heart of a continent the rainfall may be very heavy where the 
sea-wind blows across a great plain before striking the mountains, as is 
illustrated by the eastern slope of the Andes. Rainfall is, however, one 
of the most inconstant elements of meteorology, and the actual rainfall 
of any year may differ very widely from the average. The practical 
value of exact statistics of rainfall is, however, greater than that of any 
other climatological condition; for the water supply and the fertility of 
