No. 4.] WEATHEE AND ITS INFLUENCE. 45 



so drying winds from over some mountain range, and if 

 oceanic, whether they will not produce too much fog and too 

 chilling conditions at certain seasons of the year. Experi- 

 ments show that a wind blowing at the rate of ten miles an 

 hour, other things being equal, causes almost four times as 

 great evaporation as during calm weather, while blowing 

 thirty miles an hour there is over six times the evaporation. 

 Hence it is not hard to find the cause for the aridity of some 

 of our western plains or even the upland farms of our own 

 district. 



Generally speaking, the climate of any place may be put 

 under one of three heads : continental, insular and mountain. 

 If one be given the latitude and the general surroundings of 

 any place, its climate and weather may be easily written in 

 general terms, and, further, the characteristics and leading 

 occupation of the inhabitants can be determined. We must 

 know the general arrangements of the great air and ocean 

 currents to determine this, and also the intensity and average 

 path of movement of the local atmospheric disturbances. 



The heat from the sun is the cause of all the air move- 

 ments, as well as the source of all life and power on the 

 earth. The air warms tiistest in the equatorial regions, 

 and, expanding as it warms, it is forced up by the denser, 

 cooler air on either side. The ascending current cools and 

 flows out toward the poles, and, cooling more, settles to the 

 earth again, part flowing back toward the equator and part 

 moving on as a surface wind toward the poles. Then, gen- 

 erally speaking, the great air currents are well marked, but 

 they are varied and magnified by the toi)ography and by the 

 difl'erent seasons. There is an area of nearly calm but 

 ascending air around the earth near the equator, called the 

 doldrums. Another area, narrower and less apt to be 

 entirely calm for any length of time, are the so-called horse 

 latitudes, near the tropics. Between these two belts and 

 flowing toward the equator, from the north-east in the north- 

 ern and the south-east in the southern hemisphere, because 

 of the revolution of the earth, we have the trade winds. 

 Outside of the tropics are the prevailing westerlies, which 

 move toward the poles ; but because the winds are passing 

 from a section of the earth's surface which has a rapid mo- 



