212 



THK EFFECT OF .Sl.Ol'E KXl'OSrRE. 



It was iinfoj-tunate that the observations on the two slopes 

 were not begun simultaneously, those on the southern slope being 

 half-an-hour later than the others. A study of the table shows 

 that the two slopes receive very similar intensities of sunlight 

 throughout the morning up to about 11 a.m., the slightly greater 

 readings on the southern slope probably being due to the differ- 

 ence in the time of the exposure. From 11 a.m. on, however, 

 the northern slope receives a decidedly greater intensity of sun- 

 light, and the difference becomes more and more pronounced 



Table I. — Sunlight Intensity. 



throughout the afternoon, being particularly noticeable between 

 2 p.m. and 4 p.m. The difference in the total insolation received 

 by the two slopes during the day is, of course, more marked at 

 this season of the year, when the sun is north of the equator, but 

 it must be remembered that this is also the dry season, and there- 

 fore a critical period for vegetation. The differences in such 

 factors as siuilight intensity during the winter season are conse- 

 quently of very great importance. 



2. Evaporation. The rate of evaporation was measured by 

 means of porous cup atmometers, mounted on burettes so that 

 readings could be taken every hour. 



The ' ' evaporating power of the air ' ' is greater on the northern 

 slope throughout the day, and the difference becomes particularly 

 pronounced during the afternoon from 2 p.m. onwards. On the 

 southern slope the maximum evaporation occurred between 1 p.m. 

 and 2 p.m., whereas on the northern slope the maximum was not 

 reached until between 2 p.m. and 3 p.m. The difference between 

 the rates of evaporation on the two sides was accentuated by a 

 slight northerly breeze which sprang up about noon and from 

 which the southern slope was sheltered. 



