254 LIGHT SCIENCE FOR LEISURE HOURS. 



Now we have in the phenomena of the zone of calms 

 a crucial test of Sir J. Herschel s theory as to the origin 

 of the equatorial rains. It appears to me that this test 

 altogether negatives Herschel s theory. If the moisture 

 to which these equatorial rains are due came from the 

 trade-wind regions, we should certainly not expect the 

 fall of these rains to be associated in any marked degree 

 with the progress of the equatorial day ; or, if at all, 

 then the cooler parts of the day, when the point of 

 saturation is lower, would be the time of precipitation. 

 With the mid-day heat would come a cessation of pre 

 cipitation. As a matter of fact the contrary is the 

 case. The sun (we are told by Dove, Kaemtz, Hum- 

 boldt, Maury, Buchan, and many more) rises commonly 

 in a clear sky in equatorial regions. As the day proceeds 

 clouds form, and towards mid-day they grow dense. It 

 is at noon that heavy showers fall, and towards evening 

 the skies again become clear. Now, any one who has 

 noticed what happens on calm summer days in any well- 

 watered region can see that the equatorial phenomena 

 represent the same processes on a greatly enlarged scale. 

 On a summer s day in such regions we see how scattered 

 cumulus clouds begin to form in early morning, become 

 larger and more numerous as the day proceeds,, and in 

 the afternoon begin to be transformed into cumulo- 

 stratus. The explanation is simple. The sun s heat 

 has caused aqueous vapour to rise into the air, until 

 there is so much that not very far above the earth s 

 level the saturation point is reached. The further rise 

 of the vapour is followed by the process of condensation 



