DEPICTING DIAGRAMMATICALLY THE CHARACTER OF SEASONS. 65 



The exhalation from the vegetation must be a most important 

 factor in the drying of the soil. Otherwise the rate of drying 

 would rapidly decrease if the storage below could be drawn upon 

 by the aid of capillary attraction alone. The effect of the vegeta- 

 tion however, is probably to make the rate of soil-drying a much 

 more even one. 



After continued dry weather, grass and herbs begin to wither 

 and may fail altogether, while the moisture below may be still 

 sufficient to keep deep-rooted plants, shrubs and trees in a 

 flourishing condition. After the ground has been thoroughly 

 desiccated a succession of light rains may have the result of 

 putting grass and herbage in good condition, but at the same time 

 be too sparing in quantity for any of it to find its way down to 

 any great depth. With my method the temporary revival of 

 grass and herbage as the effect of light rain during the progress 

 of a drought may be shown. 



Mr. H. C. Russell gives the mean rates of evaporation from 

 water for each month calculated from a long series of years as 

 follows : — 



Jan. Feb. Mar. April May June July Aug. Sept. Out. Nov. Dec. 

 •22 -16 -14 -09 -07 -05 -06 -08 -13 -20 -22 -24 



The total mean evaporation from water surface amounts to 48-9 

 inches per annum. 



After consulting with Mr. Russell I decided to adopt half the 

 rates in the above table as the rates at which evaporation from 

 the surface of the soil would probably begin to take place. These 

 are as follow : — 



Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 

 •11 -08 -07 -045 -035 -025 -03 -04 -065 -10 -11 -12 



My method, which I will now explain, rests on the fact that as 

 soon as rain leaves off the drying process commences and continues 

 till rain falls anew and makes up in whole or in part for loss 

 previously sustained. On the diagrams, (Plate 3) periods of 

 time are measured horizontally, and each division of iV inch 

 represents a day. 



E— July 5, 1899. 



