70 



G. H. KNIBBS. 



Whatever the possibilities of its determination, such a phenomenon 

 is without doubt a definite function of several meteorological as 

 well as physical conditions : — -(a) the quantity and rate of rainfall; 

 (b) the character and circumstances of the soil and surface on 

 which the fall takes place ; and (c) the evaporative conditions ; 

 these last including temperature, radiation, humidity, wind, the 

 presence of vegetation, and to a very slight extent barometric 

 pressure ; that is to say all circumstances which affect the drying 

 of the soil. Compared with other factors, the element of pressure 

 is probably so small as to be quite negligible : it will not further 

 be referred to. 



2. Essential features of Mr, Deane's solution. — In Mr. Deane's 

 suggested solution of the problem indicated, the general principle 

 of which must I think be accepted as valid, the presence of soil 

 susceptible of instant saturation is tacitly assumed, and also 

 certain rates of exhaustion of the contained water, 1 such rates 

 depending upon average monthly conditions of temperature, 

 humidity, wind, etc., and the length of time such conditions have 

 continued. The first derivatives of Mr. Deane's evaporation-curves 

 are negative, and continually diminish with the increase of the 

 abscissae representing duration; that is to say the rate of evapor- 

 ation is assumed to continually diminish, as doubtless it does, 

 throughout unbroken periods. There is an admitted discontinuity 

 in Mr. Deane's graph, every time that the saturation line— the 

 axis of abscissae denoting time — is reached ; and since as long as 

 rain continues, no exhaustion of the degree of saturation is assumed, 

 and since also all fall is considered positive whatever its rate, the 

 curve is really reconstituted only at the end of the rainfall. That 

 is to say the curve is identical with the saturation line for such 

 periods. This method applied to the determination of the contents 

 of a reservoir, in which no losses could occur while the supply or 

 influx continued, would correctly represent the facts, provided the 

 consideration was limited to the reservoir itself : once full, all 

 further supply would run off and be lost. 



1 These have an empirically assumed relation to the evaporation of 

 water in exposed vessels. 



