72 G. H. KNIBBS. 



average conditions for these must be discussed. Here, it must be 

 confessed, lies one of the greatest practical difficulties in the way 

 of even approximate deductions. It is evident also that evapor- 

 ative and other conditions must similarly be locally considered, 

 and also treated by the method of averages. The nature of the 

 difficulties of so doing will incidentally be more fully specified 

 hereinafter. 



5. Both quantity and rate of rainfall essential factors. — Revert- 

 ing to §1, there is no serious difficulty in regard to the measure- 

 ment of the first element, viz. (a); and measurements of the rate 

 of rainfall, together with the time, give by integration the 

 quantity of fall — see §21, hereinafter. In practice the rate is 

 derived from measurements of quantities received in short periods ; 

 the discontinuities and irregularities of the natural phenomenon 

 rendering other methods unnecessary. It will be seen hereinafter, 

 that the rate of fall greatly affects the question of the degree of 

 saturation produced by a given amount ; so that if one makes this 

 latter the measure of the intensity of a drought — as Mr. Deane 

 has done — and probably no better measure could be suggested — it 

 is necessary to take account of, not the total fall, but that part of 

 it, depending on the rate of fall, on the permeability, and on the 

 general conditions and characteristics of the soil, which serves to 

 produce saturation : in other words, of that part which does not 

 run off. The determination of the amount that runs off is impor- 

 tant to engineers in the design of waterways. 



6. Permeability. — The permeability of the materials of the 

 earth's surface probably never absolutely reaches zero : the rate 

 of flow in some however is so insensible in relation to the slowest 

 rate of deposition of water as to justify their treatment as imper- 

 meable. In the sensibly permeable materials however, there is 

 every gradation, viz., from material which will instantly absorb 

 the heaviest tropical shower, to that from which the lightest rain 



will flow off. 



i 



7. Laws of flow in permeable strata. — Experiments on the flow 

 of liquids through permeable strata of uniform fineness, shew that 



