Van Yean Water Works. 193 



soil, combmed with slope, is unfavourable to loss from evapor- 

 ation on its surface. The rainfalls also being mucn less 

 frequent, and consequently heavier than in England, it is 

 evident that under similar circumstances there would be less 

 loss from evaporation, than if more diffused, as in England. 



It is reasonable therefore to conclude, that if 57*6 per cent, 

 of rainfall is lost by surface evaporation in England, under- 

 circumstances highly favorable to evaporation, a much smaller 

 per centage of the rainfall will be lost when the same circum- 

 stances are very unfavorable, as in the basin in question. 

 We should hence be justified in estimating a much less per 

 centage of loss from evaporation in the basin of the Plenty, 

 were it not that another important question must enter into the 

 calculation, namely the difference of temperature, equal ^ to 

 ten degrees in favour of evaporation in the Plenty basin, which 

 must act to some extent as a counterpoise against its un- 

 favourable character, for same in other respectsnotwithstanding 

 that the action on evaporation does not last nearly as long as 

 the English temperature. 



Viewing therefore the conditions of the surface drainage of 

 England relatively with those of the basin of the Plenty, we 

 are'of opinion that the English per centage of evaporation of 

 57-6, if correct, embraces that of the Plenty basin, if it does 

 not exceed it. We hence proceed in our calculations on this 

 assumption, leaving it further on to be shown how it is borne 

 out by the facts. 



This per centage of 57-6 will therefore give 17-856 inches 

 on 31 inches rainfall, the supply from rainfall of 31 inches and 

 loss on same by evaporation will therefore stand thus. 



Rainfall of 31 inches on basin of 60 square ") 



miles, superficial extent, or 185,856,000 Y 160,042,666 cub. yards. 



square yards ... ... .■• ••• ^ 



Loss due to surface absorption and eva-*) 



poration over same extent, to depth ofh 92,184,564: cub.yards. 



17-856 inches 



Balance delivered into the Plenty ... 67,858,102 cub. yards. 



Having thus ascertained the amount discharged into the Plenty, 

 on thil assumption, it is next necessary to determine the 

 amount of loss entailed, by the passage of its. western and 

 eastern arms through the swamps. 



The swamp on the western arm as before stated, has_ a 

 superficial extent of 787-000 square yards, over which its 

 waters are spread, thus exposing them to evaporation almost 



Y 



