78 On tTie Botany Watershed. 



Botany watershed. In order to obtain fairly an approximate 

 average, I have long felt the importance of a station on the 

 windward or southern slope of the northern boundary of the 

 watershed, because of the very heavy flow of water which 

 occasionally passes off from the Lachlan Swamp, when the flow 

 is very light in comparison at Botany. But the difiiculty of 

 getting these gauges properly tended, and preventing them being 

 tampered with, where thex-e are no habitations to protect them 

 from mischievous persons has prevented for the present the 

 establishment of a station at this place. In considering what 

 proportion of the ascertained quantity of rainfall is available 

 for storage, it is generally admitted that the proportion varies 

 with the character of the soil, whether absorbent or otherwise, 

 with the acclivity of the gathering ground, with the amount of 

 evaporation, and with the character of the rainfall, whether 

 more or less intermittent or continuous. If, in providing storage 

 room we construct dams and form open ponds or reservoirs for 

 the purpose, we must of course expect to submit to some loss 

 from evaporation, as they do in all climates. The precise 

 amount of evaporation — at least with respect to large reservoirs 

 — has yet to be practically determined. It is an acknowledged 

 fact that experiments on evaporation, on a small scale, carried 

 on at different observatories, give re ^ults considerably in excess 

 of the actual loss of large reservoirs from that cause. Many 

 eminent engineers, who had paid great attention to the subject, 

 and vvrho had been practically engaged all their lives chiefly in 

 the supply of water to towns, had arrived at that conclusion, and 

 had most sti'ongly expressed their opinions to that effect. The 

 result of such small scale experiments are vitiated by the 

 impossibility of guarding against the effect of the heat radiated 

 by the small vessels in which they are conducted. Mr. Hawksley, 

 C.E., eminent more especially in connection with the supply of 

 water to towns, gave a rule for calculating the loss by evaporation 

 in large ponds or reservoirs — by which it would appear that it 

 amounts to from 9 to 16 inches only per annum. Mr. Bateman, 

 C.E., also celebrated in connection with the supply of water to 

 towns, says that at Manchester the proportion of flow to rainfall 

 actually used by him at that place is "617 ; but he is of opinion 

 that three-fourths can be made available. Mr. Copeland, C.E., 

 also of similar note, says that he uses at Paisley, "S^S of the 

 rainfall, but is of opinion that 84 per cent, can be utilised when 

 required. The report of the Water Commission states that at 

 the Tan Yean Eeservoir, in Victoria, the evaporation is believed 

 not to exceed two feet per annum Now everything we know 

 in practice tends to confirm these statements, and the gentlemen 

 I have mentioned, who have been engaged all their lives in 

 waterworks, and have become eminent in that branch of 



