On tlie Botany Watershed,. 77 



service mains issuing from the tunnel have been largely 

 extended, aud consequently the draught has been greater, and 

 the flow of water through the tunnel has been increased from 

 that cause. The quantity which now passes through the 

 tunnel amounts to 1,200,000 gallons per diem. The cor- 

 rectness of this quantity was proved lately during a stoppage 

 in the tunnel, when connections were made between tlae 

 service pipes usually supplied from the tunnel and those 

 which are supplied by the low-level reservoir, in order 

 that the whole district might be supplied from the latter source. 



"Where the supply of water is obtained from surface collection, 

 the first point to be ascertained is the average rainfall over the 

 entire area of the watershed. The indications of the rain 

 gauge are found to vary so materially at diflerent points within 

 a very short distance of each other, and within the same 

 gathering grounds, that the determination of an approximate 

 average is a very intricate operation. The comparative amount 

 of rainfall on the same watershed is affected by elevation as well 

 as character of surface, whether it be wooded or otherwise. 

 The aspect of the slopes of the watershed in respect to the 

 direction of the prevailing winds affects the rainfall, more rain 

 falling at equal heights on the windward margin of the basin 

 than on the opposite one. Formerly the only rain-gauge in the 

 neighbourhood of Sydney was that kept at the South Head, 

 near the Lighthouse, some two or three miJes distant from the 

 nearest point of the Botany watershed. Tor the last few years 

 this gauge has ceased to furnish any returns. They have been 

 issued from the Observatory on the Flagstaff Hill instead, 

 where a gauge was established by the late Astronomer at an 

 elevation of 155 feet above the sea. About two years since I 

 established a rain-gauge at No. 1 Dam, near Botany, at an 

 elevation of 21 feet above the sea, which is very carefully tended 

 by the man in charge of the dams. At the same time I 

 established another gauge in the flat at Double Bay, at an 

 elevation of 10 feet above the sea, to which I have attended 

 myself. It will be seen that only one of these gauges is within 

 the area of the Botany watershed, that at Double Bay being 

 situate at the foot of the northern slope of the hill which forms 

 the northern boundary of that watershed. The gauge at the 

 Flagstaff is about one and a half (1-|) mile west of the station 

 at Double Bay. The following is the return from these three 

 stations for the year 1869 : — Observatory, 48"13 rainfall ; Double 

 Bay, 56-65 ditto ; No. 1 Dam, Botany, 45-87 ditto. And to the 

 31st August, 1870 : — Observatory, 45'59 inches ; Double Bay, 

 53-57 ditto ; No. 1 Dam, Botany, 43-37 ditto. The difference 

 in these returns shows the necessity for more gauges in the 



