ON THE SUPPLY OF WATER TO TOWNS. 75 



the state of cultivation, to physical features and geological structure, to make 

 such a deduction for the loss by evaporation and vegetation, as, in the abs- 

 ence of correct experiments, may under the circumstances appear to be just. 

 But in estimating this quantity as a supply to towns, it is not safe to calculate 

 upon an average of seasons. It is scarcely possible to provide storage which 

 will equalize the extremes of wet seasons and dry ones. The average of two 

 or three successive dry years should be taken as the standard. 



The storage requisite for equalizing the supply afforded during this period 

 should be provided with a due regard to the continuance of drought and the 

 quantity of water which will flow off the ground in extreme wet seasons. 

 No water should be allowed to run to waste. Experience has shown that in the 

 regions of comparatively moderate rain in this country, the storage to effect 

 this object should vary from 20,000 or 30,000 cubic feet to 50,000 or 60,000 

 cubic feet for each acre of collecting ground, the smaller quantity being about 

 sufficient for an available annual rain-fall of perhaps 18 inches, and the larger 

 for one of about 36 or 40 inches. Or in estimating the storage by time, it 

 should be sufficient to afford the average daily supply of the district for 100 

 or 120 days where the available rain is 40 inches per annum or upwards, and 

 where the rain is frequent and heavy ; and for 200 or 250 days where the 

 rain is less, and where the annual available quantity will not exceed 8 or 12 

 inches, due allowance in every case being made for the produce of the 

 springs in protracted droughts. 



The year 1852 was a remarkable year, not only in its meteorological fea- 

 tures, but as affording valuable information for the guidance of the hydraulic 

 engineer. In that year there occurred probably one of the longest droughts 

 of which we have any correct record, and the heaviest falls of rain within 

 short periods. The total annual fall was but an average, and reservoirs for 

 a town's supply should have been able to collect nearly all the water which 

 flowed off the ground during the periods of excessive wet, to have afforded 

 a full daily supply throughout the whole duration of the drought. In the 

 Manchester W ater-Works, the rain was just an average, the average being 

 about 50 inches per annum. Rather more than half the whole quantity 

 fell in the two first and two last months of the year. The quantity of 

 water which flowed from 18,900 acres between the 1st of January and the 

 9thofFebruary exceeded 800,000,000 cubic feet. The rain in the same period, 

 taking the average of what was indicated by the gauges, was 12 inches. The 

 flow from the ground, accurately measured through reservoirs, equalled 12i 

 inches, the rain-gauges evidently indicating less than the real fall. From the 

 evening of the 4th of February to the morning of the 5th, the quantity of 

 water received into the reservoirs was equal to a depth over the whole sur- 

 face of the ground of 2-^-^ inches. This excessive rain was followed by a 

 drought of 110 days in duration, occasional wet days having occurred during 

 this period, which would reduce the net duration of the drought to 105 

 days. In the year 1850, at the Whittle Dean Water- Works, which supply 

 Newcastle-upon-Tyne, the reservoirs went down constantly for 240 days, 

 the whole available produce of the district being but 6^ inches in the year, 

 out of 17f inches of rain-fall. At Warrington, in the year 1854, there was 

 no appreciable supply of water for 230 days, the reservoirs and the springs 

 constantly decreasing during that period. The total produce of the year was 

 but 8 inches out of 27 inches of rain-fall. 



These are a few of the points which require to be considered in connexion 

 with the system of obtaining water from " gathering grounds." The amount 

 of information now existing in a scattered and unpublished form is very 

 large, and if properly brought together, would form a valuable contribution 



