The use of rain-water for drinking purposes is only justified in isolated country houses where no better source is available; and under these circumstances the greatest care should be taken to prevent contamination with lead or organic impurities. The amount of water falling on any impervious material obtainable from rain can easily be estimated, if the amount of rainfall and the area of the receiving surface are known. The average annual rainfall in this country is 33 inches (see page 236). We may assume the amount practically available to be 20 inches per annum, and the area of the receiving surface 500 square feet. Multiply the area by 144, to bring it into square inches, and this by the rainfall, and the product gives the number of cubic inches of rain which fall on the receiving area in a year. One cubic foot, or 1,728 cubic inches, of water being equivalent to 6·23 gallons, the number of gallons of water can be easily calculated. To calculate the receiving surface of the roof of a house, do not take into account the slope of the roof, but merely ascertain the area of the flat space actually covered by the roof. This may be done roughly by calculating the area of all the rooms on the ground floor, and allowing an additional amount for the space occupied by the walls. It has been estimated that, even if a rain-water supply for towns were desirable, the amount collected from the roofs of houses would scarcely average two gallons per person daily—assuming the average rainfall to be 20 inches, and that there was a roof area of 60 square feet for each individual. The amount practically available from rain falling on different soils varies with their porosity and slope. Thus, according to Professor Rankine, the proportion of the total rainfall available is as follows:— Nearly the whole on steep surfaces of granite, gneiss, and slate; From three to four-fifths on moorland hilly pastures; From two-fifths to half on flat cultivated country; and None on chalk. By available rainfall is meant the amount remaining after allowing for percolation, etc., which can be stored in reservoirs. 5. =Upland Surface Water= is the water collected in hilly districts, as on moorlands, at the head of a river. By its utilisation for drinking purposes, the sources of water for the river are interfered with, and any water company or local authority using such a source is, therefore, required to run into the stream a quantity of water equal to a third of the available rainfall. The limited and regular supply thus furnished to the stream is found to be advantageous for industrial purposes as its flow is equalised, and the violence of floods mitigated. In the utilization of upland surface water the water from the surrounding hills is collected at the bottom of a valley, in an artificial, strongly-constructed lake; or in a natural lake, as in Loch Katrine (from which Glasgow is now supplied).