The weekly average of 1 in. equals 1 cub. ft. upon every 12 ft. of surface, or 3630 cub. ft. upon an acre, weighing about 113 tons. Upon a roof 40 ft. by 40 ft., 1600 sq. ft., it would be 133 cub. ft., 1037 gal., or about 26 barrels of 40 gal. each. A cistern 8 ft. across and 10 ft. deep would contain 502 cub. ft.; and one of 10 ft. across and 10 ft. deep, 785 cub. ft., or 6120 gal.--about the average fall upon a roof of the above size for 6 weeks; while the smaller cistern would hold 3900 gal., or a little less than 4 weeks’ rainfall. The weekly supply of 1037 gal. is equal to 148 gal. per day, or nearly 15 gal. to each individual of a family of 10. This is certainly enough, and more than enough, if used as it should be; but where water is plentiful it is wasted, and in our capricious climate, whether we depend upon wells or cisterns, it is wise to waste no water at all, at least during the warm summer months, and lay by not for a wet but a dry day. For this country, Field estimates 2-3 gallons of tank capacity for every square foot of roof area. [Illustration: 3. Rain-water Tank.] In Fig. 3 _a b c d_ show the excavation that must be made for the cistern, and supposing the diagram to exhibit, as it does, a section of the cistern, the receptacle for the water will be, when finished, taking the relative proportions of the different parts into consideration, just about 9 ft. wide and 4½ ft. deep. Of course, the excavation must be made greater in breadth and depth than the dimensions given, to allow for the surrounding walls and the bottom. The walls may be of brick, cemented within, and backed with concrete or puddled clay without, or of monolithic concrete; but the bottom, in any case, should be made of concrete. The trench _e f g h_ running across the bottom of the cistern is 2 ft. broad and 2 ft. deep. In the middle of this opening is built up a 9 in. brick wall, or a party-wall of concrete, _i k_. Along the bottom of the wall openings _l_ are left at intervals. The party-wall divides the entire space into the larger outer cistern _m_, and the smaller inner cistern _n_. Supposing the breadth from _e_ to _f_ to be 2 ft., and the wall 9 in., spaces of 7½ in. will be left on each side of the wall. These are filled to ¾ the height, or for 18 in., with lumps of charcoal, smooth pebbles, 1-3 in. in diameter, being laid along the top of the charcoal till the trench is filled up. The cistern is so constructed that the water from the roof enters _m_; it passes downwards through the stones and charcoal, as shown by the arrow at _f_, goes through the opening and forces its way upwards in the direction of the arrow at _e_ into the cistern _n_, in which it rises to the level of the water in _m_, to be drawn thence for use by a small pump.