TANKS AND CISTERNS. 



515 



mates with this indispensable element 

 throughout the year. A cottage has a 

 limited surface of roofing, but it contains 

 also a limited number of inmates; a man- 

 sion contains more inmates, and thus re- 

 quires a larger supply, but it also presents 

 a proportionate extent of roofing surface. 



A very simple and effective filter may 

 be constructed according to the annexed 

 section, fig. 730, which should be built 



[1 Fi £- 730. 



with brick and cement, or, if upon a small 

 scale, of pavement sides, well jointed and 

 puddled behind and at bottom, a is a 

 space of about 3 or 4 inches, covered with 

 a perforated tile, slate, or stone at b ; the 

 filtering material — say small gravel, char- 

 coal, or coarse gravel — is placed at c ; a 

 filtering-stone of some porous kind — or, 

 in default of that, a plate of lead, cast-iron, 

 or, best of all, a thick plate of glass, per- 

 forated with numerous very small holes, 

 not more than one-twentieth part of an 

 inch in diameter — is placed at d. The 

 water from the receiving-tank / passes 

 through the small opening e, and is forced 

 up by its own pressure through a b c and 

 d 9 and enters into the reserve-tank h 

 through the opening g. From the above 

 diagram it will readily be seen that, when 

 the water in the receiving-tank rises above 

 the level of the opening g, it will force 

 itself into the receiving-tank h, and this 

 force will be increased if the tank / be 

 carried higher than h ; and we think it 

 should be so, to the extent of 1 foot, or 

 perhaps 2. A man-hole should be left in 

 the top of the filtering space, so that it 

 may be cleaned out occasionally. This is, 

 however, not shown in the diagram, but 

 may be supposed. This filter may even be 

 considerably simplified by placing in it 6 or 

 8 inches of small round stones ; above these 

 a corresponding layer of sand and char- 

 coal mixed ; and over that a flat piece of 

 sponge, or several thicknesses of coarse 

 drugget or carpeting, kept in its place by 



a wire grating fixed to the sides. The fil- 

 tered water must be drawn up for use 

 from the reserve-tank by means of a 

 pump, unless the apparatus is placed so 

 high above the points of distribution that 

 the water may flow to them through a 

 pipe by its own gravity. 



Mallet's tank is thus described in the 

 " Encyclopaedia of Cottage, Farm, and 

 Villa Architecture," to the talented editor 

 of which work it was communicated by 

 Mr Mallet, along with the annexed figs. 

 731, 732, and 733. This tank is calcu- 

 lated to save expense — first, by using a 

 figure of maximum capacity and mini- 

 mum surface ; and next, by being able to 

 dispense entirely with the centering, 

 which, according to the present practice, 

 is used for arching over tanks. Mr Mallet 

 proposes, for very large tanks, to adopt a 

 spherical form ; but for any of less than 

 5 or 6 feet in diameter, a short cylinder 

 with hemispherical ends, as shown in 

 fig. 731. "The excavation being made, 



Fig. 731. 



the building is commenced, either with a 

 single brick at the bottom a, or, better, 

 with a circular piece of stone laid on a 

 layer of tenacious clay, tempered as dry 

 as possible, well beaten together, and pre- 

 viously mixed with some salt to prevent 

 the worms from working through it " — a 

 precaution we think almost superfluous. 

 "This layer of clay, b, completely sur- 

 rounds the brickwork in every part, to 

 make it retain the water. The bottom 

 part is now built all with common mor- 

 tar, in the form of an inverted dome, 

 9 inches thick ; then the perpendicular 

 part c ; and, lastly, the upper dome. Now, 



