404 COTTAGE, FARM, AND VILLA ARCHITECTURE. 805. Mallet's Tank, fig. 822, is calculated to save expense ; first, by using a figure of maximum capacity and minimum surface ; and, next, by being able to dispense entirely with the centring, 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 five or six feet in diameter, a sliort cylinder with hemispherical ends, as shown in fig. 822. The excavation being made, 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 previously mixed with some salt to prevent tlie worms from working through it. This layer of clay, b, completely surrounds the brickwork in every part, to make it retain the water. The bottom part is now built all with common mortar, in the form of an inverted dome, nine inches thick ; then the perpendicular part, c ; and, lastly, the upper dome. Now, any common arch may be built without centring as far up as where the courses lie at an angle of about thirty- two degrees, or what is called the angle of repose for masonry ; that is, where the bricks will first begin to slip off; but a brick dome may be built of any size, entirely without centring, for the following reason : — Referring to fig. 823, d d are two bricks supposed to belong to part of the course of bricks next above that at the angle of repose. Each of these is to be considered, with the mortar in which it is embedded, as a quadrangular prismatic frustum, whose sides all incline towards the centre of the hemisphere at e : now, the upper surfaces of these two bricks form an internal or reentering angle with one another, from the position they lie in on the preceding courses ; that is, they lean against each other, as if tlicy lay on opposite inclined planes, as shown in fig. 824. If, then, these 822 823 bricks slip, they must do so in the line e f; but, in doing so, they must approach each other ; but they are already in contact, therefore they cannot slip. This demonstration applies to any greater number of bricks, until the whole course is finished, when the bricks are sustained by their lateral thrust. There is a limit to the weight of the voussoir (the overhanging part of an arch, looking up from under it) which will support itself in this way, as must bo obvious to every one from the common jjrinciples of gravitation. It is also obvious that a dome may thus be eitlicr left open, or closed at top. To make the tank perfectly watertight, it is finally coated over two or three times with coal tar inside. A manhole is shown at </, in fig. 822, for getting in to clean it out occasionally. This plan of building without centring is applicable to constructing large architectural domes, provided they be of brick, and that they be afterwards plastered outside with Roman cement, which would stand as well on a dome as on a wall ; and, the great expense of heavy domical centring being got rid of, domes on our large public buildings might be luore common than they are at present. The hollow bricks invented by Mr. Frost might be here advantageously used. The usefulness of this plan of building without centres, in constructing ice-houses, fruit-cellars, ovens, kilns, sewers, &c., is obvious. Mr. Mallet adds, " I have built one tank on this plan, which holds sixty hogsheads : it was built by one mason in four days, and never leaked a droj), although one side of it stood close to an area wall, where the least moisture would have been visible." 806. Ponds formerly were thought essential to farm yards, partly for sui)plying water for the cattle to drink ; but chiefly for the liorses to wade through, in order to wash their
