219. There are many curious methods of constructing floors with short timbers, which cannot be passed over without notice, and yet are scarcely worthy of it; because they are seldom applied, as long timber may always be had. To those, however, who are more inclined for curious than useful information, the following notices respecting such floors may be acceptable.

Let A B C D, Fig. 73, represent the plan of a room, and let four joists be mortised and tenoned together at a, b, c, and d, in the form shown in the figure; then it is evident that these joists will mutually support one another. Each joist being supported at one end by the wall, and at the other by the middle of the next joist. This is one of the most simple forms, and will sufficiently explain the principle of constructing a floor of timbers shorter than will reach across the room.

The same thing may be done by mortising and tenoning the joists together as in the form represented by Fig. 74; and various other forms will readily suggest themselves, the method being once understood.

* Evelyn's ' Silva,' Dr. Hunter's edit., vol. ii., p. 217.

Fig. 73.

A design for this kind of floor was given by Serlio;* and the celebrated mathematician, Dr. Wallis, has entered very fully into the investigation of the strength and disposition of these floors, in the first volume of his mathematical works.

Fig. 74.

* Tutte, 'L'Opera d'Architettura di Serlio da Scamozzi Vineg.,' 1600, lib. i.

The researches of Dr. Wallis have been reprinted in Nicholson's 'Architectural Dictionary,' art. Naked Flooring. The Dutch manner of framing these floors is given in Krafft's 'Recueil de Charpente,' part ii.; and several forms are exhibited in Rondelet's ' L'Art de Batir,' tome iv.

220. Perhaps the most singular floor that was ever constructed on a large scale is one executed in Amsterdam, for a room 60 feet square, which has no joists whatever. There are very strong wall-plates on each side of the room, firmly secured with iron straps at the angles, and rebated to receive the flooring. The flooring consists of three thicknesses of 1 1/2-inch boards. The first thickness is laid diagonally across the opening, the ends resting in the rebates of the wall-plates; and rising about 2 1/2 inches higher in the middle than at the sides of the room. The second thickness of boards is also laid diagonally, but the direction is the reverse of the first thickness; and the two thicknesses are well nailed together. The boards of the third thickness are laid parallel to one of the sides of the room, and form the upper side of the floor, being well nailed to the boards below. All the boards are grooved and tongued together, and form a solid floor 4 1/2 inches in thickness.* This example shows how much may be accomplished by a well-disposed bond and firm connection of parts. Such a floor partakes of the nature of a thin plate supported all round the edges: the strength of plates supported in this manner is directly as the square of their thickness, and they are equally capable of supporting a weight in the middle, whatever the extent of bearing may be; but when the load is uniformly distributed, the strength is inversely as the area of the space it covers.†

* Rondelet's 'L'Art de Batir.'

† Emerson's ' Mechanics,' 4to, sect, viii., prop. 73, cor. 5.