1310

THEORY AND PRACTICE OF ENGINEERING.

Book II.

lines with exactness; a very small portion of a tie-beam left at the end is sufficient towithstand the horizontal thrust of a principal rafter, and blocks may be used at the endswhere the rafters abut to give additional strength.

Scarfing a Timber in a perpendicular Direction -When

the top surface is divided into nine squares, if four are cutdown, the other five serve as tenons to enter into as manyvacant spaces left in the piece of timber placed upon it; ortwo may be cut away, as in the side figure, to receive atenon left on the upper piece.

Partitions and Framing for the outside of Buildings , §*c.,are a species of timber walls, usually covered with lath andplaster, and formed of upright posts, mortised into ahead and sill, braced in different directions, and filled inwith quarters: the posts are placed at the extremities, aswell as at the sides of all doors and openings. When apartition dividing two or more rooms has a bearing whichis perfectly solid throughout, it is better without braces;the posts or quarters have only then to be maintained in anupright position, which is effected by driving pieces betweenthem horizontally, so as to strut them, and prevent theirbending. Where they rest upon joists, which are liable toshrink, and yield to a weight placed upon them, the par-tition should be trussed in a manner to throw its load on theparts able to sustain it: in most houses we find greatneglect upon this subject, which occasions cracking in thecornice, inability to open and shut the doors, and many otherinconveniences.

The thickness given to partitions which do not exceed 20 feet in length is 4 inches:the posts are then 4 inches square, and the other timbers 4 by 3 : when they are of greaterextent, they should be increased in thickness : when it is required to make a doorway in themiddle, the truss may be formed by the braces, the inclination of which should be atan angle of about 40° with the horizon. When the doors are at the sides, the truss maybe formed over the heads; the posts should all be strapped to the truss, and the braceshalved into the upright posts.

Fig. 2091.

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Fig. 2092.

The weight of a square of quartered partition may be estimated at from 12 cwt. to 18cwt., and every precaution should be taken to discharge its weight from the floor on -whichit is placed, to the walls, which are its best points of support: in ancient timber houses,mills, &c., the fronts or external sides are formed of upright posts, placed at a distanceequal to their scantling; these are mortised and tenoned into a top and bottom plate,■which serves also to carry the floors: the posts at the angles are of a larger scantling, andinto these, which form openings for doors and windows, are framed horizontal pieces, whichserve for heads and sills: braces are then introduced, crossing each other, like a St. An-drew’s cross: above the lintholes, and beneath the sills, short quarters or punchionsfill in the space, and the whole are mortised, tenoned, and pinned together. The framingshould be placed on brickwork, or a wall of masonry, so as to be kept quite clear of theground.

Floors -When the bearings are equal, if joists of the same width, but of different

depths or thicknesses, are used, their strength is increased in proportion to the squares oftheir vertical thickness: when the joists are but 6 inches deep, they are in strength tothose of 8 inches in depth, as 36 to 64, the square of 6 being 36, and that of 8, 64. The