The mould X. Fig. 1, is the mould for the radial bars, traced from the plane below ; Fig. 5. shews the method of applying it to the plane, which is exactly the same as if it were a hand rail.
Fig. 6. shews the method of tracing the veneers of the cod or circular bar. Fig. 7. is the concave veneer. Fig. 8. the convex veneer. Fig. 9. is the method of forming the edges of the veneers, which are to be cut out to their thick ness afterwards.
Stairs.
Stairs and hand-rails are most important branches in joinery ; but before we enter upon their construction, it will be useful to point out some of the leading principles, without regarding the materials of which stairs are con structed.
The breadth of steps in general use is from 9 to 12 inches, or about 10 inches at the medium. In the best staircases, the bre:with might never to be less than 12 inches, nor more than 18. it is a general maxim, that a step of greater breadth requires less height than one of less breadth : thus a step of 12 inches in breadth will require a rise of 62 inches ; which may be taken as a standard by which to regulate those of other dimensions ; so that multi plying 12 inches by 61, we should have 66 ; then supposing a step to be 10 inches in breadth, the height should be =64 inches, which is nearly, if not exactly, what common practice would allow. The proportion of steps being thus regulated, the next consideration is the number requisite between two floors or stories : to ascertain this, we have only to suppose the breadth of the steps to be given, say 10 inches each, as depending on the space allowed for the staircase, and this, according to the rule laid down, will re quire a rise of 7 inches nearly. Suppose then the distance from floor to floor to be 13 feet 4 inches — 160 inches ; then = 221, which would be the number required. But as the steps must be equal in height, we should rather take twenty three rises, provided the staircase room would admit of it.
Stairs have several varieties of structure, which de pend principally on the situation and destination of the building.
Dog legged stairs, are those that have no opening or well hole; the rail and balusters of both the progressive and re turning flights fall in the same vertical planes.
Geometrical stairs, are those which have an opening down the middle, and of winch every step derives its support from that immediately below, and from the wall of the staircase.
The steps of a stair consist of two parts, one being paral lel, and the other perpendicular to the horizon. The part which is parallel is called the tread of the step, and the other pail which is perpendicular, is called the riser.
The rough timber work which is used in the support of a stair is called the carriage.
The string board, is a board fitted against the ends of the steps next to the well-hole, so as to make a complete finish; and the string which terminates the ends of the winders, is a veneer made in the form of a spiral back, with thick wood, so as to make it sufficiently strong.
The most certain method of carrying up a stair, whether of stone or wood, is to provide a rod of sufficient length to reach from one floor to the other, divided into as many equal parts as the risers are in number, and thereby to try every step as the woi k advances.
Hand A hand rail is the upper part of the fence in a geometri cal stair. In order that the hand may glide easily along the rail without straining the body, it is evident that the rail ought to follow the general line of the steps, and to be quite smooth and free from inequalities.
The principle of hand railing depends on the method of finding the section of a right cylinder, cylindroid, or prism, according to three given points in or out of the surface, that is, the section made by a plane through three given points in space.
The cylinder, cylindroid, or prism, is hollow, and equal in thickness to the breadth of the rail that is to the horizontal dimension of its section, and the ends or bases, the same as the plane or projection upon the floor.
The halal rail of a stair may always be formed of a por tion of this cylinder, cylindroid, or prism, the base of which is the plane of the stair ; for the hand rail itself must stand over the plane, it will therefore be contained between the vertical surface of the cylinder, cylindroid, or prism. And as the hand rail is got out in portions, so that each portion may stand over a quadrant of the circle, or ellipse, which forms the plane, we may also suppose such a portion con tained between two parallel planes, so that the portion of the hand rail may he thus contained between the two cy lindlical, or cylindroidic surfaces, and the two parallel planes. The parts which are to be joined together to form the rail, are to be prepared in such a manner, that when set upon their place, all the sections which may be sup posed to be made by a vertical plane passing through the axis of the cylinder, or cylindroid, may be rectangles, and this is called the squaring of the rail ; which is all that can be done by geometrical rules.