The method of construction shown in Figures 31 and 32 is the one em ployed in the Sangerhalle, built in 1865 at Dresden. This will be referred to when describing Plate 19. The peculiarity of this roof-truss, constructed by the architect Edward Milller of Dresden, consists not only in the use of lattice-rafters composed of stays set together and purlins spanned be tween to support the weaker parts (fig. 32), but chiefly also in the combi nation with pairs of wire cables, which, suspended to large poles, proceed diagonally downward and are finally fastened in the earth. To provide against any injurious side-thrust, a counter-cable connecting the lower ends of the lattice-rafters is also used. The span of the roof-truss between the large poles is 148y., feet; the distance between every pair of main couples is 33 feet.
Rooprztsses zelith Plank Rollers, the details of which are shown in Fig ure 33, are not always used without roof-beams, but the latter are at least not to be regarded as an essential part of the roof-truss, for the plank raft ers, on account of their construction, exert but little horizontal thrust upon the enclosing walls. The characteristic point in this construction, which was used first by the French architect Delorme as early as the sixteenth century, consists in the fact that from three to five layers of short arched planks are nailed together, with broken joints. They thus form arch-ribs of great stiffness and resisting- power. The joining in the length consists, as the Figure shows, of horizontal pieces mortised in and bolted on the upper and lower sides of the rafters, as well as wedged in by the pieces running through them. These plank rafters are chiefly used in the con struction of roofs with rounded surfaces, particularly cupolas, the material intended to serve as a support to the roofing material being brought directly in contact with the plank rafters. An interesting example of this class—in which, also, the illustration given belongs—is the cupola, Ito feet in diameter, of the Catholic church at Darmstadt built by Von Moller.
In the three systems of construction which have been already discussed, the simple saddle roof alone has been under consideration, and the dome is mentioned only in the last example; space does not permit the treatment of roof-trusses for other forms—namely, the platform, hip, pavilion, man sard roof, etc., for which the reader is referred to the numerous special works on carpentry and building.
117ooden large mansions, and in other situations where ele gance and convenience are the chief objects of consideration, winding stairs are never introduced when it is possible to avoid them. The best archi tectural effects are produced by the use of rectangular stairways with orna mental railings and newels. In structures of the Gothic style, no other kind can be introduced for a principal staircase with propriety and har mony of design. Modern architecture, however, admits of great latitude in this respect, the stairway frequently ending in a circular form, and the railing continued, beginning from either a scroll or a newel. When there is a wall at each end, the stairs are merely built in at the time the edifice is constructed; but if they are supported at one end only, they are styled " geometric" stairs, and depend entirely on being securely wedged into the wall, as they rely solely for stability upon the wall and the support which each derives at one edge from the step below. If square in section,
they are styled " solid " steps; but, as the under side, or soffit, is then irreg-ular, it is usual to make the steps somewhat triangular in shape, so as to present a continued soffit. In this case they are styled "arris," or " feather-edge," steps. Care is required that there should be no sudden or irregular changes in the curves.
The rules governing, the ratio of ascent in stairways, and also the gen eral styles of construction, have already been considered in the section devoted to the description of stone stairs, and the same remarks apply to the erection of wooden stairways. In respect to the construction of the latter, attention need be bestowed only upon those which are composed of posts and boards, for the reason that those formerly, built of beams, after the manner of stone stairs, and known as " block stairs," have become practically obsolete, on account of the great quantity of wood necessary to their erection and the serious danger from fire consequent upon their use.
&Worts ana' stairs obtain their support from "strings," the steps being either housed in (pl. 3, fig. 34) or saddled (fig. 35). The former plan is the more generally employed. Each step consists of a hori zontal portion, or tread, and a vertical portion, or riser. The former is usually made from 'Y., to 2 inches thick, and the latter from 3/4. to r inch, while the thickness of the strings is from 2 to 3 inches, according to their length and the size of the stair. In stairs with housed-in steps, the latter are set into grooves frotn 3/4. to inch deep, which correspond to the step profile, while in those with the saddled tread the strings are to be cut out above, in accordance with the tread and riser, which are then screwed on. For the rest, as regards the form of the string-boards, they are straight in stairs having a straight flight, but in the case of winding stairs, as in the illustrations, they are curved once when the ground-plan has straight borders (fig. 34) or are double curved if the borders are curved. These curved string-boards are used also where the stair does not wind around a colutnn or spindle. Such a curved piece is shown in elevation in Figure 34a, and in ground-plan at b. All curved string-pieces must be cut out of the timber beam, and the pieces must be connected by screw-bolts at the joints, which are generally vertical.
Doors ana' ingenuity, intermingled with a number of modern improvements, lias been displayed in connection with the construction of doors and windows. At early stages of erection pro vision must be made for the apertures in which they are to be placed, and the proper performance of this task forms an important feature of the labors of builders. Doorways are fitted with jamb linings and architraves, or pilasters.