slipjoints.

mortar with generous top and bottom joints, to allow for shrinkage of the more frequent joints behind them; otherwise, they are apt to be shattered. Such unconstructional designs had, however, better be avoided. In no case should a Avail be built of part iron uprights and part masonry; one or the other must be strong enough to do the work alone; no reliance could be placed on their acting together. In frame walls, care should be taken to get the amount of "cross" timbering in inner and outer walls about equal, and to have as little of it as possible. Timber will shrink "across" the grain from one-fourth to one-half inch per foot. "Where the outer walls are of masonry, and inner partitions or girders are of wood, great care must be taken that the shrinkage of each floor is taken up by itself. If the shrinkage of all beams and girders is transferred to the bottom, it makes a tremendous strain on the building and will ruin the plastering. To effect this, posts and columns should bear directly on each other, and the girders be attached to their sides or to brackets, but by no means should the girder run between the upper and lower posts or columns. If there are stud-partitions, the head pieces should be as thin as possible, and the studs to upper partitions should rest directly on the head of lower partitions.

In masonry, all beds should be as nearly level as possible, to avoid unequal crushing. Particularly is this the case with cut stonework. If the front of the stone comes closer than the backing (which is foolishly done sometimes to make a small-looking joint), the face of the stone will surely split off. If the back of a joint is broken off carelessly, and small stones inserted in the back of a joint to form a support to larger stones, they will act as wedges, and the stone will crack up the centre of joint and wall. Stones should be bedded, therefore, perfectly level and solid, except the front of joint for about three-fourth inches back from the face, which should not be bedded solid, but with "putty"mortar. Light-colored stones, particularly lime-stones, are apt to stain if brought in connection with cement-mortar. A good treatment for such stones is to coat the back, sides and beds with lime-mortar, or, if this is not efficacious, with plaster-of-Paris. All stones should be laid on their "natural beds"; that is, in the same position as taken from the quarry. This will bring the layers of each stone into horizontal positions, on top of each other, and avoid the "peeling" so frequently seen. Ashlar should be well anchored to the backing. The joints should be filled with putty-mortar, and should be sufficiently large to take up

Shrinkage of timber.

All beds level.

Cement stains.

Natural bed.

the shrinkage of the backing. Stones should not be so large as to risk the danger of their being improperly bedded and so breaking. Professor Rankine recommends for soft stones, such as sand and lime stones, which will crush with less than 5000 pounds pressure per square inch, that the length shall not exceed three times the depth, nor the breadth one-and-a-half times the depth. For hard stones, which will resist 5000 pounds' compression per square inch, he allows the length to be from four to five times the depth, and the breadth three times the depth. Stones are sometimes joined with "rebated" joints, or "dove-tail" joints, the latter particularly in circular work, such as domes or light-houses.

All sills in either stone or brick walls should be bedded at the ends only, and the centre part left hollow until the walls are thoroughly set and settled; otherwise, as the piers go down, the part between them, not being so much weighted, will refuse to set or settle equally with them, and will force up the centre of sill and break it. Where there are lintels across openings in one piece, with central mullion, the lintel should cither be jointed on the mullion, or else the mullion bedded in putty at the top. Otherwise, the lintel will break; or, if it be very strong, the mullion will split; for, as the piers set or settle, the lintel tends to go down with them, and, meeting the mullion, must cither force it down, or else break it, or break itself.

Walls of uneven height, even where of the same material, should be connected to each other by means of a slip-joint, so as to provide for the uneven shrinkage. Slip-joints must be so designed that while they allow independent vertical movement to each part, neither can separate from the other in any other direction. Figures 71 to 73 give a few examples.

Figure 71 shows the plan of a gable-wall connected with a lower wall, by means of a slip-joint. Figure 72 shows the corner of a front stone-wall connected similarly with side-wall of brick. Figure 73 the corner of a tower or chimney connected with a lower wall. The joint must be built plumb from top to bottom. Where the higher wall sets over the tongue above lower wall, one or two inches must be left hollow over the tongue, to allow for settlement or shrinkage of the higher wall and to prevent its resting on the tongue and possibly cracking it off. Where iron anchors are used in connection with slip-joints, they should be so arranged as to allow free vertical movement.

Size of stones.

Sills bedded hollow.

Slip-Joints.

Slip Joints

Fig. 7 1.

Such joints and.anchors must be designed with reference to each special case. In stepped-foundations (on shelving-rock, etc.), or in walls of uneven heights where slip-joints are impracticable, the foundations or walls should be built up to each successive level and be allowed to set thoroughly before building further. A hard and quick-setting cement should be used, and the joints made as small as possible. In no case should one wall of a building be carried up much higher than the others, where slip-joints are not to be used. When building on top of old work, clean same off thoroughly or the mortar will not take hold (clinch). In summer, soak the old work thorough". Where new work has to be built against old work, a slip-joint should be used, if possible, or else a straight joint should be used with slip-anchors, and after the new work has thoroughly set, bond-stones can be cut in. In such cases, the foundations should be spread as much as possible, to avoid serious settlements. In all work involving old and new walls combined, the quickest and hardest-setting cements should be used. Sometimes it is advisable not to load walls until they have set, unless all walls are loaded alike, as the uneven weights on green walls are apt to crack them. All walls should be well braced, and wooden centres left in till they have set thoroughly.