may be necessary to make small covered drains under them to convey it off, and to place a stratum of loose stone between the sides of the foundations and the surrounding earth to give it a free downward passage.

Tt may be laid down as a maxim in building, that mortar which is exposed to the action of frost before it has set, will be so much damaged as to impair entirely its properties. This fact places in a stronger light what has already been remarked, on the necessity of laying the foundations and the structure resting on them in hydraulic mortar, to a height of at least three feet above the ground; for, although the mortar of the foundations might be protected from the action of the frost by the earth around them, the parts immediately above would be exposed to it, and as those parts attract the moisture from the ground, the mortar, if of common lime, would not set in time to prevent the action of the frosts of winter.

In heavy walls the mortar in the interior will usually be secured from the action of the frost, and masonry of this character might be carried on until freezing weather commences; but still in all important works it will be by far the safer course to suspend the construction of masonry several weeks before the ordinary period of frost.

During the heats of summer, the mortar is injured by a too rapid drying. To prevent this the stone, or brick, should be thoroughly moistened before being laid; and afterwards, if the weather is very hot, the masonry should be kept wet until the mortar gives indications of setting. The top course should always be well moistened by the workmen on quitting their work for any short period during very warm weather.

The effects produced by a high or low temperature on mortar in a green state are similar. In the one case the freezing of the water prevents a union between the particles of the lime and sand; and in the other the same arises from the water being rapidly evaporated. In both cases the mortar when it has set is weak and pulverulent.

FRAMING.

487. FRAMING is the art of arranging beams of solid materiais for the various purposes to which they are applied in structures. A frame is any arrangement of beams made for sustaining strains. 488. That branch of framing which relates to the combinations of beams of timber is denominated Carpentry.

489. Timber and iron are the only materials in common use for frames, as they are equally suitable to resist the various strains to be met with in structures. Iron, independently of offering greater resistance to strains than timber, possesses the farther advantage of being susceptible of receiving the most suitable forms for strength without injury to the material; while timber, if wrought into the best forms for the object in view may, in some cases, be greatly injured in strength.

490. The object to be attained in framing is to give, by a suitable combination of beams, the requisite degree of strength and stiffness demanded by the character of the structure, united with a lightness and an economy of material of which an arrangement of a massive kind is not susceptible. To attain this end, the beams of the frame must be of such forms, and be so combined that they shall not only offer the greatest resistance to the efforts they may have to sustain, but shall not change their relative positions from the effect of these efforts.

491. The forms of the beams will depend upon the kind of material used, and the nature of the strain to which it may be subjected, whether of tension, compression, or a cross strain.

492. The general shape given to the frame, and the combinations of the beams for this purpose, will depend upon the object of the frame and the directions in which the efforts act upon it.

In frames of timber, for example, the cross sections of eacn beam are generally uniform throughout, these sections being either circular, or rectangular, as these are the only simple forms which a beam can receive without injury to its strength. In frames of cast iron, each beam may be cast into the most suitable form for the strength required, and the economy of the material.

493. In combining the beams, whatever may be the general shape of the frame, the parts which compose it must, as far as practicable, present triangular figures, each side of the triangles being formed of a single beam; the connection of the beams at the angular points, termed the joints, being so arranged that no yielding can take place. In all combinations, therefore, in which

the principal beams form polygonal figures, secondary beams must be added, either in the directions of the diagonals of the polygon, or so as to connect each pair of beams forming an angle of the polygon, for the purpose of preventing any change of form of the figure, and of giving the frame the requisite stiffness. These secondary pieces receive the general appellation of braces. When they sustain a strain of compression they are termed struts; when one of extension, ties.

494. As one of the objects of a frame is to transmit the strain it directly receives to firm points of support, the beams of which it is formed should be so combined that this may be done in the way which shall have the least tendency to change the shape of the frame, and to fracture the beams. These conditions will be best satisfied by giving the principal beams of the frame a position such that the strains they receive shall be transmitted through the axes of the beams to the fixed supports; in this manner there can be no tendency to change the shape of the frame, except so far as this may arise from the contractions, or elongations of the beams, caused by the strains; and as all unnecessary transversal strains will in like manner be avoided, the resistances offered by the beams will be the greatest practicable.

495. Whenever these conditions cannot be satisfied, the strains on the frame should be so combined that those which are not transmitted to the points of support shall balance, or destroy each other; and those beams which, from being subjected to a cross strain, might be either in danger of rupture, or of being deflected to so great a degree as to injure the stability of the frame, should be supported by struts abutting either against fixed supports, or against points of the frame where the pressure thrown upon the strut would have no effect in changing the shape of the frame.

496. The points of support of a frame may be either above, or below it. In the first case, the frame will consist of a suspended system, in which the polygon will assume a position of stable equilibrium, its sides being subjected to a strain of extension. In the second case the frame, if of a polygonal form, must satisfy the essential conditions already enumerated, in order that its state of equilibrium shall be stable.

497. The strength of the frame and that of its parts, and their consequent dimensions, must be regulated by the strains to which they are subjected. When the form of the frame and the direction and amount of the strain borne by it are given, the direction and amount of the strain which the different parts sustain can be ascertained by the ordinary laws of statics, and, from these data, the requisite dimensions and forms of the parts.

498. The object of the structure will necessarily decide the general shape of the frame, as well as the direction of the strains

to which it will be subjected. An examination, therefore, of the frames adapted to some of the more usual structures will be the best course for illustrating both the preceding general principles, and the more ordinary combinations of the beams and joints.

499. Frames of Timber. These are composed either entirely of straight beams, or of a combination of straight beams and of arches formed by bending straight beams.

Pieces of crooked timber are used either where the form of the parts requires them, or else where a strong connection is necessary between straight pieces that form an angle between them.

500. As has already been stated, the cross section of each beam is generally uniform and rectangular. This will, in some cases, give more strength than the character of the strain resisted may demand; and will, also, throw a greater amount of pressure on the points of support, than if beams of a form more strictly adapted to the object in view were used: but it avoids cutting the fibres across the grain, or making, as it is termed, grain-cut beams, and thereby materially injuring the strength of the piece. This objection, however, is only applicable to the parts of a frame formed of single beams. Wherever several thicknesses of beams are required in the arrangement of any part, the advantage may be taken of giving the combination the most suitable form for strength and lightness combined.

501. Frames for Cross Strains. The parts of a frame which receive a cross strain may be horizontal, as the beams, or joists of a floor; or inclined, as the beams, or rafters which form the inclined sides of the frame of a roof. The pressure producing the cross strain may either be uniformly distributed over the beams, as in the cases just cited, arising from the flooring boards in the one case, and the roof covering in the other; or it may act only at one point, as in the case of a weight laid upon the beam.

In all of these cases the extremities of the beam must be firmly fixed against immoveable points of support; the longer side of the rectangular section of the beam should be parallel to the direction of the strain, on account of placing the beam in the best position for strength.

If the distance between the points of support, or the bearing, be not great, the framing may consist simply of a row of parallel beams of such dimensions, and placed so far asunder as the strain borne may require. When the beams are narrow, or the depth

h

Fig. 56-Represents a cross section of horizontal beams a a braced by diagonal battens b.

of the rectangle considerably greater than the breadth, (Fig. 56.)

chort struts of battens may be placed at intervals between each pair of beams, in a diagonal direction, uniting the bottom of the one with the top of the other, to prevent the beams from twisting, or yielding laterally.

When the bearing and strain are so great that a single beam will not present sufficient strength and stiffness, a combination of beams, termed a built beam, which may be solid, consisting of several layers of timber laid in juxtaposition, and firmly connected together by iron bolts and straps,-or open, being formed of two beams, with an interval between them, so connected by cross and diagonal pieces, that a strain upon either the upper or lower beam will be transmitted to the other, and the whole system act under the effect of the strain like a solid beam.

502. Solid built Beams. In framing solid built beams, the pieces in each course (Fig. 57) are laid abutting end to end with

Fig. 57-Represents a solid built beam

of three courses, the pieces of each course breaking joints and confined by iron hoops.

a square joint between them, the courses breaking joints to form "a strong bond between them. The courses are firmly connected either by iron bolts, formed with a screw and nut at one end to bring the courses into close contact, or else by iron bands driven on tight, or by iron stirrups (Fig. 58) suitably arranged with screw ends and nuts for the same purpose.

Fig. 58-Represents an iron stirrup, or hoop a with nuts or female screws c which confine the cross piece of the stirrup b.

When the strain is of such a character that the courses would be liable to work loose and slide along their joints, the beams of the different courses may be made with shallow indentations, (Figs. 59, 60,) accurately fitting into each other; or shallow rec

Fig. 59-Represents a solid built beam of three courses arranged with indents and confined by iron hoops.

a

b

Fig. 60-Represents a solid built beam, the top part being of two pieces b, b which abut against a broad flat iron bolt a, termed a king bolt.

tangular notches (Fig. 61) may be cut across each beam, being