by a number of writers on the theory of the equilibrium of arches, and tables for effecting the necessary numerical calculations have been drawn up from their results to abridge the labor in each case.

517. In cloistered arches the abutments may be less than in an ordinary cylindrical arch of the same length; and in groined arches, in calculating the resistance offered by the abutments, the counter resistance offered by the weight of one portion in resisting the thrust of the other, must be taken into consideration.

518. When abutments, as in the case of edifices, require to be of considerable height, and therefore would demand extraordinary thickness, if used alone to sustain the thrust of the arch, they may be strengthened by the addition to their weight made in carrying them up above the imposts like the battlements and pinnacles in Gothic architecture; by adding to them ordinary, full, or arched buttresses, termed flying buttresses; or by using ties of iron connecting the voussoirs near the joints of rupture below the keystone. Tie-rods are evidently the safest expedient. The employment of these different expedients, their forms and dimensions, will depend on the character of the structure and the kind of arch. iron tie, for example, cannot be hidden from view except in the plate-bande, or in very flat segment arches, and wherever its appearance would be unsightly some other expedient must be tried.

The

Circular rings of iron have been used to strengthen the abutments of domes, by confining the lower courses of the dome and relieving the abutment from the thrust.

519. In a range of arches of unequal size, the piers will have to sustain a lateral pressure occasioned by the unequal horizontal thrust of the arches. In arranging the form and dimensions of the piers this inequality of thrust must be estimated for, taking also into consideration the position of the imposts of the unequal arches.

520. Precautions against Settling. One of the most difficult and important problems in the construction of masonry, is that of preventing unequal settling in parts which require to be connected but sustain unequal weights, and the consequent ruptures in the masses arising from this cause. To obviate this difficulty requires on the part of the engineer no small degree of practical tact. Several precautions must be taken to diminish as far as practicable the danger from unequal settling. Walls sustaining heavy vertical pressures should be built up uniformly, and with great attention to the

ARCHES.

259

bond and correct fitting of the courses. The materials should be uniform in quality and size; hydraulic mortar should alone be used; and the permanent weight not be laid on the wall until the season after the masonry is laid. As a farther precaution, when practicable, a trial weight may be laid upon the wall before loading it with the permanent one.

up

in

Where the heads of arches are built into a wall, particularly if they are designed to bear a heavy permanent weight, as an embankment of earth, the wall should not be carried up higher than the imposts of the arches until the settling of the latter has reached its final term; and as there will be danger of disjunction between the piers of the arches and the wall at the head, from the same cause, these should be carried dependently, but so arranged that their after-union may conveniently effected. It would moreover be always well to suspend the building of the arches until the season following that in which the piers are finished, and not to place the permanent weight upon the arches until the season following their completion.

be

521. Pointing. The mortar in the joints near the surfaces of walls exposed to the weather should be of the best hydrau lic lime, or cement, and as this part of the joint always requires to be carefully attended to, it is usually filled, or as it is termed pointed, some time after the other work is finished. The period at which pointing should be done is a disputed subject among builders, some preferring to point while the mortar in the joint is still fresh, or green, and others not until it has become hard. The latter is the more usual and better plan. The mortar for pointing should be poor, that is, have rather an excess of sand; the sand should be of a fine uniform grain, and but little water be used in tempering the mortar. Before applying the pointing, the joint should be well cleansed by scraping and brushing out the loose matter, and then be well moistened. The mortar is applied with a suitable tool for pressing it into the joint, and its surface is rubbed smooth with an iron tool. The practice among our military engineers is to use the ordinary tools for calking in applying pointing; to calk the joint with the mortar in the usual way, and to rub the surface of the pointing until it be comes hard. To obtain pointing that will withstand the vicissitudes of our climate is not the least of the difficulties of the builder's art. The contraction and expansion of the stone either causes the pointing to crack, or else to separate from the stone, and the surface water penetrating into the cracks thus made, when acted upon by frost, throws out the

pointing. Some have tried to meet this difficulty by giving the surface of the pointing such a shape, and so arranging it with respect to the surfaces of the stones forming the joint, that the water shall trickle over the pointing without entering the crack, which is usually between the bed of the stone and the pointing.

522. The term flash pointing is sometimes applied to a coating of hydraulic mortar laid over the face or back of a wall, to preserve either the mortar joints, or the stone itself, from the action of moisture, or the effects of the atmosphere. Mortar for flash pointing should also be made poor, and when it is used as a stucco to protect masonry from atmospheric action, it should be made of coarse sand, and be applied in a single uniform coat over the surface, which should be prepared to receive the stucco by having the joints thoroughly cleansed from dust and loose mortar, and being well moistened.

No pointing of mortar has been found to withstand the effects of weather in our climate on a long line of coping. Within a few years a pointing of asphalte has been tried on some of our military works, and has given thus far promise of a successful issue.

523. Stucco exposed to weather is sometimes covered with paint, or other mixtures, to give it durability. Coal tar has been tried, but without success in our climate. M. Raucourt de Charleville, in his work Traité des Mortiers, gives the following compositions for protecting exposed stuccoes, which he states to succeed well in all climates. For important work, three parts of linseed oil boiled with one-sixth of its weight of litharge, and one part of wax. For common works, one part of linseed oil, one-tenth of its weight of litharge, and two or three parts of resin.

The surfaces must be thoroughly dry before applying the compositions, which should be laid on hot with a brush.

524. Repairs of Masonry. In effecting repairs in masonry, when new work is to be connected with old, the mortar of the old should be thoroughly cleaned off wherever it is injured along the surface where the junction is effected, and the surface thoroughly wet. The bond and other arrangements will depend upon the circumstances of the case; the surfaces connected should be fitted as accurately as practical ble, so that by using but little mortar, no disunion may take place from settling.

525. An expedient, very fertile in its applications to hydraulic constructions, has been for some years in use among the French engineers, for stopping leaks in walls and renew

ing the beds of foundations which have yielded, or have been otherwise removed by the action of water. It consists in injecting hydraulic cement into the parts to be filled, through holes drilled through the masonry, by means of a strong syringe. The instruments used for this purpose (Fig. 83 a) are usually cylinders of wood, or of cast iron; the bore uniform, except at the end, which is terminated with a nozle of the usual conical form; the piston is of wood, and is driven down by a heavy mallet. In using the syringe, it is adjusted to the hole; the hydraulic cement in a semi-fluid state poured

into it; a wad of tow, or a disk of on top before inserting the piston. by repeated blows on the piston.

leather being introduced The cement is forced in

526. A mortar of hydraulic lime and fine sand has been used for the same purpose; the lime being ground fresh from the kiln, and used before slaking, in order that by the increase of volume which takes place from slaking, it might fill more compactly all interior voids. The use of unslaked lime has received several ingenious applications of this character; its after expansion may prove injurious when confined. The use of sand in mortar for injections has by some engineers been condemned, as from the state of fluidity in which the mortar must be used, it settles to the bottom of the syringe, and thus prevents the formation of a homogeneous mass.

527. Effects of Temperature on Masonry. Frost is the most powerful destuctive agent against which the engineeer has to guard in constructions of masonry. During severe

winters in the northern parts of our country, it has been ascertained, by observation, that the frost will penetrate earth in contact with walls to depths exceeding ten feet; it therefore becomes a matter of the first importance to use every practicable means to drain thoroughly all the ground in contact with masonry, to whatever depths the foundations may be sunk below the surface; for if this precaution be not taken, accidents of the most serious nature may happen to the foundations from the action of the frost. If water collects in any quantity in the earth around the foundations, it 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.

It 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.