quires peculiar care in segmental, askew, and rampant arches. In the first, the thrust of the arch being very great, it will be well, in heavy arches, to make the joints of the interior courses of the abutment, for some courses at least below the impost, oblique to the horizon to counteract any danger from sliding. The top stone of the abutment, termed the cushion stone of the arch, should be well bonded with the stones of the backing, and its bed, or bottom joint should be so far below the impost joint, that the stone shall offer sufficient strength to resist the pressure on it. In the askew arch the abutments are not uniformly loaded, and the entire thrust of the arch will not be received by the abutments if the arch is constructed in the usual manner. Each of these points requires peculiar attention: the first demanding the thickness of the abutment to be suitably regulated; the second that the arch be so built that the thrust may be thrown, as nearly as practicable, parallel to the planes of the heads. To effect this last point, the portion of the arch above the upper joints of rupture (Fig. 54) must be divided into several zones, each of these zones being built without any connection with the two adjacent to it, but with their ends so arranged that this connection may be formed, and the arch made continuous after the centres are struck By this plan the settling will take place after uncentring without causing cracks, and the thrust will be thrown on the abutments in the direction desired. In rampant arches, the impost joint being oblique to the horizon, care must be taken, if this obliquity be not less than the angle of friction of the stone used, either to cut the impost into steps, or else to use some suitable bond, or iron cramps and bolts to prevent disjunction between the arch and abutment. 474. The abutments of right and of slightly oblique cylindrıcal arches are made of uniform dimensions; but when the ob liquity is considerable, it may be necessary to increase the thickness of a portion of each abutment where there is the greatest pressure. In conical and conoidal arches the abutments will in like man in dimensions with the span. ner vary 475. In cloistered arches the abutments will 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. 476. 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 key stone. The employment of these different expedients, their forms and dimensions, will depend on the character of the structure and the kind of arch. The 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. 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. 477. When abutments sustain several arches above each other, like relieving arches in tiers, their dimensions must be calculated to sustain the united thrusts of the arches; and the several portions between each tier must be strong enough to resist the thrust of their corresponding arches. 478. 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. 479. 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 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. 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 carrried 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 up independently, but so arranged that their after-union may be 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. 480. Pointing. The mortar in the joints near the surfaces of walls exposed to the weather should be of the best hydraulic 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 becomes 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 enter ing the crack which is usually between the bed of the stone and the pointing. 481. 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. 482. 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, onc 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. 483. 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. The bond and other arrangements will depend upon the circumstances of the case; the surfaces connected should be fitted as accurately as practicable, so that by using but little mortar, no disunion may take place from settling. 484. 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 renewing 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. 55) 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 leather being introduced on top before inserting the piston. The cement is forced in by repeated blows on the piston. 485. 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. 486. Effects of Temperature on Masonry. Frost is the most powerful destructive agent against which the engineer 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 |