would be unsuited to the elementary character of this little work.

76. Centering. The centering of an arch is the temporary framework which supports it during its erection, and is formed of a number of ribs or centres, on which are placed the planks or laggings on which the work is built,

77. In designing centres, there are three essential points to be kept in view. 1st, that there should be sufficient strength to prevent any settlement or change of form during the erection of the arch. 2d, that means should be provided for easing or lowering the centre gradually from under any part of the arch. 3d, that, as the construction of centres generally involves the use of a large quantity of timber merely for a temporary purpose, all unnecessary injury to it should be avoided, in order that its value for subsequent use may be as little diminished as possible.

78. Where the circumstances of the case do not admit of piles or other supports being placed between the piers, it becomes necessary to construct a trussed framing resting on the piers, and of sufficient strength to support the weight of the arch. The tendency of this form of centre to rise at the crown, from the great pressure thrown upon the haunches during the erection of the arch, renders it necessary to weight the crowns with blocks of stone until it is nearly completed. Centres of this kind are always costly, and afford little facilities for easing.

79. Abutments. The tendency of any arch to overturn its abutments, or to destroy them by causing the courses to slide over each other, may be counteracted in three ways. 1st, the arch may be continued through the abutment until it rests on solid foundation, as in fig. 24. 2d, by building the abutments so as to form a horizontal arch, the thrust

Fig. 25.

is practicable, by joggling the courses together with bed-dowel joggles, so as to render the whole abutment one solid mass.

80. Wing Walls.-Where the wing walls of a bridge are built as shown in fig. 26, the pressure of the earth will always have a tendency to fracture them at their junction

with the abutments, as shown by the lines a b, c d. Equal strength with the same amount of material will be obtained by building a number of thin longitudinal and cross walls, as shown in fig. 27, by which means, the earth being kept from

the back of the walls, there is no tendency to failure of this kind.

81. Vaulting. The ordinary forms of vaults may be classed under three heads, viz., cylindrical, cored, and groined A cylindrical vault is simply a semicircular arch, the ends of which are closed by upright walls, as shown in fig. 28.

When a vault springs from all the sides of its plan, as in fig. 29, it is said to be coved. When two cylindrical vaults in

tersect each other, as in fig. 30, the intersections of the vaulting surfaces are called groins, and the vault is said to be groined.

82. In the Roman style of architecture, and in all common vaulting, the vaulted surfaces of the several compartments are portions of a continuous cylindrical surface, and the profile of a groin is simply an oblique section of a semi-cylinder.

83. Gothic ribbed vaulting is, however, constructed on a totally different principle. It consists of a framework of light stone ribs supporting thin panels, whence this mode of construction has obtained the name of rib and pannel vaulting. The curvature of the diagonal ribs or cross springers, and of the intermediate ribs, is not governed in any way by the form of the transverse section of the vault, and in this consists the peculiarity of ribbed vaulting. This will be understood by a comparison of figs. 30 and 31.

84. Domes are vaults on a circular plan. The equili brium of a dome depends on the same conditions as that of a common arch, but with this difference, that, although a dome may give way by the weight of the crown forcing out the haunches, failure by the weight of the haunches squeez

ing up the crown is impossible, on account of the support the voussoirs of each course receive from each other.

MASONRY-BRICKWORK-BOND.

85. The term masonry is sometimes applied generally to all cemented constructions, whether built of brick or stone; but generally the use of the term is confined exclusively to stone-work.

86. There are many kinds of masonry, each of which is known by some technical term expressive of the manner in which the stone is worked; but they may all be divided under three heads.

1st. Rubble work (fig. 32,) in which the stones are used without being squared.

2nd. Coursed work (fig. 33,) in which the stones are squared, more or less, sorted into sizes, and ranged in

3d. Ashlar work* (fig. 34), in which each stone is squared and dressed to given dimensions.

87. Different kinds of masonry are often united. Thus a wall may be built with ashlar facing and rubble backing; and there are many gradations from one class of masonry to another, as coursed rubble, which is an intermediate step between rubble work and coursed work.

88. In ashlar masonry, the stability of the work is

*In London, the term "ashlar" is commonly applied to a thin facing of stone placed in front of brickwork.