Chap. XXIV.

MASONRY.

1441

The acute angles are objectionable in this arrangement; greater strength is obtained byadopting the form laid down in the small figure ABCD.

Fig- 2527. VAULTS OF DIFFERENT DIAMETERS CONNECTING OBLIQUELY,

Descending Vaults , intersecting at right Angles. — These are adopted to support the stepsof a staircase, as well as to form its ceiling: by a reference to the letters on the planand sections, as well as to the several stones shown in isometrical perspective, its constructionmay be understood. The springing stone, fig. 2531., the third voussoir, fig. 2529., the counterkey, fig. 2530., and the key, fig. 2528., may have their several places found in the arch, byobserving the letters on the plan and sections corresponding with them.

It has always been the practice with masons, when greater strength and security wererequired than could be obtained by the stone itself, to use metal cramps of different forms,adapted for the purpose to which they were applied; dowel, dovetail, and cauked orcogged are severally used to secure the joints both vertically and horizontally that arelikely to be acted upon by the sliding or moving of any of the stones after they arebedded.

The Dowel Cramp is generally a piece of round iron, varying from J inch to 3 inches indiameter, and from 1 to 9 inches in length, according to the dimensions of the stones ; twoholes are drilled into the contiguous faces of the work, exactly opposite to each other, anda little larger than the dowells to be inserted, which are either fixed in soft fluid plasterof Paris and water, or run with very hot molten lead : when the latter method is adopted,a small channel is cut out between the faces of the contiguous stones to introduce the lead,and a small cup or funnel is formed at the end of the channel with damp clay, into whichthe lead is poured from a ladle : care must be taken to cut this channel in a perpendiculardirection, or obliquely to the back of the work, so that it may not be seen after itscompletion.

Dovetail Cramps are made of flat pieces of cast or wrought-iron, with the ends spreadingout like the tail of a dove or swallow ; a cavity of the same form is sunk half into onestone and half into the other, so that the cramp may be sunk rather more than its thicknessinto both, after which it is run with lead.

Descending Vault, intersecting obliquely -In this example we are required to make

another section to indicate the length of the joints, and obtain the development of thesoffites; the great vault being oblique to this projection, the section through its centre isexpressed by portions of ellipses, determined by perpendiculars raised from all the points

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