1476
THEORY AND PRACTICE OF ENGINEERING.
Book II.
bm of the mould, and trace its contour for the cylindrical section in which the stringis comprised : the upper and lower arrises are drawn by means of plumb-lines, and a level
Fig. 2690.
corresponding with the section of the steps at fig. 2687. and 2691. The string is sunk toreceive the ends of the steps within it.
Geometrical staircases are now usually formed without a string, each step being partlysupported by tightly wedging its end into the wall, and it is further assisted by restingupon the step below it. The outward end of such steps have the nosing returned on them :the under joint, where two steps unite, is made perpendicular to the soffite of the staircase,and the depth of the step, perpendicularly through the middle, must depend upon thequality of the stone ; when of a hard quality, like the Cragleith, the following equation maybe applied. Suppose w = the greatest uniform load on a square foot, including the weightof the stone itself = 300 pounds: the length of the flight of stairs, measured on its soffite,and which we will call C D = 10 feet, and the length of the step 6 feet, represented by B D;
then,
: C D* x D B a „ . /
(I or\/ -
300 x 100 x 36
= -55 feet will be the depth of the
/(CD' + DB 1 ) 26000 x (100 + 36)"
step, from the middle of the tread to the soffite, in a perpendicular direction. Where thestep is inserted in the wall, from 4 inches to 8 may be the depth of the indent; butthis must depend upon the length the steps project from the face: in staircases wherethere are windows, considerable care is required to work the soffite in a regular and easy