STAIRCASES.

187

for the corresponding points 1, 2, 3. It will be sufficient to lay offthe angles AEl, A Eli, AEIII, with the bevel to find the inclina-tion of the face of the point EF, in each of the points 1, 2, 3, fig. 16,of the contiguous steps.

The principles of this construction may oe made veryclear, if recourse for explanation can be had to a model,either in wood or in plaster.

Staircases considered as continued surfaces, belong, atleast for their lower surfaces, to spiral surfaces, whichpossess great interest for the arts; and fall to be treat-ed of in the Twelfth Lesson,