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THEORY AND PRACTICE OF ENGINEERING.

Book II.

springing, as at C. To determine the lower circle of the first row, a semicircle is firstdescribed, through which the summit of a spherical triangle is traced; having then dividedthe angle formed by this circle, we must proceed as shown in the figure, drawing thespherical angles upon each caissoon, which is a tedious process. For octagonal and lozenge-c

CAISSOONS IN SPHERICAL VAULTS.

shaped coffers a similar system must be adopted, taking care to project the several linesupon the spherical diagonals.

In setting out the caissoons of a dome, like that of the Pantheon at Rome , after havingdecided the number, we must commence with the plan, and set out the lower tiers, with alltheir sinkings and ribs between: perpendicular lines drawn up from these divisions to thebase line of the elevation of the dome will determine the commencement of the severalcurved lines that are to bound the caissoons ‘in their vertical position: to mark out theirdirection, an inner circle, of the diameter of the dome, must be drawn on the plan, at theheight of the first caissoon, divided in a similar manner, and then the perpendicular linesbe proceeded with as before.

Domes that are elliptical on their plan must, in order that the caissoons may not producea disagreeable effect, have their rows comprised between horizontal ellipses similar to thatof the base. The projection of the sides should also form right lines meeting at the centre;these lines, which vary in length, are the great axes of vertical semi-ellipses, which givedifferent measure, and curves to the developed width of each row of caissoons, and even forthe upright edges of each.