t 156 )

this opening fweep the arch AB. On the center 2, with the-
compafies extended to B, fweep the arch B C. On the center 3,
with the compafies extended to C, fweep the arch C D. On the
center 4, with the compafies extended to D, fweep D E, and the
volute will be turned once round. For the fecond time round,
begin at center 5, and extend the inftrument to E, and fweep
EG.. On the center 6, with the compafies at G, fweep GIL
On the center 7, with the compafies at H, fweep HI. On the
center 8, with the compafies at I, fweep IK, and the. volute will
be turned twice round..

The third time round begins with center 9, from which ex-
tend the compafies to K, and fweep KM. On the center 10,
with the compafs foot at M, fweep MN. On the center 11,
with the compafs foot at N, fweep NO. And laftly, on the
center 12, with the compafs foot at O, fweep OP, which wilf
complete the volute three times round.
Obferve, that the whole volute is composed of twelve qua-
drants of a circle, drawn from twelve, centers^ and gradually
contracted in by means of the diagonal lines in the eye, fee Fig. 3.
Therefore, as there are three complete turnings in the whole
convolution, each of thefe turnings is made, up of four qua-
drants.

4

How: