Note 3 — PART SECOND.

I ought perhaps to have introduced this reasoning on the 5th. figure by observing, that every projection of every part of a screw, on a plane at right angles with the axis of such screw, is a circle; and that therefore the chord z, or the line g d, is the true projection of a proportionate part of any line, B C, fig. 4, when wrapped round a cylinder of equal diameter with the circle a b, fig. 5.