No. CXVIII
This is the way to draw the spiral—
Tie a piece of strong thread with a loop at its end round the upper part of the windings of a screw. Drive the screw into a board, through the middle of a card, wind the thread down the screw so that its loop just reaches the card, place a pencil in this loop, and draw the spiral freely, unwinding the thread from the grooves of the screw, and keeping it always taut. A perfect spiral is the result.
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