The volute compas is similar in principle to a beam compas except that the pencil in the slider is not fixed but moves with respect to the stationary point as the compas is turned. The compas operates by turning the beam round the fixed point in such a manner that the large screw head remains in one orientation. If this is the case the slider will be moved down the beam at a rate in proportion to the beam's rotation, causing a spiral to be drawn. The various drums (pictured in the foreground) give various spirals. The apparatus was designed in 1760 by David Lyle.
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