Turning the wheel round precisely one half is rendered certain by an index, suggested by Mr. Francis Barrow, and shown on an enlarged scale fig. 175. Two short lines distinguishable one from the other by dots, are engraved upon the circular edge of the brass eccentric, at the termination of a diametrical line, at right angles to a line drawn through its center, and those of its two eccentric holes; a fixed reading point, is attached to the upper edge of the arm. Previously to shifting the wave, the screw of the slide rest is turned, until one or other of the marks on the edge of the eccentric, is opposite the reading point; the radial arm of the spiral apparatus is then raised, and the wheel on the arbor turned round until the opposite mark agrees with the reading point, upon which the two wheels are replaced in gear. Placing the two wheels in gear for the first cut upon the work, with one of the marks on the eccentric agreeing with the index, when the tool has also been placed opposite its starting point; ensures that the extreme end of the ornament commences with the center of a wave.
The ornamentation resulting from grouping the waved lines, every line shifted consecutively, less than half the length of a wave, is obtained by placing the two wheels out of gear as before, and then, moving round the slide rest screw the required proportion by its micrometer; taking care that the wheel on the arbor is not disturbed, which also may be observed by the index to the eccentric.
The waved line when placed on the surface, fig. 181, diminishes in its undulations from the circumference to the center; this additional variety in the line, happily showing itself in like manner upon all curved solids, the depth of the wave decreasing, as the line travels from the greater to the lesser diameter.
The surface spiral apparatus fig. 172, is required for using the reciprocator upon the surface, upon cones and upon curves having a surface character; the wheel hitherto on the slide rest screw, is then carried upon the left hand side of the socket. The connection with the slide rest screw, being made by the pair of 30 and 60 mitre, or round edged wheels, as before. The arrangement of the apparatus is sufficiently apparent; but the length of the waves obtained by any pair of wheels, is doubled by the fraction of the second pair of wheels introduced by the surface spiral apparatus.