American School of Correspondence · Chicago, American school of correspondence · leaf 145
perly, are secured permanently bydowel pins. The lower end of the arms is provided with teeth,or threads, which work in the threads of a geared scroll, H, thepitch of which is one-half inch. The scroll turns in a recess inthe arch which is concentric with the cylinder. Around theperiphery of this… … Fig. 11. Section and elevation of scroll. It will be seen that, as the pitch of the scroll is one-halfinch, two revolutions will be necessary to give the arms and bendone inch movement, radially, and, as the scroll has 110 teeth, togive it two revolutions, would require twenty turns of the…
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