Secondly, ye Joynt use of ye Projection and Common Dyall, is, yt ye one may and ye other in ye true placing of ye Dyall or finding a meridian-line, as ye Author shows in <s>ye <\s> his 5th? proposition; but there is too much trouble and uncertainty therein. Of ye Ruffe. To remedy ye Inconvenience mentioned , there is a Circle or Ruffe, contrived to slide on, and draw off, upon ye Axis, ye Radius where of being Radius, there is a naturall Tan gent of 23 1/2 days graduated thereto, on ye Cock or Axis both upwards and downwards. The Circle itself represents ye Parallel of ye Suns declination, and ye chief use of it is suddenly to find a True meridian-line, by turning ye dyall about (standing Horizontall) till ye shaddow of ye Ruffe fall on ye Tangent of ye Declinaon on ye Cock, then may a Me ridian-line be traced by ye side of ye Dyall, whereby to place it, when ye Ruffe is laid aside, the which together wth ye Cock is a Dyall of itself, without ye bottom-plate or projec tion, and may be placed by ye meridian line so found.
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Description of the double horizontal dial presented to the Royal Society by John Collins, 1667. From The Royal Society, CLP/24/6
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