Description of the double horizontal dial presented to the Royal Society by John Collins

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                                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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Manuscript details

Author
John Collins
Reference
CLP/24/6
Series
Cl.P
Date
1667
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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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