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

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                                under severall Denominaons, and as an 
cient as Ptolemy; also fully demonstrated 
by Clavius in Astrolabio, and by Aquilonius 
in its own nature. It is also to be seen in an 
Ancient Author, printed, before ye late contest 
about its Invention happen’d. Upon ye 
Supposition mention’d, it follows, if all ye 4. Projections treated off in ye Sector on a qua 
drant, albeit of different contrivances, and a 
5th for findg ye Azimuth in ye limb, wth ye 
Summer Tropick downwards, are no other 
yn ye Western sides of ye Horizontall Projec 
tion, fill’d with more Parallels, and ye Demon
straon of ye whole Stereographick Projection 
may be reduced to a few Proposions more 
easy than hitherto, rules in likelyhood to be ex 
spected in ye posthumous works of Tacquet 
now in ye Press at Anwerp.
Here we may observe, yt this Projection 
is one of ye most easy and speedy Instrunts 
in its Delinaon, and ye most usefull for 
findg ye Hour and Azimuth, yt can be made 
on a sudden w<sup>th<\sup> compasses, especially if thereto
be added an exact way for finding Semidia 
meters of ye Parallels, wth out finding their whole 

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John Collins
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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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