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diftinfl likewife. This inflrument promifes alfo the faireft trial 

 to the wire micrometer, which has, I fear, too haftily been laid 

 afide. The prifmatic, the Caflegrain, the divided objed glafs, 

 the divided eye-glafs micrometers, have all their particular difad- 

 vantages, deriving error from metaphyfical as well as phyfical 

 fources ; not that I mean to infer that the wire micrometer is 

 unobjedionable. 



The dome of the upper room here is to be fimilar to that 

 of the equatorial room, and is defigned for this one inftrument. 

 Two obfervers in the fame room are too many for fuch obfer- 

 vations as are to be made here : He muft be a fteady pradifed 

 obferver, whofe imagination wall not be affe£led when another, 

 with a more perfedl inftrument, gives evident figns of his obfer- 

 vation being compleated, whilft to the former it has not yet 

 taken place. Thus let us fuppofe two telefcopes of different 

 powers applied to an emerfion of one of Jupiter's fatellites, the 

 better telefcope may perhaps fhcw it fourteen or fifteen feconds 

 fooner than the other ; now if the obferver with the better telef- 

 cope rife to write down his obfervatiori, he will not only 

 difturb the other with the noife, but the imagination of the 

 latter may even perfuade him that the emerfion has taken place 

 to his eye, when with fuch a telefcope it was impofTible. To 

 avoid this inconvenience, the lower room is defigned to permit 

 occafional obfervers to pradice, and mafonry is contrived to 

 fupport other telefcopes with proper ftability. The northern wing 

 may be applied to the ufe of a zenith feflor if required *, and an 



apparatus 



* Or of a pillar quadrant, if that fhould by any one hereafter be confidericl 

 as an expedient method for determining refraftions. 



