ninth an Equatorial Micrometer. 335 
refraction, will arrive together at the horary circle LX at the 
(ame time, and with the fame difference of declination, as if 
ho refraCtion had taken place. It is therefore only the difference 
of refraCtion which takes place in objeCts at different heights 
in the fame field , that can alter their relative fituations : how- 
ever, it appears neceffary to examine what this may amount 
to. 
Let the letters in fig. 2. denote the fame things as before: 
to which we will add, that a. A, B, C, D, denote the parallel 
horary wires of the micrometer, and AA , BB, the declina- 
tion wires, denoted A and B in the tables : now from the celcftial 
globe we fhail alfo readily obtain the horary angle VLP — 54T 
— L be. Let now an objeCt pafs along the wire A A from the 
horizontal line at d to the vertical line at h \ in this it will pafs 
through a difference of refraCtion, according as it gets more 
and more elevated above the horizontal line HO; and let the 
elevation Lb be half a degree or 30 minutes : then, according 
to Dr. Bradley’s Table of RefraCtion*, the difference of 
refraCtion betwixt the 78th and 79th degrees of zenith diffance 
is 23". 6, half of which 1 i'A8, may be effeemed the difference 
of refraCtion for a difference of half a degree of altitude at 
78°! zenith diffance, or of u°| altitude: the objeCt, there- 
fore, in paffing from the horizontal line at d to the vertical line 
at b paffes through every difference of refraCtion from o x/ to 
n'hS ; and the queffion is, how much it is at a medium , that 
is, when it arrives at the middle wire at the point c ? F rom 
this point let fall the perpendicular ce . Now, the proportion 
of the {ides of the triangle db L being given from conffruction, 
they may be taken off by a fcale, viz* 
* Inferred in Dr. MasxelYne’s Obfervations, Vol. I. ]>. 13. 
Vol. LXXVII. C c c Sup- 
