[ 270 ] 
eonfideratlon of the figures, that, as the effeCt of par- 
allax depreftes the Moon directly towards the hori- 
zon, fo it will always encreafe her apparent diftance 
from a ftar, except in the cafe reprefented by Fig. 
5th i that is to fay, unlefs the zenith diftance of the 
Moon be lefs than that of the ftar, and, at the fame 
time, the arch M P be greater than r M or half 
the diftance of the Moon from the ftar. Q^E. D. 
Remarks on the ufe of the two foregoing rules. 
It has been remarked, after the rule for refraCtion 
above, that if the altitudes of the Moon or ftar are 
under 10 degrees, the zenith diftances muft be firft 
lefiened by 3 times the refractions correfponding to 
their refpeCtive altitudes before the effeCt of refrac- 
tion be computed. 
But in order to compute the effeCt of parallax from 
the fecond rule, the obferved diftance of the Moon 
from the ftar muft be firft corrected by adding the 
effeCt of refraCtion to it found by rule the firft ; 
as muft the obferved altitudes of the Moon and ftar 
be alfo corrected by taking from them their refpec- 
tive refraCtion in altitude, and the corrected arches 
thus found muft be made ufe of in computing the 
parallax. Only, if the altitudes of the Moon and 
ftar are both 10 degrees or more, part of the calcu- 
lation of rule the fecond may be faved, and arch the 
fecond, found by rule the firft, taken for arch A in 
the fecond rule without any fenfible error. In this 
cafe, it will be moft convenient to obferve the fol- 
lowing order of computation inftead of that before 
4 prefcribed 
