108 ESSAYS anp OBSERVATIONS. 
moon’s orbit, and may always be ex~ 
prelied by a conftant number, viz. that of 
the 2ds of a degree equal to the radius of 
a circle, The diftance of the vertex from 
its point on the difk may be conceived to 
be made up of two parts, pofitive, or ne~ 
ative: The firft of which is the perpendi~ 
cular diftance of the centre of the given 
parallel circle from the plane of the difk 5 
the fecond part is the perpendicular di- 
{tance of the given vertex from a plane 
pafling through the forefaid centre, and 
parallel to the difk. 
5 i Pof. 7 when the Jat. pl. 
rt Pane’ lth x Sel. © § : 
Neg. § and dec], © are 
of $5 neo ipecies, 
different 
Cof. lat: pl. X cof. decl_ © X cof. deg. 2 noon 
See RR. oe ee 
Nene if the given. hour from: noon be - nea than 6+ 
8 
12. It would be eafy,. from the fore~ 
going rules, to conftruct tables of the pa- 
rallaxes for any given latitude. Such ta- 
bles would be fufficiently exact, Gf calcus 
lated for every third degree of the fun’s 
longitude, and every quarter of an hour 
from 
