252 Mr. Herseuer’s Obfervations — 
meafures. muft he corrected for the latitude of the {pot; next, — 
they muft be reduced to a heliocentric obfervation, which will 
alfo correét them from the difference occafioned by the different 
fituation of the planets when they were taken. ‘This. being 
done, we may feleé&t two obfervations at a proper diftance; from 
which, by trigonometry, we fhall have the, node and inclinas 
tion ofi the axis.. When thefe elements are obtained, it, will 
be eafy to fee how other obfervations agree with them ; which 
will afford the means of correcting or verifying the former 
calculations. par oh a 
‘Let T, fig. 29. (tab. X) be the earth; & Q gq ¥8 the eclipticas 
feen from Ts Pthe point of the heavens towards which the 
north pole of the earth is directed ;, M the place of the orbit of 
Mars » 7M, where an obfervation of the poles of that planet has 
been made, which isto be reduced to its hehocentric meafures 
And, firtt, fuppofe it to have been made; at the time of the 
oppolition of that planet... Then, the place Mor Q in. the 
ecliptic being given, we have the fides Q &, Go Ps whence 
the angle Q, of the right-angled triangle P 35 Q, is found, 
This being added to, or taken from, the obferved angle of pas 
fition of the axis of Mars, according to circumftances. eafily 
to be determined, reduces it to.its heliocentric pofition, But 
if this obfervation, was not made at the time_of an oppefition, . 
but at fome other place m, a fecond correétion is to be applied 
in the following manner. 
Let the angle g, of the triangle P & g, be found as before, - 
and. properly \applied to the pofition of, the axis of Mars now 
at m3; then make the angle mSu, at the fun S, equal.to the 
angle Sm'T, and » will be the heliocentric place, where the 
angle of pofition; when feen from 5, will; appear.to beas it 
was found at mw, after the. application. of the, Arlt correétion : 
‘ for! 
