MATHEMATICAL SECTION. 45 
TOMO P cote ; : : 
The expression 7; will in the actual computation be written in 
the form 
do 365.25 k m’ 0 
dt Ya(l+m)sini2= ~~” 
in which 
j CG a Tp. > Sq. 
. , ° , , , 0 Nie 
Using the values 0’ = 14°36’, a’ = 0.200, 7’ = 7°,and m’ = a p9009 
derived by me some time since for the intra-Mercurial planet, the 
perturbation of the node of Venus’ orbit is found to be 
60 = — 3.92120 
in a century. 
Mr. Hix, commenting on this paper, discussed briefly the prob- 
able mass and density of Mercury on grounds of probability and 
analogy, pointing out that its density would on such grounds appear 
to be far less than the ordinarily accepted value. 
18tH MEETING. APRIL 15, 1885. 
In the absence of the Chairman, Mr. DooiTrLe was made Chair- 
man pro tem. 
Present, ten members and one guest. 
Minutes of the sixteenth and seventeenth meetings were read and 
approved. 
Mr. Marcus Baxer then read the following paper on 
A GROUP OF CIRCLES RELATED TO FEUERBACH’S CIRCLE. 
In any plane triangle the middle points of the sides and the feet 
of the perpendiculars drawn from the vertices to the opposite sides 
are six points in the circumference of a circle. This circle also 
bisects the segments of the perpendiculars between orthocenter and 
vertices, thus making nine noteworthy points. These properties 
33 
