m the Rotation af the Planets^ Ike, 
Third monthly period, 
D. H. M. S. 
May 13 It 25 51 
June 17 p 9 20 
/ " T _ 
34revol. 34 21 43 29 
m 1 ' ■ 1 - 1 ..— . m i .X 
t revolution = 24 b. 38' 20^,3 
This laft is, perhaps, as likely to be near the truth as 21174 
fince the fame fpot was here obferved for the third time, and 
therefore its motion become more familiar. 
Here we have three longer periods that agree to fifteen 
feconds, which is quite fufficient for extending the interval of 
•time to thofe ebfervations that were made in the year 1 777* 
But as thefe are the fynodical revolutions, it will be neceflary 
firft to reduce them to fydereal rotations. 
In figure 24. let us fuppofe the orbit of Mars, mabc, 
to be in the fame plane with the orbit of the earth, edfg ; 
and the axis of Mars to be perpendicular to his orbit. Let 
M, e, m, e, be the fituations of Mars and the earth on the 
13th of May and 17th of June ; then will the line em, that 
eonnedts the centers of Mars and the earth, point out the geoceir- 
tric place of Mars on the 13th of May ; and the line em, the 
geocentric place of the fame planet on the 17th of June. Draw 
er and ms parallel to er ; then will er point out the geocentric 
place of Mars on the 13th of May ; and the angle sme is equal 
to the angle mer. Now, by an ephemeris * the geocentric 
place of Mars, May 13. at 11 h. 26' was 7 f. 20 d. 59' 21" ; 
* The Nautical Almanac gives the geocentric place of Mars only to every fixth 
day ; for which reafon I ufed white’s Ephemeris, where it is given for every day, 
though perhaps not with fo much exadtnels as I could with, 
S 2 
and 
