132 Mr. herschel’s Agronomical Obfervations 
and on the 17th of June, at 9 h. 9', it was 7 f . I2 d. 2 j' 22", 
by which we obtain the difference or angle rem — ems ~ 
8 d. 3 i r 5 9 / h 
Now a fpotonMars, fituated in the direction me, will have 
made a fydereal revolution when it returns to the fame, or a 
parallel diredlion ms. From which we gather, that the fpot on 
the 17th of June, after coming to the line me 9 where it finifhes. 
the fynodical revolution, will have to go through an arch of 
8. d. 3 1' 59 // , in order to arrive into the diredtion of the line 
ms y where it finifhes the fydereal rotation. The time k will 
take to go through this arch, at the fydereal rate of 24 h. 39/ 20 // 
to 360 degrees, or 4^,109 'per minute of a degree, will be 
35' 3", 8 ; this being divided by the numbers of revolution 
34, gives 1' 1 // ,B ; which, added to, 24 h. 38' 20", 3, gives us 
24 h. 39 / 22 ;/ ,i for the fydereal revolution of Mars, as found by 
the third of the monthly periods. This quantity will help us 
to find a proper divifor for the three following long biennial 
periods. 
It is to be obferved, that Mars has been retrograde in the. 
above example, for which reafon the meafure of the angle- ems 
was to be added to the fynodical revolution when we wanted to 
find the fydereal rotation ; but if he had been diredl, or if his 
place had been more advanced, in the ecliptic than that to which, 
we compared it, as at y, then the line ycr parallel to EM would 
be the direction to which the fpot fihould return, in order to. 
accomplifh a fydereal revolution, and therefore the quantity of 
the angle cyr — yrr, or difference of the geocentric places ought 
to he fub trailed from the fynodical revolution to obtain the 
fydereal one* 
