732 Mr. herschel’s Agronomical Obfervatlons 
and on the 17th of June, at 9 h. 9', it was 7 f . 12 d. 27' 22", 
by which we obtain the difference or angle rem = ems = 
8 d. 3 ! ' 59^. 
Now a fpot on Mars, fituated in the direction me, will have 
made a fydereal revolution when it returns to the fame, or a 
parallel direction ms. From which we gather, that the fpot on 
the 17th of June, after coming to the line me, where it finifhes 
the ly nodical revolution, will have to go through an arch of 
8 d. 3 1 7 59", in order to arrive into the direction of the line 
ms, where it finifhes the fydereal rotation. The time it will 
take to go through this arch, at the fydereal rate of 24 h. 39' 20" 
to 360 degrees, or 4 // ,io9 per minute of a degree, will be 
35' 3^,8 ; this being divided by the numbers of revolution 
34, gives 1' i ;/ ,8 ; which, added to 24 b. 38' 20", 3, gives us 
24 h. 39' 22", 1 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 
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 fy nodical revolution when we wanted to 
find the fydereal rotation ; but if he had been dir eft, or if his- 
place had been more advanced in the ecliptic than that to which 
we compared it, as at p, then the line pa- parallel to em would 
be the direftion to which the fpot fhould return, in order to 
accomplifh a fydereal revolution, and therefore the quantity of 
the angle <rpe = per, or difference of the geocentric places ought 
to be fubtrafted from the fy nodical revolution to obtain the 
fydereal one* 
periods 
