i 34 Mr. herschel’s Agronomical Obfervations 
D. H. M. s. 
Third biennial period, 1777, April 26 9 5 o 
1779, June 19 8 40 22 
783 23 35 22 
S. D. ' " 
Geocentric places 6 1 24 36 
7 12 31 48 
i 1 r 712 
Turned into time 783 23 35 22 
and fubtrafled — 2 45 15,6 
763 revol. 783 20 50 6,4 
1 revolution = 24 h. 39' 23", 04. 
As thefe three periods are fupported by obfervations of equal 
validity, I fhall take a mean of them all for the neareft approxi- 
mation to the true fydereal revolution of Mars on his axis, 
which therefore is 24 b. 39' 
It remains now only to fee how far we may depend upon this 
determination of Mars’s diurnal rotation as coming near the 
truth ; and looking over thofe caufes which may poffibly pro- 
duce any errors, we find, firft of all, that in the long biennial 
periods a miftake in the number of revolutions would produce 
a confiderable deviation from truth. Secondly, in the obferva- 
tions of a fpot which moves fa flow, we are alfo liable to fome 
confiderable miftake in eftimating the time when it comes to a 
certain place ; and the more fo, if that place is not the center. 
Laftly, the time itfelf is liable to inaccuracy. 
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