134 Mr. herschel/'s Agronomical Obfervations 
D. H. M. S. 
Third biennial period, 1777, April 26 9 5 o 
1779, J une I 9 8 40 22 
783 23 35 22 
S. D. / " 
Geocentric places 6 1 24 36 
7 12 3 1 48 
1 11 7 12 
Turned into time 783 23 35 22 
and fubtra&ed — 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 h. 39' 21", 6 y. 
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 miflake in the number of revolutions would produce 
a confiderable deviation from truth. Secondly, in the obferva- 
tions of a fpot which moves fo 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. 
3 
As 
