i 3 6 
Mr. herschel’s Agronomical Obfervations 
circle, and which way it deviated from It. As the di reft ion of 
the motion of a fpot on Mars is known, 1 thought the perfons 
who were to judge of the place of the points were in titled to 
be acquainted with the line in which they were placed, which 
for that reafon was always to the right and left only. The 
points that anfwer to the excentricity of 1 5 and 20' are indeed , 
fo vifibly out of the center, that 1 believe we may fafely fay, 
that any nf flake, in eftimating the time of a fpot on Mars 
coming to the center, cannot well exceed a quarter of an hour 
at the outfide. 
As for the third and lafl occafion of error, the time itfelf, I 
believe my manner of obtaining and keeping it in the year 
1779 will appear htisfadory, and may, I think, be depended 
upon to a few feconds ; but the obfervations of the year 1777, 
indeed, are far from having the fame advantage. I was not 
then provided with an altitude inflrument, therefore fet my 
clock by a good fun-dial, with the equation of time contained 
in the Nautical Almanac, and found it to agree generally to a 
minute or two with the time calculated for the eclipfes of Ju- 
piter’s fir ft fatellite, as I deduced it for Bath from the Nautical 
Almanac. However, it was certainly liable to an error of 
feveral minutes ; therefore, allowing no lefs than io' for the 
clock in 1777, and 20' for an error in eftimating the fituation 
of a fpot in 1779, it will both amount to half an hour : then, 
if we take a mean of the three numbers, whereby we have 
divided the three biennial periods, we have 766.7 ; and half an 
hour, divided by 766 7, will therefore give us the quantity to 
which, it feems, can amount, all the uncertainty in the fydereal 
diurnal rotation of Mars, which is 2" ,34. 
A nearer approximation to truth I hope to obtain at the next 
oppoftion, which will happen about the middle of July 1781. 
5 I have 
