on the Rotation of the Planets, &c. 1^5 
As to the fi'rft. It appears from the three monthly periods 
obfervedin the year 1779, when the proper allowances for the 
geocentric places are made, that the fydereal revolution of Mars 
cannot well be lefs than 24 h. 39' 5", nor more than 
24 h. 39' 22' ' ; but if we fhould divide any one of the three 
biennial periods by a fuppofed number of revolutions, only one 
more or one lefs than we have done, the difference would be fo 
confiderable, that nothing but a miftake in every one of the 
three monthly periods, of at leaft one whole hour, could juftify 
fuch a fuppofition ; and that fuch a miftake in the fituation of 
a fpot on Mars cannot have been made in thofe obfervations, I 
think, is evident enough from the exadfnefs with which they 
were made, and from their agreement with each other. 
The fecond caufe of error, which is the uncertainty in 
affigning the exaft time when a fpot comes to the center, is of 
fome. force. But it feems to me highly probable, from the 
manner in which I have feen the fpots on Mars pafs over the 
difk of that planet, that there can hardly be fo great an error 
as 1 o' in an obfervation of any remarkable fpot’ s coming to 
the center. However, not being willing to truft more to the 
eye than I ought to do, I had recourfe to the following experi- 
ment. I drew feveral circles of one inch radius; taking dare to. 
make no vifible impreftion of a center; and placed in each a 
fine point at the feveral diftances of .0424, .0636, .0848, in ten 
thoufands of an inch from the real center ; fome to the right, 
others to the left. Thefe meafures are the lines to radius one, 
of 2 0 26', 3 0 39' , and 4 0 52', which are the arches a fpot on 
Mars paftes over in 10, 15, 20' minutes relpedlively. I ex~ 
pofed them to feveral perfons unacquainted with my deiigns^ 
and found, that not one of them made a fingle miftake in fay- 
ing whether the point was, or was not, in the center of the 
circle* 
