on the Rotation of the Planets , &c; i : r 
As to the firfl, it appears from the three monthly pern is 
obferved in 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' f'\. 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 
eonfiderable, that nothing but a miflake in every one of the 
three monthly periods, of at lead: one whole hour, could juflify 
fuch a fuppofition ; and that fucli.. a miflake in the fituation of 
a fpot on Mars cannot have been made in thofe obfervations, I 
think, is evident enough from the e'xadtnefs with which they 
were made, and from their agreement with each other. 
The fecond caufe of error, which is the uncertainty in 
affigning the ex aft time when a fpot comes to the center, is of 
feme 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 1.0' in an obfervation of any remarkable fpot’s coming to 
the center. However, not being willing to trufl more to the 
eye than ! ought to do, I had recourfe to the following experi- 
ment. I drew feveral circles of one inch radius, taking care to 
make no vifible impreffion of a center ; and placed in each a 
fine point at the feveral diftances- of .0424, .0 636, .0848, in ten 
thoufands of an inch from the real center; fome to the right, 
others to the left... Thefc meafures are the fines to radius one, 
of 2 0 2 6fi, 3° 39',. and 4° 5c 7 , which are the arches a fpot on. 
Mars paffes over in .10, 15, 20 7 minutes refpedlively.. I ex- 
pofed them to feveral perfons unacquainted with my defigns,. 
and found, that notone of them made a fmgie miflake in iay-. 
ing whether the point, was, or- was, not, in the center of the 
circle,,. 
