on the Rotation of the Planets , &c. 1 1 9. 
piter’s revolutions, deduced from fpots on his dilk that may 
have changed their fituation ; for if we fuppofe the rotation of 
Jupiter, according to Cassini, to be. 9 h. 56', then fo me fpots 
that I have obferved mutt have been carried through about 6o° 
of Jupiter’s equator in 22 of his revolutions or days. This 
would certainly be a very great velocity in the clouds, which is, 
however, not unparalleled by what has happened in our own 
atmofphere. 
But to return to my purpofe : on the planet Mars we fee fpots 
of a different nature ; their conftant and determined fhape, as 
well as remarkable colour, (hew them to be permanent and 
fattened to the body of the planet. Thefe will give the revo- 
lution of his equator to a great certainty, and by a great num- 
ber of revolutions, to a very great exadtnefs alfo. Suppofing 
then, that, by a method I fhall hereafter defcribe, we can de- 
termine whether a fpot on the difk of Mars is, or is not, in 
the line which joins the center of the earth and the center of 
that planet, to half an hour’s time with certainty (I believe 
ten or twelve minutes will be found fufficient for that pur- 
pofe), in this cafe we fhall in 30 days have the revolution true 
to a minute ; and, by continuing thefe obfervations for three 
months, we fhall have it to 20" . When we are fo far certain, 
we can eafily arrive to a much greater degree of exaflnefs ; for 
as we now can no longer miftake a whole revolution, if we take 
the time of any particular fpot’s being in the line which joins the 
centers of the planets during one oppofition of Mars, and take 
the fame again at or near the next oppofition, we fhall have an 
interval of about 780 days, which will give the diurnal motion 
of that planet true to about 2" . The next oppofition will give 
it to one, and fo forth ; by which means, and by taking a proper 
number of fuch periods, we may determine the rotation of 
Mars 
