1 1 8 Mr. herschel’s Agronomical Obfervations 
of Jupiter 9 h. 56', of Mars 24 h. 40', how true foever It 
may be in a general way, was much too inaccurate for this cri- 
tical purpofe. The gyration of Venus was ftill lefs to be 
depended upon, being only noted to the hour without the 
minutes : it became, therefore, neceflary to proceed to obferva- 
tions of a more determinate kind. From what I had already feen 
of the rotation of the planets, I concluded, that Mars on feve- 
ral accounts would be the moft eligible planet for my purpofe : 
for the fpots on Jupiter change fo often that it is not eafy, 
if at all pofilble, to afcertain the identity of the fame appear- 
ance, for any confiderable length of time. Nor do the dark 
fpots only change their place, which may be fuppofed to be 
large black congeries of vapours and clouds fwimming in the 
atmofphere of Jupiter ; but alfo the bright fpots, though they 
may adhere firmly to the body of Jupiter, may undergo fome 
apparent change of fituation by being differently covered or 
uncovered on one fide or the other, by alterations in the belts. 
It will be feen hereafter, that I have obferved the revolution of 
a very bright fpot, not fufpedted of any change of fituation, to 
befirft, by one fet of obfervations, at the rate of 9 h. 51' 45", 6 ; 
and afterwards, by another fet immediately following at the 
rate of 9 h. 50' 48". 
As the principal belts on Jupiter are equatorial, and as we 
have certain conftant winds upon our planet, efpecially near 
the equator, that regularly, for ceitain periods, blow the fame 
way it is eafily fuppofed, that they may form equatorial 
belts by gathering together the vapours which fwim in our 
atmofphere, and carrying them about in the fame direction. 
This will, by analogy, account for all the irregularities of Ju- 
* See A&a Eruditorum, 1 687. Dr. halley’s Account of periodical Winds. 
4 piter’s 
