7o6 ESSAYS AND OBSERVATIONS 



No phaenomenon feems more extraor' 

 dinary, relating to Jupiter, than that, ac- 

 cording to Caflini, fome of his fpots re- 

 volve in lefs time than others ; the dif- 

 ference amounting to 4^. This is a phae- 

 nomenon, of that kind, of which it is per- 

 haps beft not to attempt any expHcation, 

 till the fame be confirmed by more ob- 

 fervations. 



It is worthy of our notice, that, from 

 feveral phaenomena, aftronomers con- 

 clude the fatellites to revolve on their 

 axes, in the fame time that they revolve 

 around their primaries ; by which means 

 the exceeding great tides that would be 

 produced in them are avoided. Thofe 

 arifing from their various diltances from 

 the primaries, in their apftdes^ may be 

 fufficient for agitating their waters. The 

 tides that would be produced in our 

 moon, from this clrcumftance , alone, 

 ought to be conGderably greater than the 

 tides produced in our ocean ; and pof^ 

 fibly, by a careful attention to the limits 

 of thofe bjack parts of the moon v/hich 



formerly 



