liijirument for exhihiiing Jtiplttr and his Moans, 13 1 



1 7. Hence it will appear, that an eclipfe feen from the earth is an occultatioii feen from 

 the fun. 



18. That an eclipfe is never vifible at the fun. 



19. And that the fliadow of a fatellite, as feen from the earth, falls on Jupiter fome- 

 times before, fometimes after, and fometimes at its tranfir, according to the relative fitua- 

 tions of Jupiter, the fun, and earth. 



20. If the central candle only be fuffered to remain, and the little balls 'themfelves be 

 -viewed in an oblique dire£lion, the reafon vv'ill be apparent why tl^e fatellites arc feen 

 iunated from Jupiter : 



21. And alfo Jupiter Iunated when feen from them ; 



22. But neither of them Iunated when feen from the earth or fun. 



23. It will alfo be evident why the fun is frequently eclipfed to the inhabitants of Jupi- 

 ter by the fhadows of his fatellites. 



24. And, lafliy, why the fatellites, as feen from the earth, do not eclipfe one another, 

 Thefe, and perhaps other phenomena not fpecified here, will be illuftrated by the fatel- 



lltian in fo diflin£l a manner, as to convey to a fpeftator, who may compare the apparent 

 motions of the fliadows to the real motions of the little balls, a clear conception of tlie re- 

 lation that the apparent motions of Jupiter's fatellites, as viewed from the earth with a te- 

 lefcope, have to their true circular motions, as viewed by the inhabitants of Jupiter. But 

 though a clear conception may thus be formed by a fpe£lator of the true and apparent 

 motions of the fatellites as they regard Jupiter himfelf, yet he mud be informed that the 

 real track in which they move round the fun along with Jupiter, the progreffive centre of 

 fheir detached fyftem, is neither circular nor elliptical, but in a line which croffes Jupiter's 

 track in a fmuofe manner, more or lefs frequently as their periods are (horter or longer. 

 As the velocity of the firft and fecond fatellites exceeds the velocity of Jupiter, they are 

 not only apparently, but really retrograde in fome part of every revolution ; on which ac- 

 count their tracks form loops which are alternately concave and convex towards the fun ; 

 the concavity being greater than the convexity : but as the velocity of Jupiter exceeds that 

 of the third and fourth, thefe are not really but only apparently retrograde at the inferior 

 femicircles : on this account their tracks are always concave, except at their llationary 

 points, which project and divide the concave f^aces. Thefe real tracks will eafily be com- 

 prehended, if, while the fatellites are in motion, Jupiter himfelf be alfo conceived to have 

 a dire£t motion flower than the two firft, but quicker than the two laft. 



If now, after what has been f\ud, we conceive ourfelves, like the philofophical poet *, 

 conveyed into the regions of Jupiter and his fatellites, we (hall with him find caufe for 

 adoring the power, wifdom, and goodnefs of the Almighty Creator ! When we confidcr 

 that Jupiter, the diameter of which planet is more than tentimes larger than that of our 

 earth, has a rotation on its axis in the fmall fpace of nine hditrs and fifty-fix minutes, we 



■" remote from day's all-cliccring fource. 



" Large Jupiter performs his coiiftaiu courfc ; 



'' Four friendly moons with borrow 'd luftre rife, 



-" iicftow their beams bcniijn, and light his Ikies."'' BAteCK-JjC^/'fer/f. 



»S 2 mud 



