Bijnmetii for exhibiting Jupiter and his Moont. 127 



fonaner the grand equation of the fatellites, which depends upon Jupiter's anomaly at the 

 time of redtification. 



The lad moft favourable time was on 0£tober i r, 1794, and the next leaft favourable 

 will be o'l July 5, j8oo: the former will recur on Augufl 21, i8c6, and the latter on 

 May 17, 1 812; and two of each will follow at nearly fix years diftance from each other 

 in each revolution of Jupiter afterwards. 



If the diameters of the cyphers in the nautical almanac, which are too fmall, were fo fau 

 augmented as to bear an * exa£t proportion to the greateft elongations of the fatellites there 

 exhibited, which appears to be not the cafe, their apparent places, meafured by thofe dia- 

 meters,, might be eafily afcertained without further trouble on any given day, as well as 

 their true places of mean motion at the fultable times above fpecified. 



The configuration fig. 7. is for half paft fix P. M. on Odlober 1 1, 1 794, where it appears, 

 from what has been already remarked, th^t the firft fatellite is approaching Jupiter in its 

 fuperior femicircle near its greateft weftern elongation ; the fecond receding in itsinferior }, 

 and the third and fourth receding each in its fuperior ; tha third being near its greateft 

 caftern elongation. The correfponding places of the fliadows on the fcrecn, when the fa— 

 tellitian is re£tified for this time, will appear as in, fig. 8. 



The fliadows may be made to fall a little higher in their inferior femicircle than in their.- 

 fuperior, by giving the inftrument a fmall, recllnation with a thin wedge placed under the 

 end next the candle, or by an adjufting fcrew preffiiig againft the table,^ which efFedl is 

 greateft with the fourth. 



In this reprefentation the fatellites are very nearly in their true places of mean motion, 

 as well as in their apparent places : therefore, if the fatellitian be rectified for Saturday 

 October 11, 1794> at half paft fix in the evening,, by this latter configuration, and the 

 handle be turned till the month-hand comes to the end of February 1796, before it be put 

 back a day, and thence forward to the prefent time (1798), provided the firft and fecond 

 arms be alfo adjufted for their errors in motion, as already direcled, the redification will. 

 be more accurate, for mean, motion at prefent than if made by the. almanac at a!ny other 

 time. 



From' 1794 to 1800 the difference between mean and apparent motion of the fatellites, 

 rejefting the fmaller equations, is increafing ; and from 1800,. when it will be a maximum, 

 to 1 806, it will decreafe, the apparent being fafter than the mean, according to the year 

 f^r which the fatellitian is to be ufed : from 1806 to 1812 it will- again increafe, after 

 which time a decreafe will commence ; and, in a little lefs than every fix years, the 

 increafe and decreafe will continue to be alternate for the time to follow.. 



Laftly : Put the index of the candleftick to the fun's place in the ecliptic, and the hello* 

 centric longitude of Jupiter taken from a nautical almanac or White's Ephemeris, imme-- 

 diately between the central ftem and Jupiter in the machine, by means of a thread ftretched 

 and tied to both, which will ferve as an index,, as well as a guide for the diftance of the 



• The diameter of Jupiter is reprefcnted in the oppofite extreme, in the Encyclopaedia Britaonica, Ijoth in 

 Sg. 1 8. plate 6z. and in fig. 177. plate 79. though it is faid, in vol. ii. part 2. p. 577. that " the. orbits of 

 Jupiter's moons are drawn in true proportion to his diameter'' in tlie latter diagram, and are. evidently in. 

 tended to be fuch in the f«rmer,— -W, P, 



candJeftickj^ 



