126 liijlruinent for exhibiting Jupiter atid his Afoons. 



•the central candle and Jupiter form a right angle at the other candle, the two fliadofls 

 •will cover one another •juft fo much as is reprefented i\\ fig. 4. or fig. 6. -, but if this is not 

 cxaftly the cafe, the candleftick muft be brought nearer, or removed farther, till this ap- 

 pearance is produced ; and then the fcreen mufl be fecured in its place, and the candleftick 

 be fufl'crcd 'to remain. In the nest place, -with a pair of bow-compafles defcribe a circle 

 coincident with that fhadow which is occafioned by the central candle; and within that, 

 three other concentric circles, the largeft of the three to touch the edge of the other fha- 

 dow when the arm remains unmoved ; the next to touch the fame when the arm is 46' 

 from conjundtiou with Jupiter and the central candle ; and the fmalleft to touch the fame 

 when the arm is 24" from a fimilar fituation. 



Here then the central candle will reprefent the fun ; the outermoft circle In fig. 4. or 5. 

 tlie diametrical fe£lion of the fun's ftiadow at the orbit of the firfl fatellite ; the fecond 

 circle is fuppofed to reprefent the appearance of the fame at the orbit of the fecond ; the 

 third, the appearance at that of the third ; and the fmall one, the appearance at that of 

 the fourth, as viewed in perfpe£tive from the earth ; whilft the candle carried by the arm 

 will be the reprefentative of the earth. Remove now the central candle, and Jupiter's 

 remaining fliadow will be his fituation as viewed from the earth, coinciding with his real 

 fhadow, reprefented by the concentric circles, as far as the orbit of the fecond fatellite, and 

 then difunltin^. 



Hitherto the fatellltes themfelves have been difregarded, and mufl; next be placed in their 

 proper fituations by the help of a * nautical almanac, thus : Look for the day, for which 

 the fateUitian is to be rectified, among the configurations given in the lad page of every 

 month, and the relative apparent fituations of all the fatellites, at the hour fpecified at the 

 top, will be found to the right and left of the central cypher which reprefcnts Jupiter. 

 When the numerical figure ftands between the reprefentative point of any fatellite and Ju- 

 piter, the fatellite is approaching him ; but when the point is put the nearer, It is reced- 

 ing from him : alfo, when a fatellite is approaching on the right hand of Jupiter, or re- 

 ceding on the left, it is in its fuperlor femicircle, and is placed above the centre of the cy- 

 pher -, but when receding on the right, or approaching on the left, it is in its inferior one, 

 and ftands higher than the centre. The relative fituations and diredlon of motion of each 

 fatellite being obferved, put them all to their places as near as the eye can guefs by the 

 lliadows compared to the points in the almanac ; then, having prevloufly marked the great- 

 eft elongation of the fourth with little points on the fcreen, by means of a fe£lor or dia- 

 gram of fimilar triangles, make the diftatice of each fhadow, from Jupiter's centre on the 

 fcreen, bear the fame proportion to the diftance of each point from the centre of the 

 cypher in the almanac, that the greateft elongation of the fourth on the former bears to 

 the greateft elongation of the fame in the latter: this will be attended with no difficulty.— 

 The re£tification for the true places of mean motion will however be the more accurate the 



• The reftification may be made mod accurately by calculating the time of a conjunftion of each fatellite, 

 tneanot apparent, as may be required, by the help of Wargentin's Tables, in any given month: but every 

 jcadcr cannot be fuppofed to have fuch Tables ; and therefore the configurations are here fubftituted as af- 

 ■fwdjng a more general, as well as fwriiliar method.— —W. P. 



o fmaller 



