hijirument far e:<htbit'wg Jupiter and his Moons. tij 



36J equal parts, therefore five revolutions of the month-hand, viz. 5 X 73, will complete 

 the year. The 365 days in the fpiral are divided into calendar months, the initials of each 

 of which ftand immediately over the beginning of the firft day of each ; fo that when the 

 inftrument is redified for the month, day, and hour at any particular time, the different 

 hands will preferve tlieir refpcftive fituations to the end of the year, and fo on from year 

 to year, provided the month-hand be put back one day in the fpiral at the latter end of 

 February in every leap year. This putting back, however, will not in the lead affeft the 

 motions of the fateilites; for the hands, being put upon the circular ends of the axles, will 

 turn in adjuftment without giving motion to the wheels. 



As the error in the firft and fecond fateilites will amount in one year to about three 

 hours and forty-four minutes motion of each, they muft alfo after that fpace be put to 

 their right places again, which is very eafily done thus : Turn the handle in a retrograde 

 direction till all the fateilites have come back 4h. 44;min. then hold the firft and fecond 

 in that particular pofition while the handle is again turned to its original fituation, and 

 tlien let them be at liberty. — If this is done at a quarter of a year after reftification, the 

 error will be fomewhat lefs than an hour, and it may render a new re£lification by the 

 nautical almanac, as will be defcribed hereafter, needlefs. 



If at any time the letter pointed to in the week circle, and' the correfponding day of the 

 month in the fpiral, in the firft, fecond, third, fourth, or fifth line, as the cafe may be, be 

 both attentively obferved, it will appear by infpedlion for what particular time the fatel- 

 litian ftands redificd in a given year after it has been out of ufe ; for, as the month-hand 

 lies over the five lines, that particular line is the proper one, the numerical point of which 

 anfwers to the initial of the day to which the week-hand points : for example, in the plate 

 the redification of the hands is for noon of Wednefday June 14, 1797. Without a regard- 

 to fuch an infpeclion, a new re£tification would become necelTary every time the inftru- 

 toent was to be ufed, and in fome cafes probably when the means of redlification might 

 not be immediately attainable. 



If the orbits of the fateilites were not neceflarlly too large in the fatellitian to be 

 proportional to thofe of the earth and Jupiter, when the latter is placed at fuch a diftance 

 from them as that a candle in its place may produce a diftin£t (hadow of each, 5-r\ times 

 the diftance between the two fockets, viz. 15-;% inches, would be the proper diftance of the 

 central candle from Jupiter, in which pofition the angle at Jupiter in the fatellitian fubtend- 

 cd by the diftance between the fockets, viz. 3 inches, would be equal to the parallax of the 

 earth's annual orbit feen from Jupiter j but as the exa6l proportions of all their orbits 

 cannot be preferved, a mechanical adjuftment becomes requifite to preferve the due ar- 

 rangement of the Ihadows upon the fcreen. 



The re£lification ought always to be very minutely attended to, and is thus effe£led : 

 Fix the fatellitian firmly on a table, and, fetting the centre of the candleftick jud four feel 

 from Jupiter, place a fteady burning candle, or rather a lamp, in the central focket, upon 

 the fame level with Jupiter in the inftrument, and a fimilar one in the focket fupported by 

 the arm at the fame height ; by this means two fliadows of Jupiter will fall upon the 

 fcreen when placed behind the fatellitian : Aide the fcreen as near Jupiter as a revolution 

 of the longeft arm will fuffer it to approach, which will be cpwaids of 2-| inches j and if 



the 



