liijlrumentfor exhihittng Jiipitey and his Aloons. tjj 



In figure l of plate VI. is exhibited a lateral view of the wheels of this hiftrument, 

 which are calculated for producing the mean motions of the four fatellites, and contained 

 in the brafs box ABCD, 8-^'^ inches long within, 4 broad, and i^^ deep. Figure 2, EF, 

 reprefcnts the cover of this box, on which the different faces for the hour, week, and 

 month-hands are graduated and filvered : the fmall concentric wheels marked 8, 18, 27, 

 and 39 in fig. i« are all fixed on the fame revolving axle, placed on the point H, which 

 carries the hour-hand on its upper end once round in 24 hours ; but the larger correfpond- 

 ing wheels, 134, 129, 96, and 69, are fixed on the ends of as many feparate tubes or hol- 

 low axles of brafs, which juft turn eafily round within one another, and are fupported by a 

 fhoulder in the perpendicular fixed ftem that holds Jupiter over the point S ; the inner- 

 moll being the longed, and the next to the innermoft the next in length, as in a common 

 planetarium : thefe tubes have each a broad ring of a proper diameter to Aide a little way 

 upon their upper ends, to which are riveted the crooked arms which fupport the fatellites ; 

 and the rings are turned round the tubes in the adjuftment of the fatellites to their rela- 

 tive fituations ; the tubes alfo have flioulders each for the fupport of its next fuperior 

 wheel, and are kept in their places by the lower edges of the rings. 



All the parts of the fatcllitian are laid down in the plate in their proper proportions 

 and full fize, according to the following table, from which an artift of but little ingenuity 

 may eafily conftruft it, provided he is furniflied with, or has accefs to, a cutting engine for 

 making the wheels, the plate of which is properly divided for the required numbers. An 

 eafy method for dividing a plate is laid down in Mr. Fergufon's " Seleft Exercifes," in 

 fubftance thus : As all the teeth in any wheel are to 360**, fo are any odd number of teeth, 

 to the number of degrees to be marked out, and divided into that odd number; after wliich 

 the remaining even number may be very eafily divided by continued halves. 



Table of Dimenfions *. 



» The wheels in the table are proportioned thus : — As the fum of the teeth in any two corf efponding wheel* 

 is to the diftance between their centres, or fum of their femidiameters, fo is the number of teeth in each, 

 taken feparately, to its femidiameter : an allowance being afterwards made for the driving wheel being fome- 

 what larger in this proportion than the driven one, to make them work more eafily.— —-W. P. 



R2 ThougU 



