530 CORRESPONDANCE. 1684. 



ters whilfl: I was in England; but now you may take the following account from 

 me as an eye-witness, but you mufl: think I could not hâve time in that f hort period 

 while I ftayed with him, (wich in ail was not above an hour and a half,) to look 

 nicely on every part in't, nor could I civilly be fo inquifitive, and afk fo many 

 queftions of one that was a perfeét ftranger to me, at my firft vifit, as thoroughly 

 to make me underiland the motion and ail its parts ; fo you mufl: be content with 

 what I obferved then, and can remember now, which is as follows: — It fliands up 

 againfl: the wall like a clock 4). Theoutward dial-plate, where the great circle of 

 the ecliptick is defcribed, containing within it ail thofe of the planets, placed ac- 

 cording to the hypothefis of Copernicus, is about two foot fquare. This fhows 

 you at once the minute, hour, day of the month and year, with the exa6l pof- 

 tures and afpedts that ail the planets bear to the fun and one another at that very 

 moment, and alfothefite of the fatellites in refpeél of their middle planets, viz. the 

 three of "b, and the four of % — thefe and ail the planets abfolve their courfe,jufl: 

 in the famé time as they do in the heavens. Hère you hâve ail rétrogradations, ex- 

 centricitys, and other irregular motions defcribed. He told me that in 1682, when 

 Saturn and Jupiter were in conjunélion feveral timesby their rétrograde motion s), 

 that they were fo jufl: as oftcn in his machine as in the heavens. He has not hère 

 (for he could [not] with convenience) made the planets, and the diameter of thefe 

 circles they defcribe anfwer according to that proportion they truly bear to one 

 another; but this he has donc by a fmaller fcheme in one corner of his plate, ac- 

 cording to the latefl: obfervations he and the Parifian afl:ronomers could make. He 

 complained much that the motion of Saturn was not yet certainly regulated by 

 any of the aftronomers. I obferved its motion was far more excentrick on his plate 

 than any of the refl: of the planets. In the infide 't is a very plain pièce of work, 

 confifliing of not above eight or nine wheels. As I could difcern, thechiefor 

 principle motion that fets thefe agoing is not a pendulum, but a balance regulated 

 by fprings, as thofe of pendulum watches are. He can with a key turn the whole 

 engine about, (wich he did,) and fet ail the wheels agoing, fo as to put it to any 

 day that is to come for thefe three hundred years, and fo find out how the pofl:ure 

 of the planets will be then. Yet after ail this, 'tis nothing more than an ingénions 

 curiofity, and is fo far from being fo exaél as to fupply the place of ephemerides 

 as I imagine 'tis of little or no ufe atall; for I afked him could he by help of it 

 exaétly détermine an ecclipfe, and I obferved he would not give me a pofitive anf- 

 wer, as being loath to confess the imperfections of his contrivance to me that fee- 

 med to admire it fo much as I did. You may fee fuch a fort of an engine defcribed 

 in your lafl: volume of the , Journal des Scavans", in the firfl: month ^). After this he 

 carried me into his garden and fhowed me every particular thing belonging to his 



'^) Voir la figure de la Lettre N°. 2289. s) Voir la Lettre N°. 2278. 



*') Le numéro du 19 janvier 1682. 



