. 



** And I have reason to think," says Mr. rluygens, " tlie** 

 are one or two more still behind. For between the 

 fourth and fifth l!;ev- is a di tmice not at all proportion- 

 able to that, between till the others. Hence it is [proba- 

 ble, there -may be a sixth. And there may not impro- 

 bably be another, without the tilth, whkh has hitherto 

 escaped us. For we can never see the -fifth but in that 

 part of its orbit, which is towards the west. 7 ' 



Saturn himself revolves with them round the sun in 

 about thirty years. tJUi is supposed to be :iifteen times 

 bigger than the earth. 



If we Compute the magnitude of the planets in num.- 

 Jjer of miles, the diameter of the moon is supposed to be 

 2 1 75 miles, that of Mercury .2748, that o! Mars 4875, 

 -of the earth, (and nearly of Venus) 7967, of Saturn 

 345 1, of Jupiter 130653 ; and that of the sim 822 148. 



With regard to their distance- from the earth, there is 

 ,$uch an immense difference in. the calculations of astro- 

 jiomers, even with respect to the distance of the sun 

 (which some demonstrate to be ninety millions of miles, 

 Bothers to be not three millions ftom the earth), that it 

 is the wisest to .cnutess our ignorance,, and to acknow- 

 ledge we have nothing to rest on here, but mere micer- 

 ,-tain conjectuie 



.9- Comelsr are opuke bodies, which emit numerous 

 ^ray>, sometimes forwards, someiimes backwards, some- 

 ; limes all round the body of the. comet. Now they sink 



near the body of the sun : then they rise far beyond the 

 ,orb of Saturn, ^ome suppose' them to be imperfect 

 .planets, or such a chaos oi unformed matter, as may 

 .hereafter be forme J into an earth like outs. Probably 

 .those rays which they emit, are only vapours, by which 



the rays of the sun, are refracted to us. 



Hence tiiey have a difterenr appeara.uce, according 

 ,<is tliey are (Jhierenlly situated with regard to the comet: 



the nearer they ,are to tlie sun, the more those rays are 



increased, and the farther they recede from it, the more 

 .those are diminished. And hence some imagine that. 

 ,iixd stars, covered with vapours .and .spots, become co 



