VOL. I.] PHILOSOPHICAL TRANSACTIONS. 3^ 



sistency of a pap, letting it stand for two days, that the water may extract all 

 the salt that is in the earth : then they pass this water into another pit, in 

 which it crystallizes into saltpetre. When they would have it whiter and purer, 

 they boil it in a cauldron once or twice, scumming it continually, and emptying 

 it out, whilst it is hot, into earthen pots which hold each 25 or 30 lbs. These 

 they expose to clear nights, and if there be any impurity remaining it falls to 

 the bottom. Afterwards they break the pots, and dry the salt in the sun. 



An Account of Hefelius's Prodromus Cometictis, ivith some Animad- 

 versions made upon it, hy a Frejich Philosopher. N° 6, p. 104. 



Hevelius, in his Prodromus, so called, because it is as an introduction to his 

 Cometography, gives an account of the observations he has made on the first 

 of the two late comets ; reserving those on the second for that great treatise, 

 in which he also intends to enter more into detail on the first. 



In this account he represents the rise, place, course, swiftness, phases and 

 train of this comet, and the causes of the generation of comets in general. 



He finds its apparent motion was not made in an exact great circle, but 

 deviating considerably from it. 



He is positive, that without the annual motion of the earth no rational ac- 

 count can be given of any comet, but that all is involved in perplexities, and 

 deformed by absurdities. 



He inquires what kind of line they describe by their motion, whether circu- 

 lar, or straight, or curved, or partly straight and partly curved ? And if curved, 

 whether regular or irregular? if regular, whether elliptic, parabolical, or hyper- 

 bolical ? He answers, that this motion is conical ; and is of opinion that by the 

 conic path all the phaenomena of comets can, without any incoveniency, be 

 readily solved. 



He puts it out of doubt that they are in the sky itself, producing reasons for 

 it that are very considerable, and alleging among others, that it is clearly 

 evinced by the parallaxes, which he finds far less in comets than in the moon, 

 and sometimes than in the sun itself. Where he also represents that he has 

 deduced the horizontal parallax of this very comet from one observation only, 

 by which he found that then it was distant from the earth 5000 semidiameters 

 of the same. From this distance from the earth, he deduces, that on that day 

 when it was so remote from the earth, its true diameter was 2d6o German 

 miles, which is three times larger then the diameter of the earth, and almost 

 six times larger than that of the moon, whose diameter, according to his 

 theory, is 442 German miles. 



