Some there are, who thinke that these Comets are nothing else, but exhalations from our earth, carried up into the higher parts of the Heaven. So _Peno_, _Rothmannus_ & _Galilæus_,[1] but this is not possible, since by computation ’tis found that one of them is above 300 times bigger than the whole Globe of Land and Water. Others therefore have thought that they did proceed from the body of the Sun, and that that Planet onely is _Cometarum officina, unde tanquam emissarii & exploratores emitterentur, brevi ad solem redituri_: The shop or forge of Comets from whence they were sent, like so many spies, that they might in some short space returne againe, but this cannot be, since if so much matter had proceeded from him alone, it would have made a sensible diminution in his body. The Noble _Tycho_ therefore thinkes that they consist of some such fluider parts of the Heaven, as the milkie way is framed of, which being condenst together, yet not attaining to the consistency of a Starre, is in some space of time rarified againe into its wonted nature. But this is not likely, for if there had beene so great a condensation as to make them shine so bright, and last so long, they would then sensibly have moved downewards towards some center of gravity, because whatsoever is condenst must necessarily grow heavier, whereas these rather seemed to ascend higher, as they lasted longer. But some may object, that a thing may be of the same weight, when it is rarified, as it had while it was condenst: so metalls, when they are melted, and when they are cold: so water also when it is frozen, and when it is fluid, doth not differ in respect of gravity. But to these I answer: First, Metalls are not rarified by melting, but molified. Secondly, waters are not properly condensed, but congealed into a harder substance, the parts being not contracted closer together, but still possessing the same extension. [Sidenote 1: _Tycho Progym. l. 1. cap. 9._]