Natural Questions Chapter 3

DE NUBIBUS Grandinem hoc modo fieri si tibi affirmauero quo apud nos glacies fit, gelata nube tota, nimis audacem rem fecero. Itaque ex his me testibus numero secundae notae, qui uidisse quidem se negant; aut, quod historici faciunt, et ipse faciam: illi cum multa mentiti sunt ad arbitrium suum, unam aliquam rem nolunt spondere sed adiciunt: "Penes auctores fides erit". Ergo si mihi parum credis, Posidonius tibi auctoritatem promittit tam in illo quod praeteriit, quam in hoc quod secuturum est: grandinem enim fieri ex nube aquosa iam et in umorem uersa sic affirmabit tamquam interfuerit. Quare autem rotunda sit grando, etiam sine magistro scire possis, cum adnotaueris stillicidium omne glomerari, quod et in speculis apparet, quae umorem halitu colligunt, et in poculis sparsis aliaque omni leuitate; non minus foliis si quae guttae adhaeserunt, in rotundum iacent. "Quid magis est saxo durum? Quid mollius unda? Dura tamen molli saxa cauantur aqua." Aut, ut alius poeta ait: "stillicidi casus lapidem cauat." Haec ipsa excauatio rotunda fit; ex quo apparet illud quoque huic simile esse quod cauat: locum enim sibi ad formam et habitum sui exsculpit. Praeterea potest, etiamsi non fuit grando talis, dum defertur, corrotundari et, totiens per spatium aeris densi deuoluta, aequabiliter atque in orbem teri. Quod nix pati non potest, quia non est tam solida, immo quia fusa est, et non per magnam altitudinem cadit sed circa terras initium eius est: ita non longus illi per aera sed ex proximo lapsus est. Quare non et ego mihi idem permittam quod Anaxagoras? Inter nullos magis quam inter philosophos esse debet aequa libertas: grando nihil aliud est quam suspensa glacies, nix [in] pruina pendens [congelatio]. Illud enim iam diximus, quod inter rorem et aquam interest, hoc inter pruinam et glaciem nec non inter niuem et grandinem interesse.

Si je t’affirmais que la grêle se forme dans l’air, de même que la glace parmi nous, par la congélation d’une nuée entière, ce serait par trop de témérité. Range-moi donc dans la classe de ces témoins secondaires qui disent : « Je ne l’ai pas vu, certes, mais je l’ai ouï dire. » Ou encore, je ferai ce que font les historiens : ceux-ci, quand ils ont, sur nombre de faits, menti tout à leur aise, en citent quelqu’un dont ils ne répondent pas, et dont ils laissent à leurs auteurs la responsabilité. Si donc tu es disposé à me croire, Posidonius s’offre pour garant tant de ce que j’ai dit ci-dessus que de ce qui va suivre. Il affirmera, comme s’il y eût été, que la grêle provient de nuées pleines d’eau, ou même déjà changées en eau. Pourquoi les grêlons sont-ils de forme ronde ? Tu peux le savoir sans maître, si tu observes qu’une goutte d’eau s’arrondit toujours sur elle-même. Cela se voit sur les miroirs qui retiennent l’humidité de l’haleine, sur les vases mouillés, et sur toute surface polie. Vois même les feuilles des arbres, les herbes, où les gouttes qui s’y arrêtent demeurent en globules. Quoi de plus dur qu’un roc ? quoi de plus mou que l’onde Qui laisse au dur rocher une empreinte profonde ? Ou, comme a dit un autre poëte : Goutte à goutte en tombant l’eau creuse enfin la pierre Et ce creux est sphérique. D’où l’on voit que l’eau qui le produit l’est aussi, et se taille une place selon sa forme et sa figure. Au reste, il se peut, quand les grêlons ne seraient pas tels, que dans leur chute ils s’arrondissent, et que, précipités à travers tant de couches d’un air condensé, le frottement agisse sur tous et les façonne en boules. Cela ne saurait avoir lieu pour la neige ; elle est trop peu ferme, trop dilatée, et ne tombe pas d’une grande hauteur, mais se forme non loin de la terre. Elle ne traverse pas dans les airs un long intervalle ; elle se détache d’un point très rapproché. Mais pourquoi ne prendrais-je pas la même liberté qu’Anaxagore ? car c’est entre philosophes surtout qu’il doit y avoir égalité de droits. La grêle n’est que de la glace suspendue ; la neige est une congélation flottante parmi les frimas. Nous l’avons déjà dit : entre l’eau et la rosée il y a la même différence qu’entre le frimas et le glaçon, comme entre la neige et la grêle.

I should be somewhat too bold if I were to assure you as on oath that hail is formed in the sky much in the way ice is with us, only that in the former case a whole cloud is frozen. So I may regard myself as a witness only in the second degree--one of those who say not that they have actually seen but have been informed. Or, I may, for once, do as the chroniclers do. After lying at large to their heart’s content, they fix on some one point for which they refuse to vouch, adding: Evidence of this will be found in the authorities. So, if you do not believe me, Posidonius will vouchsafe to you his authority both for the statement I have made, and for one that I am going to make. He will assure you, as confidently as if he had witnessed the process of formation, that hail is formed from a cloud that is charged with rain, and has already turned into moisture. You can discover without a tutor why the hail is round if you observe that drops of all kinds tend to become globular. This is seen, for example, in looking-glasses, which gather moisture from the breath, as well as in cups, and any other smooth surface bedewed with it. So, too, in the leaves of grass or trees, any drops that adhere take a circular form. What is harder than rock, what softer than water? Yet the hard rock is hollowed by drops of the soft water; or, as another poet tells us: The drip by its fall hollows the stone: and this hollow is itself round. Whence it is evident that its shape resembles this drip which hollows it out, sculpturing the spot to its own form and character. Besides, the hail, even were it not of this shape, might be rounded in its fall, and worn equally on all sides into globular form as it is again and again whirled round in its descent through the space of thick air it traverses. Snow, on the contrary, cannot be affected thus, because it is not so solid, being indeed very much scattered, and falling from no great height. It has its source in the neighbourhood of the earth, and its descent is of no great distance through the air, but starts from a point quite close by. Why should I not allow myself the same licence as Anaxagoras in differing from my authorities? Nowhere can equality of rights be claimed with more propriety than among the philosophers. Hail is simply ice held suspended in mid-air; snow is a floating congealed mass of the nature of hoar-frost. I have already said that the difference between water and dew is reproduced in the difference of hoar-frost and ice, and, in like manner, in that between snow and hail.