cédés d’isolement en beaucoup de cas sont impuissants, et ces cas sont ceux où l’effet, étant produit pair un concours de causes, ne peut être divisé en ses éléments. Les méthodes d’isolement sont alors impraticables. Nous ne pou-
terial change in the pre-existing circumstances. It is observed that dew is never copiously deposited in situations much screened from the open sky, and not at all in a cloudy night, but if the clouds withdraw even for a few minutes, and leave a clear opening, a deposition of dew presently begins, and goes on increasing… Dew formed in clear intervals will often even evaporate again, when the sky becomes thickly overcast. The proof, therefore, is complete that the presence or absence of an uninterrupted communication with the sky causes the deposition or non-deposition of dew. Now, since a clear sky is nothing but the absence of clouds, and it is a known property of clouds, as of all other bodies between which and any given object nothing intervenes but an elastic fluid, that they tend to raise or keep up the superficial temperature of the object by radiating heat to it, we see at once that the disappearance of clouds will cause the surface to cool ; so that Nature, in this case, produces a change in the antecedent by definite and known means, and the consequent follows accordingly : a natural experiment which satisfies the requisitions of the Method of Difference.
