[36] Pp. 159-162.
[37] Infra, book iv. ch. ii. On Abstraction.
[38] I must, however, remark, that this example, which seems to militate against the assertion we made of the comparative inapplicability of the Method of Difference to cases of pure observation, is really one of those exceptions which, according to a proverbial expression, prove the general rule. For in this case, in which Nature, in her experiment, seems to have imitated the type of the experiments made by man, she has only succeeded in producing the likeness of man's most imperfect experiments; namely, those in which, though he succeeds in producing the phenomenon, he does so by employing complex means, which he is unable perfectly to analyse, and can form therefore no sufficient judgment what portion of the effects may be due, not to the supposed cause, but to some unknown agency of the means by which that cause was produced. In the natural experiment which we are speaking of, the means used was the clearing off a canopy of clouds; and we certainly do not know sufficiently in what this process consists, or on what it depends, to be certain à priori that it might not operate upon the deposition of dew independently of any thermometric effect at the earth's surface. Even, therefore, in a case so favourable as this to Nature's experimental talents, her experiment is of little value except in corroboration of a conclusion already attained through other means.
[39] In his subsequent work, Outlines of Astronomy (§ 570), Sir John Herschel suggests another possible explanation of the acceleration of the revolution of a comet.
[40] Discourse, pp. 156-8, and 171.
[41] Outlines of Astronomy, § 856.
[42] Philosophy of Discovery, pp. 263, 264.
[43] See, on this point, the [second chapter of the present Book].
[44] Ante, [ch. vii. § 1].
[45] It seems hardly necessary to say that the word impinge, as a general term to express collision of forces, is here used by a figure of speech, and not as expressive of any theory respecting the nature of force.