In the beginning of this year (1862) the Duke of Manchester, in writing to the Rev. W. Airy, had said, “I wish your brother, the Astronomer Royal, could be induced to have investigations made as to whether the aspects of the Planets have any effect on the weather.”  This enquiry produced the following reply: 

A subject like that of the occult influences of the planets (using the word occult in no bad sense but simply as meaning not thoroughly traced) can be approached in two ways—­either by the a priori probability of the existence of such influences, or by the a posteriori evidence of their effects.  If the two can be combined, the subject may be considered as claiming the dignity of a science.  Even if the effects alone are certain, it may be considered that we have a science of inferior degree, wanting however that definiteness of law and that general plausibility which can only be given when true causes, in accordance with antecedent experience in other cases, can be suggested.

Now in regard to the a priori probability of the existence of planetary influences, I am far from saying that such a thing is impossible.  The discoveries of modern philosophy have all tended to shew that there may be many things about us, unknown even to the scientific world, but which well-followed accidents reveal with the most positive certainty.  It is known that every beam of light is accompanied by a beam of chemical agency, totally undiscoverable to the senses of light or warmth, but admitting of separation from the luminous and warm rays; and producing photogenic effects.  We know that there are disturbances of magnetism going on about us, affecting whole continents at a time, unknown to men in general, but traceable with facility and certainty, and which doubtless affect even our brains and nerves (which are indisputably subject to the influence of magnetism).

Now in the face of these things I will not undertake to say that there is any impossibility, or even any want of plausibility in the supposition that bodies external to the earth may affect us.  It may well be cited in its favour that it is certain that the sun affects our magnetism (it is doubtful whether it does so immediately, or mediately by giving different degrees of warmth to different parts of the earth), and it is believed on inferior evidence that the moon also affects it.  It may therefore seem not impossible or unplausible that other celestial bodies may affect perhaps others of the powers of nature about us.  But there I must stop.  The denial of the impossibility is no assertion of the truth or probability, and I absolutely decline to take either side—­either that the influences are real, or that the influences are unreal—­till I see evidence of their effects.

Such evidence it is extremely difficult to extract from ordinary facts of observation.  I have alluded to the sun’s daily disturbance of the magnet as one of the most certain of influences, yet if you were to observe the magnet for a single day or perhaps for several days, you might see no evidence of that influence, so completely is it involved with other disturbances whose causes and laws are totally unknown.