George Biddell Airy to Faraday   7 July 1861

The Grange, Keswick, | 1861 July 7

My dear Sir

Your kind acquiescence¹ in the proposal of the British Association sets me at liberty to a great extent. If it is not contradicted by subsequent communication before my return, I will so far act on it as to pass through Manchester and see the capabilities for a lecture.

The subject of Comets, on which you invite my remarks², is a most puzzling one. I am a firm believer in Laplace’s³ nebular hypothesis, and I do not doubt that he is correct in thinking comets to be stray bits of the cosmical nebulous matter: and this, in process of time, would undoubtedly assume the spherical or nearly spherical form in which we see comets when distant from the sun and in which we see their heads when they are near the sun. So, if comets would preserve their condensed form, I should be quite satisfied. But the difficulty is in the nature and motion of their tails and envelopes. The head of a comet is found to move under the mechanical laws of gravitation, exactly as if it had no appendage whatever. And the motion of the tail cannot be explained on mechanical laws, supposing it in empty space; and is still more opposed to them if in a resisting medium. It appears to me therefore that the tail of today cannot be the same as the tail of yesterday. I cannot imagine it to be any thing more real than the beam of light in a dusty atmosphere, which travels when the perforated board through which the sunlight passes is moved. But how to account for such a cometary beam (especially so crooked as it was in the case of Donati’s⁴ comet), and how to explain three covers like three glass shades successively one within another (as appears to hold with this comet) I cannot at all conjecture. It appears to me that all that has been suggested by Bessel⁵, Faye⁶, and twenty other people, has added nothing to our knowledge or our plausible suppositions.

I want to discuss with you another subject, that of thunder-storms. I have lately gone through two sharp ones. In no.1, there was one thunder clap which commenced by an instantaneous very heavy ringing sound which gave the idea that an Armstrong⁷ shot of enormous size had struck the cliff with very great force, the rest of the peal not remarkable. During the remainder of the storm we remarked that, with the utmost regularity, the peals were alternately strong and weak. (I have since thought that the weak ones might be only echoes, which in these mountains sometimes come after a long interval: but I think that here the intervals were too long). - In the storm no. 2, almost every peal began with a low noise, not very un-uniform, much resembling the sound of a military rocket, lasting perhaps 3 seconds, and then followed by very heavy rolls. Now as the light sound could not be an echo of the heavy one (because it preceded it), and as the heavy sound could not be an echo of the light one (because it was heavier), is it possible to conceive these as having any other relation than that of a train of gunpowder to a magazine?- In any case, my best thanks will be due to any body who will give me a plausible conjecture on the causes of the heavy introductory shot in No. 1 and of the light introductory fizz in No. 2.

I am, my dear Sir, | Yours very truly | G.B. Airy

Professor Faraday