To John Frederick William Herschel   21st Nov. 1861

21^st Nov. 1861

Dear Sir John

Here follows an account of the observations on Lunar Radiation referred to in my letter in the last number of the Philosophical Magazine.¹

On the roof of the Royal Institution I had a platform erected, so high as to enable me to sweep the heavens with my thermo-electric pile, without impediment from the chimney-pots of London. The pile was supported on a vertical stem with which the axis of the instrument could be made to enclose any required angle. Wires passed from the pile to an excellent galvanometer placed in the laboratory at a vertical distance of 72 feet below the platform on the roof.

On directing the pile towards the heavens so that the reflection of the instrument embraced no terrestrial object the chilling of the face turned skyward was so great,² and the consequent galvanometric deflection so large that it was quite hopeless to operate upon the needle in this position. The zenith is also intensely cold at noon. To move it a single degree would have required many hundred times the quantity of heat or cold necessary to urge it through one of the lower degrees of the galvanometer. To render it manageable I therefore operated in the following manner.

The galvanometer was a differential one, that is to say, two wires ran side by side round the astatic needle at the instrument. The end of one of these wires were connected with the pile on the roof; the ends of the second wire I connected with a second pile, which was turned toward a vessel kept at a constant temperature by boiling water. The direction of the current caused by the heat below was opposed to that caused by the cold above, and the needle was thus compelled to take up its position among the lower degrees of the scale.

I then ascended to the roof, fixed my pile at its proper angle to its stem, and directed it off the moon. I descended and observed the galvanometer—the needle oscillated between 10° and 20°; its mean position was therefore 15°.

I reascended and turned the pile on the moon in descending I found the needle oscillating between 30° and 40° the mean position being 35°.

The ascending and descending were repeated six times in succession and the following results were obtained:

Deflections

off the moon on the moon

1 15° 35°

2 27° 40°

3 33° 40°

The numbers all show cold, the deflection being much as would be produced by a refrigeration of the face of the pile turned towards the sky, and we find that the deflection where the pile was on the moon is in all cases less than when it was off. In other words less heat was lost by the instrument when directed off the moon, than when directed towards it. This, as I have stated, is equivalent to a radiation of cold from our satellite.

The note in which I referred to these observations was probably written without due reflection on the circumstances. In fact I had been reading your excellent essay on Meteorology,³ and some of its passages brought my sins of omission so forcibly to my mind that I wrote the letter off hand. I pictured the matter thus:—supposing the earth to be surrounded by an atmospheric veil of precipitated vapour, the depth to which the moon’s heat could clear away that veil would be greatest along the line drawn from the earth’s centre to the moon, and would diminish from this normal all round, thinning off to nothing where the incidence became sufficiently oblique. The escape of terrestrial heat as far as the atmosphere is concerned would be less impeded along the normal than in any other direction. Theoretically speaking, when I turned my pile off the moon I turned it towards a portion of the atmosphere, where the moon’s heat was less effective in rendering the sky clear, but whether the difference could be measured is another question. The observations made during the last solar eclipse tend, I believe, to push the height of our atmosphere enormously beyond the limits hitherto assigned to it; were the limits within which precipitated vapour can form similarly expanded the explanation which I have given might apply; but not to an atmosphere limited as we have hitherto supposed ours to be.

I attach but little importance to the explanation as in all probability the disturbing elements at hand were able to mask the influence of the moon. On referring to a map I find that on turning my pile off the moon I turned it more over the mass of London, and the diminution of the cold may have been due to the temperature of the air suspended over this vast focus of artificial heat. I will not speak with confidence as to the influence of this or any other cause till I have time to investigate to the question thoroughly: at present I am working hard at an enquiry of another kind.

Let me complete this letter by asking you to suspend your judgment on the points of difference between my friend Prof. Magnus & myself:⁴ in a very few weeks⁵ I hope to have something interesting to communicate to you on this and kindred subjects.

Believe me dear Sir John | Yours very faithfully | John Tyndall

I should be extremely obliged to you if you would favour me with line saying whether you see any objection to the publication of this letter (I have a copy) in the next number of the Philosophical Magazine.⁶

Is it not at all events clear that a hot body may so modify a medium on which it acts that a second body shall have its temperatures lowered in the presence of the hot one!

RS HS 17.389 (this is Tyndall’s own transcription)

RI MS JT/1/TYP/2/509-511