Collingwood | Nov 10/61.
Dear Sir
Many thanks for your communication of “Remarks on Radiation & Absorption”.1 The fact of air being non absorptive and vapour highly so is indeed a very capital one and must afford a key to much that has hitherto been inexplicable in meteorology. The dissipation of cloud & clearing of sky by the Lunar rays is, as you very justly observe a palpable consequence of her heat being entirely absorbed aloft (as I always imagined it was) As to the actual production of cold on your radiation instrument by directing its axis on the moon you will excuse me if I don’t quite see the force of your reasoning. The moon’s heat dissipates the vapour not only in that angular space of the sky as seen from the focus which is occupied by her disk (a circle ½° diameter) but also and equally over an immense area all around.
If the result be one you can certainly rely on and unequivocally proved by many experiments—the only explanation I can think of is that we receive radiant heat in considerable quantity from the ultra-lunar spaces. This the moon intercepts over the visible area of a circle ½° diam. while all the heat it sends is stopped aloft so that from that half degree none reaches the Earth’s surface.
But if I recollect rightly Ritchie2 did find some small heating power in the moon’s rays.* P. Smyth3 on Tenerife4 indicates also some very small effect—but then he was at 10,000 feet altitude.
I remain | Dear Sir | Yours very truly | J. F. W. Herschel
P.S. If vapor absorbs heat and air does not then the vapour should be heated by the Sun’s rays & not the air (except by conduction from the vapor) Therefore the S.G.5 of the vapor would be diminished relatively—the ascensional power increased (see Fourier?6 on the ascent of clouds7) and may it not be that an atmosphere of attenuated vapor may extend beyond the aerial one?
(*I see on reference to his paper8 that he only projected an expt. whether he ever made it, I cannot remember.)
RI MS JT/1/H/95
RI MS JT/1/TYP/2/508
Royal Society Letters (RMD 5 August 2011), 23_347 (this is Tyndall’s own transcription)
communication of “Remarks on Radiation & Absorption.”: see letter 1815.
Ritchie: William Ritchie (c. 1790–1837), Professor of Natural Philosophy at the RI from 1831 until his death (ODNB).
P. Smyth: Charles Piazzi Smyth (1819–1900), Astronomer Royal for Scotland and Professor of Natural Philosophy, University of Edinburgh (ODNB).
on Tenerife: C. Piazzi Smyth, Tenerife, An Astronomer’s Experiment: or, Specialties of a Resident Above the Clouds (London: Lovell Reeve, 1858), pp. 208ff.
S.G.: Specific Gravity, the ratio of the density of the vapour to that of air.
Fourier: Jean-Baptiste Joseph Fourier (1768–1830), French mathematician and physicist.
ascent of clouds: most likely referring not to Fourier but to Augustin-Jean Fresnel. See A. Fresnel, ‘Sur l’Ascension des nuages dans l’atmosphère’, Annal. Chem. et Phys. (1822), pp. 260–63.
reference to his paper: William Ritchie, ‘On a new photometer with its applications to determine the relative intensities of artificial light &c’, Phil. Trans (1825), pp. 141–7.
Please cite as “Tyndall1826,” in Ɛpsilon: The John Tyndall Collection accessed on