430 S. P. Langley — Temperature of the Moon. 



Theory of observation ; with typical example, showing method. 



Every observation on the moon, whether on its total heat, a& 

 observed directly in the lunar image, or on its diffused heat in 

 the spectrum, should consist in a comparison of its radiations 

 with those of the adjacent sky on either side of it. If our 

 thermometric apparatus had an absolute scale, and there were 

 no intervening atmosphere, it appears, in accordance with what 

 has already been said, that such apparatus, when directed not 

 to the moon but to "space" more or less adjacent, should indi- 

 cate the temperature of this space, which is sensibly that of 

 the absolute zero ; and then, when it k turned upon the moon, 

 supposing it to receive only the emitted and not the reflected 

 heat, it would give, also on the absolute scale, the temperature 

 of the lunar soil. In fact, with such an absolute thermometer, 

 the preliminary comparison with space would be unnecessary. 

 In reality, we use not an absolute apparatus with a natural 

 scale, but a differential apparatus with an arbitrary scale ; and 

 if we could work without an intervening atmosphere, we 

 should, even in this case, require to let the bolometer radiate 

 to space in order to determine the point on our arbitrary scale 

 which corresponds to zero. We should then observe a second 

 point corresponding to the temperature of the lunar surface, 

 and having determined the value of the units of our arbitrary 

 scale in terms of the natural one, we should evidently have the 

 quantity sought. 



The above conditions are still of ideal simplicity. The great, 

 the almost insuperable difficulty of actual observation, lies less 

 in the minuteness of the actual radiation, or even in its two- 

 fold character, than in the fact that it is masked to us by the 

 changes of an always intervening atmosphere. The case of 

 observations on the sun is totally different from the present 

 one, and would be so even if the sun were withdrawn till it 

 emitted no more heat than the moon ; for in this latter imagin- 

 ary case, the greater proportion of the solar radiation would 

 still lie in a spectral region totally distinct from that in which 

 the radiations proper to the obscuring atmosphere are found, 

 and it is the peculiar, unavoidable difficulty, at every stage of 

 this long investigation, that since the moon and the air are both 

 alike cold bodies, their invisible spectra are, in general, super- 

 posed in the same field. Let us add to this, that the (invisible) 

 spectrum of the air is usually not fixed but fluctuating, and we 

 shall see the desirability of having some separate standard 

 with which to compare it from night to night. This we obtain 

 most conveniently by filling a vessel of proper shape and size, 

 either with water or with a freezing mixture at a temperature, 

 constant for the series, and making this vessel itself the screen 

 which is interDosed between the bolometer and the lunar rays. 



