S. I*. Langley — Temperature of the Moon. 429 



fectly correct one in theory, the writer believes that the 

 temperature of the lunar soil could be determined with great 

 exactness, were it not for the intervention of the earth's atmos- 

 phere, which exercises, in this part of the spectrum as in every 

 other, a highly selective absorption, indicated here, however, 

 not by fine lines like the Fraunhofer lines of the solar spec- 

 trum, but by enormously wide " cold bands," which vary in 

 size and even in position from night to night,* rendering the 

 exact position of this maximum in a corresponding degree 

 indeterminate. 



Another chapter is occupied by an example of a single 

 night's work in detail, with a statement of some of the pre- 

 cautions and corrections employed in practice. It may be ob- 

 served here in general as to the apparatus, that while the rock- 

 salt train, as already mentioned, is of such perfection as to 

 show the Fraunhofer lines very completely in the lunar lumin- 

 ous spectrum, the accompanying bolometer and galvanometer 

 enable us to measure cold bands in the non-luminous lunar or 

 air spectrum, whose heat is otherwise inappreciably so small 

 that it corresponds to a radiation of towottto °^ a sma -U calorie 

 per second, measured by the generation of a current of 

 '000,000,001 ampere. This is the amount of heat and cur- 

 rent implied in moving the galvanometer image over l mm of 

 the scale. The image is quite steady enough under favorable 

 conditions in fact to admit of the observation of less heat than 

 this, giving deflections of fractional portions of a millimeter; 

 but owing to fluctuations in the absorption of our. atmosphere 

 which transmits this radiation, rather than to any limitations of 

 the instrument itself, it is generally found best not to note de- 

 flections of less than l mm . What has just been said refers par- 

 ticularly to measures of the diffused heat from the moon's soil 

 in the invisible lunar spectrum and of the corresponding spec- 

 tral analysis of the reflected heat. "When, however, we place 

 the bolometer directly in the lunar image formed by the 8-inch 

 aperture, the deflection throws the needle at once off the scale, 

 and is found on more careful measurement to correspond under 

 favorable circumstances to a potential deflection of about l,500 mm 

 divisions. Melloni, it will be remembered, obtained four or 

 five divisions on his galvanometer with the thermopile and the 

 meter polyzonal lens on Vesuvius, and the immense difference 

 just noted is some indication of the advance of experimental 

 physics in this matter since his day. 



* That an absorption baud may vary in magnitude will excite no surprise, but 

 that it should vary sensibly in position may appear to some in contradiction with 

 our knowledge of the fixity of lines in the upper spectrum. An explanation of 

 the anomaly will be found in this Journal for Dec, 1888. 



