NO. 8 SAMUEL PIERPONT LANGLEY — ABBOT 23 



the present means of science, and have reached no conchisions which 

 we are absolutely sure of. As regards the main point, concerning the 

 radiant heat of the moon, we know that it is divided into two salient 

 kinds, reflected and emitted heat, and that the latter overlaps the 

 former and extends probably between the deviation 40° of a rock-salt 

 60° prism (corresponding to A=i'*.03) and a deviation of over 33° 

 in the extreme infra-red (A — perhaps 50'*). Contrary to all previous 

 expectations, it nevertheless reaches us, thus bringing evidence of the 

 partial transparency of our terrestrial atmosphere even to such rays as 

 are emitted by the soil of our planet. It is probable, as remarked 

 elsewhere, that even of the heat of arctic ice some minute portion 

 escapes by direct radiation into space. 



" If 1)eyond this we can be said to be sure of anything, it is that 

 the actual temperature of the lunar soil is far lower than it is believed 

 to be; but the evidence does not warrant us in fixing its maximum 

 temperature more nearly than to say it is little above 0° centigrade; 

 but, it will be seen, the writer is sensible that this conclusion mili- 

 tates against one drawn by him from the Mount Whitney observa- 

 tions, according to which the soil of an airless planet at the moon's 

 distance would have a temperature not greatly above — 225°C. Great 

 experimental labor on this expedition was expended in ascertaining 

 the excess of temperature which a thermometer-bulb would attain in 

 space at the earth's distance from the sun, which was found to be 

 approximately 48° centigrade. From this observation, which appears 

 to be quite trustworthy, the writer drew the inference that the sunward 

 surface of an airless planet would be very greatly below the zero of 

 the centigrade thermometer, and materially colder than the moon's 

 surface appears by these observations to be. As between my obser- 

 vations and my inferences, I hold to the former ; and since later and 

 long-continued observations, of the character detailed in this volume, 

 show that the temperature of the sunward surface of the moon (which 

 is certainly nearly airless) is almost as certainly not greatly below 

 zero, I have been led to believe myself mistaken in one of the infer- 

 ences drawn from former experiments, in themselves exact, where 

 this inference is not supported by these later observations. 



" Several methods have been tried for obtaining the ratio of the 

 total radiation of the full moon to that of the sun, with results rang- 

 ing from 1/70000 to i/iioooo. The liability to error in the compari- 

 son of such diverse quantities is obvious ; but a portion of the dis- 



