NO. 8 SAMUEL PIERPONT LANGLEV — ABBOT I9 



and the idea that we should expect heat to be associated with the light 

 seems to be essentially a modern one. This modern view, until very 

 recently, has been that the light of the moon penetrated to us, while 

 the rays which give only heat were kept back by our own atmosphere. 

 Melloni, the most conspicuous early asserter of our present doctrine 

 that radiant heat and light are but different manifestations of the 

 same energy, was led to pursue his lunar heat work on Mount Vesu- 

 vius by these a priori considerations, and his perseverance was justi- 

 fied by obtaining finally most minute yet real indications of heat. Save 

 the observations of Piassi Smyth on the Peak of Teneriffe, and of 

 M. Marie-Davy in France, we shall find, however, that with the excep- 

 tion of Lord Rosse, of the persons who have sought to observe the 

 heated moon, nearly all have left only records of failure or of purely 

 imaginary and therefore misleading, successes. 



" Lord Rosse's work excels greatly in importance that of his prede- 

 cessors, as he not only obtained unquestionable evidence of lunar heat, 

 but was able to make the important generalization that since a con- 

 siderable part of this is intercepted by glass, a great deal of the moon's 

 heat is probably radiated from her soil. As regards the temperature 

 of the sunlit surface of the moon. Lord Rosse determined in his first 

 paper that it ranges through 500° of the Fahrenheit scale; but in a 

 subsequent memoir in the Philosophical Transactions of the Royal 

 Society for 1873, this range is stated to be more nearly 200° Fahren- 

 heit, a large error having crept into the previous work. The assiduous 

 labor of observation and the instrumental means employed in these 

 researches have acquired great and deserved repute ; but few per- 

 haps have noticed minutely that in the computation of the ratio of 

 solar to lunar radiation, the error of assumption is made that all, or 

 nearly all, of the invisible heat is stopped by glass, with other postu- 

 lates equally inadmissible in the light of our present knowledge. We 

 must, then, while rendering a tribute of respect and even admiration 

 to the conscientious labors of observation and reduction point out 

 that some of the values derived from them by their author must be 

 revised, as resting on assumptions which the progress of science has 

 contradicted. 



" In a previous memoir ° we have given the results of various 

 experiments in regard to the distribution of light in the lunar spectrum 



'On the Temperature of the Surface of the Moon, Mem. Nat. Acad. Sci., 

 vol. III. 



