426 & P. Langley — Temperature of the Moon. 



tain observations (carried on, however, only during a limited 

 time), we have taken advantage of the sensitiveness of our 

 apparatus to directly explore a large lunar image with the 

 bolometer, in spite of the diminished heat in such one. For 

 this purpose a special mirror, 303 mm in diameter and 3137 mm 

 focus, giving a lunar image of about 30 mm diameter, has been 

 employed. On the occasion of a lunar eclipse the last named 

 apparatus has also been used. 



We have already alluded (see Memoirs of the National Acad- 

 emy, Vol. Ill, p. 20) to the especial importance of the action 

 of the screen in these observations. This arises from the fact, 

 as will be seen later, that we shall deal with lunar heat of a 

 totally different quality from that of moonlight or sunlight. 

 It is, in a large part, radiation emanating from the lunar soil, 

 and of a quality, as we shall see, approximating that of the 

 screen itself. It must be evident, then, that the radiations 

 from this screen assume here a wholly abnormal importance. 



An investigation of the theory of the screen accordingly 

 occupies a chapter, for which the reader is referred to the orig- 

 inal. We may remark here, in passing, that the investigation 

 incidentally offers an explanation of the empirically known 

 fact that the velocity of cooling of a hot body at various tem- 

 peratures of excess varies with that of the enclosure itself. 

 The discussion also indicates what appears to be an independ- 

 ent method of determining the absolute zero; but the method, 

 although apparently correct in theory, would demand observa- 

 tions more accurate than our own casual ones to give it practi- 

 cal value. It may be, however, worth mentioning that the 

 observations, such as they are, indicate by this novel method, 

 the existence of an absolute zero at a point between —250° and 

 -300° Cent. 



A list of all the observations in the lunar spectrum extend- 

 ing from October, 1884, to February, 1887, is then given, 

 together with some collateral ones upon the " great radiator." 

 This latter instrument may be briefly described as analogous 

 to an immense Leslie cube, presenting as it does a blackened 

 radiating surface of the temperature of boiling water and of 1 

 square meter area. The object in giving it this extraordinary 

 dimension, is to enable it to still angularly subtend the whole 

 field of view of the bolometer, while it is at such a distance 

 that the intervening column of air may be supposed to exercise 

 a measurable absorptive effect. Its actual distance was 100 

 meters, and at this distance the absorption of the intervening 

 air on the dark radiant heat, emanating from its surface at 100° 

 Cent., was in fact manifest, and gave evidence, novel and 

 interesting, both as to the actual absorption by our atmosphere 



