OBSERVATIONS OF LUNAK OBJECTS. 257 



olijects on the Maria aud low-li/ing tracts are more or less variable iu 

 visibUifcy." 



In reference to Mr. Elger's remarks, an important question may bo sug- 

 gested. Is the moon surrounded by an atmosphere of elastic gas ? This 

 question has been answered in the negative, inasmuch as in the phenomena 

 of occultations no distortion or bending of the rays of light from the stars 

 occulted has been noticed. If I remember rightly, this is the basis of the 

 negation : Mr, (now Sir "W. E.) Grove, in his address to the members of the 

 British Association at Nottingham in 1866, alluded to the unsolved state of 

 the question ; and, " supposing the moon to be coustituted of similar materials 

 to the earth, it must be," he said, " doubtful if there is oxygen enough to 

 oxidize the metals of which she is composed ; * * * and it might be a fair 

 subject of inquiry whether, if there be any coating of oxide, it may not be 

 so thin as not to disguise the form of the congealed metallic masses, as they 

 may have set in cooling fx'om igneous fusion." The presence of oxygen, in- 

 ferred from oxidation, presupposes an atmosphere of permanently elastic gas 

 or gases. From the investigations of Herr Althaus, it has been approxi- 

 mately estimated that the moon's hemisphere turned towards the earth 

 attains at least a maximum temperature of 840° of Fahrenheit, upon the 

 assumption that the moon's power of absorbing heat is equal to that of quartz. 

 The heat thus attained would very closely approximate to the temperature 

 at which iron appears red in twilight, and exceeds the fusing-points of tin and 

 lead. On the other hand, the minimum is estimated to be — 92° of Fahren- 

 heit, which would give a fall of about 940° in fifteen days ; this Avould be 

 equivalent to daily increments and decrements of heat of about 63° each. 

 This enormous variation must be attended with very considerable expansion 

 and contraction of the gases, either present or liberated, and a very rapid 

 diminution of temperature upward must result. Now about the period of 

 maximum temperature of the luni-solar day the surface, whatever materials 

 may compose it, must be in a very different condition to what it existed in at 

 sunrise ; and this is so far visible to us by the different aspects of objects 

 under high illumination, so graphically described by Webb, and also by the 

 intensely glowing luminous spots, such as Aristarchus, Censorinus, Dionysius, 

 and various streaks under a midday sun. If the heat exceeds the melting- 

 points of tin and lead, it is quite possible that, long before the maximum tem- 

 perature is attained, substances may be fused and vapours given off which, 

 rising quickly to a cooler region, may be condensed and become visible to us 

 as cloud. 



As regards the streaks on Plato, it has been proved by observation that 

 the floor is irregular, although it generally appears to bo smooth and even ; 

 it is known in some places to rise slightly above a mean level and to sink in 

 others slightly below ; and this depressed character is most prominent at the 

 interior foot of the surrounding mountain-ring. The principal craterlet is 

 situated upon the highest part of the floor, and from it radiate the arms and 

 stem of the " trident." It is not at all unlikely that the great heat to which 

 the surface is subject may at times produce eruptions from this and other 

 craterlets ; indeed we appear to possess evidence that this has actually taken 

 place at least twice within the period of the observations ; and the arms, as 

 wcU as the sector and great northern streak, may owe their existence to such 

 eruptions, which of course would give them a permanent character. Sub- 

 jected to the intense cold of the lunar night, at sunrise they would only 

 reflect the same amount of light as the other portions of the floor ; but as the 

 floor becomes unequally heated; some portions absorbing heat while others 



1872. T 



