ASTRONOMICAL PHENOMENA AND PROGRESS. 



49 



a manner that a given amount of heat on the 

 anterior face of one pile would produce a devi- 

 ation equal in amount and opposite in direc- 

 tion to that produced by an equal amount of 

 heat on the anterior face of the other pile. 

 The result of the experiments may be summed 

 up as follows : The law of the variation of the 

 moon's heat is found to differ not much from 

 that of the moon's light. It may therefore be 

 inferred that not more than a small part of the 

 moon's heat comes from the first of the three 

 sources mentioned. Readings of the galva- 

 nometer taken on four different nights at the 

 time of full moon showed a deviation of about 

 six or eight divisions. The probable per cent- 

 age of the moon's heat which passed through 

 disks of thin plate-glass placed in front of each 

 pile was 8 or rather less. The greater part of 

 the moon's heat which reaches the earth 

 appears to have been first absorbed by the 

 lunar surface. The ratio of the heat of the 

 sun, which reaches the earth, to that of the 

 moon, is estimated at about 89,819 to 1. The 

 value of the readings of the galvanometer was 

 determined by comparison with those obtained 

 by using a vessel of hot water coated with 

 shellac and lampblack varnish as a source of 

 heat. The vessel was of tin, circular, and sub- 

 tended the same angle at the small concave 

 reflectors as the large mirror of the telescope. 

 It was thus found that (the radiating power 

 of the moon being supposed to be equal to that 

 of the lampblack surface, and the earth's at- 

 mosphere not to influence the result) a devia- 

 tion of 90 for full moon appears to indicate an 

 elevation of temperature at the moon's surface 

 of about 500 F. In deducing this result, al- 

 lowance has been made for the imperfect ab- 

 sorption of the solar rays by the lunar surface. 

 Of this estimated high temperature at the 

 moon the author remarks that it is quite in ac- 

 cordance with the views of Sir John Herschel 

 on the subject (" Outlines of Astronomy," sec- 

 tion 732 and preceding sections), where he 

 says that, " in consequence of the long period of 

 rotation of the moon on its axis, and still more 

 the absence of an atmosphere, the climate of 

 the moon must be most extraordinary, the 

 alternation being that of unmitigated and burn- 

 ing sunshine, fiercer than that of an equatorial 

 noon, and the keenest severity of frost, far ex- 

 ceeding that of our polar winters for an equal 

 time." And again, "the surface of the full 

 moon exposed to us must necessarily be very 

 much heated, possibly to a degree much ex- 

 ceeding that of boiling water." 



Captain ^ John Ericsson, the distinguished 

 American inventor, reported to the American 

 Association, at its meeting in August, conclu- 

 sions derived from a series of original experi- 

 ments, exactly opposite to those submitted by 

 Earl Rosse. Captain Ericsson proceeded upon 

 the generally-accepted theory that the moon 

 has no atmosphere, and sought to ascertain 

 what was the temperature of the solar rays in 

 absolute space that is, before entering the at- 

 VOL. ix. 4. A 



mosphere of the earth arguing that the heat 

 at the surface of the moon, by reason of the 

 absence of an atmosphere, would be the same 

 as that in absolute space. His experiments 

 led him to believe that the temperature of ab- 

 solute space, and, therefore, at the lunar sur- 

 face, is more than 300 below the freezing- 

 point of water. The atmosphere of the earth 

 imprisons, so to speak, the solar rays, which 

 are then absorbed into the earth, until the 

 heat of the surface is increased to the existing 

 average temperature, when other causes inter- 

 fere to prevent it going higher. In other 

 words, the heat received by the earth from 

 the sun is prevented by the, atmosphere from 

 being reflected off into space; but no such 

 cause exists to check the reflection into space 

 of the solar heat falling on the moon's surface. 

 On Captain Ericsson's theory, the moon is a 

 mass of ice, and the heat noticed by Earl Rosse 

 is reflected, as heat might be from the coldest 

 surfaces, and not radiated from it. 



Report of the Lunar Committee on Changes 

 in the Moon. The report of the Lunar Com- 

 mittee, at the August meeting of the British 

 Association, contains some items of interest. 

 Mr. Birt stated that, since the appointment of 

 the committee, in 1868, a surface of 100 square 

 degrees i. e., 10 degrees of south latitude and 

 10 of west longitude has been carefully sur- 

 veyed; the outlines of 433 objects laid down 

 on a scale of 200 inches to the moon's diameter, 

 and a catalogue prepared, containing numerous 

 notices of important phenomena bearing on the 

 questions relative to the physical aspect of our 

 satellite. The great question of continued 

 lunar change, either transient or permanent, 

 as contrasted with apparent change dependent 

 upon illuminating and visual angle, is one, he 

 remarks, for posterity to settle. Several in- 

 stances were adduced to show that the moon's 

 surface had undergone successive changes dur- 

 ing its past history. The determination of 

 these changes rests on the strong indications 

 afforded by a careful study of photograms of 

 the priority and posteriority of well-marked 

 features, which can only be realized by con- 

 templating the lunar pictures in the seclusion 

 of the study. The report alludes to certain 

 differences between the photograms, particu- 

 larly one with respect to a crater figured by 

 Lohrmann, and found on De La Rue's map, but 

 not a vestige of which can be discovered on 

 Rutherfurd's. The comparison of photograms 

 appears to open up a line of investigation of 

 great promise. Some attention has been given 

 to apparent changes of brightness and tint ; and 

 three or four somewhat conspicuous spots were 

 referred to as exhibiting these alterations. 



In a paper subsequently read before the 

 Association by Mr. Birt, he spoke of the num- 

 ber of changes which have been observed, 

 during the last 49 years, on the lunar crater 

 Plato. From records in his possession, it ap- 

 pears that no less than 24 spots have been 

 seen, at different times, on the smooth, dark 



