Correspondence 



227 



when it is, an interval of doubt may have occurred, during 

 which the faculty of discrimination may have been called 

 into play unprejudiced by previous knowledge. We have 

 no wish to offer puzzles, and our design is only to increase 

 the habit of original observation by a temporary and brief 

 concealment of the facts. 



CORRESPONDENCE. 



TO THE EDITOR OF THE "MUSEUM GAZETTE." 



E. H. has kindly supplied us with the following addenda to our 

 Memoranda as to the Moo?i. 



Page 157, par. 5. — The Materials of the Moon. If, as is believed, 

 the earth and moon were originally one mass, and the latter has 

 subsequently become detached, it must be supposed that the materials 

 of which the moon is composed are the same as those of the earth, 

 though the proportions of lighter and heavier substances evidently 

 differ in the two bodies. Perhaps, then, it would be safer to say, that 

 while the bulk of the moon is a little more than one-fiftieth that of the 

 earth its mass is less than one-eightieth of that of the same body. Its 

 superficial gravity is only one-sixth ; and, as a consequence, a man on 

 its surface could therefore leap six times as high, or throw the same 

 missile six times as far, as he can do on the surface of the earth by 

 the same effort. 



. Par. 2, page 158, might be thus continued: During half of each of 

 these long days — such half corresponding to the duration with us 

 of a fortnight — each portion of the moon successively experiences, 

 without intermission, the burning heat and fierce light of the sun 

 in a cloudless sky ; and during the other half suffers total absence 

 of sunlight accompanied by intense cold. 



It might be of interest here further to note that as the moon has 

 lengthened its days by occupying as much time in performing one 

 revolution round its axis as it does in revolving round the earth, it 

 is surmised that the earth also is very gradually lengthening its days 

 by the retardation of its axial velocity, and may be expected, ages 

 hence, to revolve upon its axis in the same time as it revolves round 

 the sun. This means that, should its separate existence continue 

 sufficiently long, and under the same conditions, the earth's future day 

 will equal in length our present year. Daylight and darkness will then 

 be each of six months' duration. The prospect is not alluring ; but its 

 possible realisation is so far distant that though it may well exercise 

 our imagination it need not excite our fears. 



Page 159, top of page. — " The best harvest moons occur on or near 

 the 2 1st of September." 



