ASTRONOMY. 403 



a lower layer of clay, under the influence of the change of seasons; 

 while at the same time a progressive slipping of one layer on the other 

 will account for the changes of level. In this connection, we may refer 

 to diurnal movements of the soil derived from astronomical observations 

 by Dr. Gould (U. S. Coast Survey Report, 18G2-'64, and Cordoba Obser- 

 vations, vol. ii, p. lii), and by Mr. Fergola (Tt. Ac. Sci. Napoli, 1871), as 

 well as to the special physical studies by D'Abbadie in France, G. Dar- 

 win in England, and others. 



THE MOON. 



Semi-diameter of the moon.* — Prof. H. M. Paul, formerly assistant at 

 the United States Naval Observatory, Washington, gives in Appendix 

 II of the Washington Observations for 1879 the results of two occupa- 

 tions of the Pleiades group by the moon, observed by himself to deter- 

 mine the occultation semi-diameter of the moon, and also the corrections 

 to the right ascension, declination, and parallax of the moon, these be- 

 ing necessarily involved with the semi-diameter. The occultations oc- 

 curred on July 6, 1877, and September 6, 1879, and were observed with 

 the 9.6-inch equatorial of the Washington Observatory. The relative 

 positions adopted for the stars were those of Wolf, with proper motions 

 from comparison with Bessel, and the general proper motion of the group 

 as given by Kewcomb. The observations of 1877 were poorly placed 

 for a determination of the correction to the semi-diameter ; but those of 

 1879 give a much more reliable result. From the later (fourteen in 

 all) the resulting correction to Hansen's mean semi-diameter (15' 33".47) 

 is — 1".69± 0".12; and the resulting value, is therefore, semi-diameter 

 =15'31".78±0".12. He gives also the results of Airy's determination 

 from 296 scattered observations from 1833 to 1860. From the immer- 

 sions and emersions at the dark limb the resulting values are larger by 

 0".9 and 0".5 than those given by Professor Paul, and from immersious 

 and emersions at the bright limb Airy's results are larger by 2".3 and 

 4".4. Professor Paul concludes that the best way to observe the actual 

 occultation at the bright limb is to use as high a magnifying power as 

 possible, so as to obtain a decided difference of color between the star 

 and the moon's limb. Neither set of occultations observed by Professor 

 Paul gives any evidence of deviation of the moon's limb from a perfect 

 circle. — (The Observatory, October, 1883.) 



The moon's heat. — The Sidereal Messenger for August, 1883, contains 

 an extract from a private letter of Professor Langley's giving some re- 

 sults of unpublished observations on the lunar heat : 



"We are measuring the heat of the moon by the bolometer, and the 

 light in its spectrum by other methods, in order to ascertain the tem- 

 perature of the lunar surface. Our preliminary measures already war- 

 rant us in announcing a different conclusion from that reached by Lord 

 Eosse, who, from the fact that a certain specimen of glass absorbed more 



* Science, vol. I, No. 20. 



