•October 7, 1898.] 



SCIENCE. 



459 



by Professor Searle and Mr. Eeed were ex- 

 Mbited, and, considering the delicacy of 

 the observations, coincided well. 



Professor Frank W. Very, in a paper on 

 ^ The Probable Range of Temperature on 

 the Moon,' described his experimental work 

 in determining the radiation from di- 

 verse substances at ordinary temperatures. 

 Special attention was called to the compen- 

 sation involved in radiation from subsurface 

 molecules, which narrows the range of ra- 

 diant emissivity, as exercised by different 

 materials. Hence, the errors committed in 

 classifying radiators in a very few, or, under 

 •certain restrictions, even in a single divi- 

 sion, are not large. Measurements of lunar 

 radiation have been carefully made at 

 various phases, and the interpretation of 

 these results in terms of absolute measure- 

 ment is now possible. The method of re- 

 duction was indicated, and the final conclu- 

 «ion was that the moon's surface tempera- 

 ture lags behind the possible temperature 

 which might be given by the sun's rays 

 throughout most of the seven days of the 

 lunar morning. A retention of heat by 

 ^he surface material of the moon is thus 

 shown, but the retention is so small that 

 almost the whole of the heat received while 

 the sun is above the horizon is lost before 

 sundown on the same day, and thus the 

 proper radiation from the surface, which in 

 the case of the earth goes out to an entire 

 circumscribing sphere, emanates virtually 

 from only half the surface of the moon 

 while it is undergoing insulation. The day 

 temperature of the moon consequently be- 

 comes extremely high — above that of boil- 

 ing water, except in the polar regions — 

 while at night the cold approaches that of 

 space. 



Professor "William H. Pickering pre- 

 sented a paper (not read) on ' Swift's 

 Comet I., 1892.' This comet, discovered 

 March 6th, was first noticed to have a faint 

 tail, by Finley, on March 26th. As discov- 



ered at Arequipa, March 29th, the tail was 

 well developed. Forty-four photographs 

 were taken of this comet and used in a recent 

 investigation of its nature. The tail was 

 plainly two- fold in character, the inner tail 

 continuously issuing from the head, and, 

 being at least 20 degrees in length a month 

 after passing perihelion, consisted of two 

 absolutely straight rays inclined at an 

 angle of 10 degrees. The outer tail was 

 formed by successive eruptions from the 

 head, later diminishing in angle. The 

 whole efiect was to produce multiple tails 

 like the comet of 1744. The photographs 

 showed the inner tail to be of a diiferent 

 character on alternate days, now appearing 

 continuous near the head and now bifur- 

 cated. The explanation is that the comet 

 rotates about a longitudinal axis directed 

 toward the sun, the rotation period deter- 

 mined being 94.4 hours. This period was 

 determined by the use of the photographs 

 made at Arequipa by the writer, and cor- 

 rected by those of Professors Barnard and 

 "Wolf. Bessel observed in Halley's comet, in 

 1835, ' a vibration from side to side ' with a 

 period of 110 hours; this was probably a 

 similar rotation of the comet with an analo- 

 gous period. The changing appearance of 

 both comets being explicable by the hypothe- 

 sis of rotation, the next endeavor was to find 

 a physical explanation of the cause of such 

 rotation. The sun furnishes a strong elec- 

 tro-magnetic field. The comet also by the 

 electrical induction of the sun receives elec- 

 trical charges, and these charges are, by the 

 gases of the tails, radially projected from 

 the head of the comet, and become in effect 

 so many electrical currents. The result of 

 the motion of these radiating currents, i. e., 

 tails, in the magnetic field of the sun is the 

 production of a spinning of the comet about 

 a longitudinal axis. Theory would thus 

 produce the rotation demanded by observa- 

 tion, and the rotation would be independent 

 of the direction of the motion of the comet, 



