Mabch 27, 1908] 



SCIENCE 



503 



Time of the Total Solar Eclipse of August 30, 

 1905," by 0. H. Tittmann. 



" Preliminary Note on an ' International Mag- 

 netic Standard,' " by L. A. Bauer. 



" The Life and Work of Maurits Snellen," by 

 E. van Everdingen. 



" Die Magnetische Observatorien des Preussi- 

 schen Meteorologischen Instituts," by Adolf 

 Schmidt. 



" Mean Values of the Magnetic Elements at 

 Observatories," compiled by J. A. Fleming. 



Letters to Editor: "Principal Magnetic Storms 

 recorded at the Cheltenham Magnetic Observa- 

 tory," by O. H. Tittmann ; " Concerning Publica- 

 tion ' Caractfere magnStique de chaque jour,' " by 

 E. van Everdingen. 



SOCIETIES AND ACADEMIES 



THE PHILOSOPHICAL SOCIETY OP WASHINGTON 



The 645th meeting was held February 15, 

 1908, President Bauer presiding. 



Professor Simon Newcomb read a paper 

 upon " The Climate of Mars." 



The establishment of Stefan's law of radia- 

 tion, as proportional at ordinary temperatures, 

 to the fourth power of the absolute tempera- 

 ture, makes possible an estimate of the 

 temperature of an opaque planet more re- 

 liable than was possible formerly. Chris- 

 tiansen, Poynting and Lowell have applied the 

 law to estimate the temperature of Mars. 

 The speaker stated that the method in which 

 these investigators had made their results to 

 depend upon the albedo of the planet, and the 

 absorbent power of its atmosphere did not 

 seem to him well adapted to the case. He 

 based his ovm conclusions upon Kirchhoff's 

 law of the equality between radiating and 

 reflecting power of matter for each separate 

 wave-length of heat, which law is, itself, a 

 corollary from the second law of thermo- 

 dynamics. Making abstraction of an atmos- 

 phere, there is a certain normal temperature 

 of a planet which would be independent of 

 this albedo, except when the latter was dif- 

 ferent for different parts of the spectrum. 

 The effect of this difference is probably small. 

 Assuming Mars to reflect the lower rays of 

 the spectrum more strongly than the higher 

 ones, there would be somewhat higher tempera- 

 ture than the normal one. The effect of an 



atmosphere like ours would be to make the 

 planet rather warmer than the normal. This 

 effect would be produced in two ways, one of 

 which is the effect of absorption by the sun's 

 heat which mix up the lower strata with the 

 higher one. This results in the earth being 

 warmer than it would be in the absence of an 

 atmosphere. Since Mars has only an ex- 

 tremely thin atmosphere, the effect in raising 

 its temperature is much less than in the case 

 of the earth. 



As the result of his estimates it was said 

 that, in general, the surface of Mars must be 

 in general below the freezing point of water 

 except in the equatorial zone. It does not 

 seem possible that the polar regions can ever 

 rise to the temperature of melting ice. Before 

 approaching this temperature the radiation, as 

 given by Stefan's law, would exceed the heat 

 absorbed from the solar radiation so that a 

 fall of temperature should be the result. The 

 disappearance of the white polar caps is easily 

 accounted for through evaporation of ice at 

 the lowest temperatures, especially under so 

 small an atmospheric pressure as prevails on 

 Mars. 



The second paper of the evening was pre- 

 sented by Dr. C. G. Abbot, of the Smith- 

 sonian Astrophysical Observatory, upon " The 

 Variability of the Sun." 



If two observers at opposite sides of the 

 earth, and greatly different altitudes, should 

 both determine the intensity of solar radia- 

 tion at mean solar distance by a process 

 sound in theory, and both should decide inde- 

 pendently that the average value of the 

 " solar-constant " is 2.10 calories per square 

 centimeter per minute, but that on March 

 1 of a certain year it was 2.20 while on March 

 1 of the following year it was but 2.00 — then 

 it is probable that interest and support would 

 be forthcoming to push the task of " solar- 

 constant " determination steadily and perse- 

 veringly enough to determine the extent and 

 character of the sun's variation. 



The evidence of solar variation thiLS far 

 attained is not so strong as this, but is strong 

 enough to warrant a persevering study of the 

 subject. Measurements on Mount Wilson in 



