392 



SCIENCE 



[N. S. Vol. XXVII. No. 68S 



penumbra and it was stated that its width. 

 could be accounted for by supposing the ex- 

 istence of matter outside of the plane of the 

 rings, or in effect by tores or thickenings of 

 the rings; the dark medial core of the band 

 being the projection of the ring proper. It 

 was explained in what way the tores could 

 have been produced and in what part of the 

 rings they should occur. In this connection 

 it was shown how the separations of the rings 

 were produced by the planet's satellites; and 

 it was stated that one should expect that the 

 tores should occur just inside of the paths of 

 the satellites. By observation the tores were 

 found to be just where celestial mechanics 

 would put them under the disturbing iniiuence 

 of the satellites. 



In reviewing the conclusions as to the con- 

 stitution of the rings, and the disintegration 

 that must be taking place as evidenced by the 

 positions of the tores, the ultimate disappear- 

 ance of the rings was predicted. 



R. L. Faeis, 

 Secretary 



DISCUSSION AND CORRESPONDENCE 



AGE OF A COOLmG GLOBE 



To THE Editor of Science: In my paper 

 on a cooling globe in Science for February 

 7, pages 231 and 232, the depth of the level 

 of isostatic compensation is stated as 71 miles 

 or 140 kilometers. This last should be 114 

 kilometers. The blunder arose in copying, 

 and the correct value was used in the com- 

 putations. 



George T. Becker 



THE earth as a HEAT-RADIATING PLANET 



To THE Editor of Science: Of the many 

 far-reaching consequences resulting from the 

 discovery announced in Science for November 

 22 and December 20, 1907, perhaps no one 

 fact stands out more clearly and strongly 

 than this — The inherent heat of the earth 

 still plays an important if not controlling part 

 in all terrestrial phenomena (as, for instance, 

 in the formation of ocean and atmospheric 

 currents, in cloud formation, and the increase 

 in temperature with increase of cloudiness, 

 etc.), for it now seems certain that without 



this inherent heat radiation the terrestrial 

 atmosphere could not exist. 



With a surface which, even at its lowest 

 known temperature, is still more than 200° C. 

 above the temperature of surrounding space 

 (ocean temperatures at great depths being 

 about 270° above) and with the temperature 

 increasing with the depth below the surface, 

 there can no longer be much reasonable doubt 

 as to the facts concerning the past or future 

 history of the earth, so far as effects due to 

 temperature changes are concerned. 



The earth is now, and has been for ages, 

 radiating heat into space, shrinking in size, 

 and, with a constantly decreasing surface tem- 

 perature, growing colder. 



The mean absolute surface temperature of 

 the earth is, let us say, 300° C. If we regard 

 this as made up of an inherent surface-tem- 

 perature of 200° C. and a stored or trapped 

 heat equivalent to a temperature of 100° C, 

 the radiation into space is such that the moon, 

 for example, receives about one twenty-seventh 

 as much heat from the earth as it does from 

 the sun. 



The sun's influence is rendered overpower- 

 ingly conspicuous because this influence is 

 zonal and varyingly differential, thereby ob- 

 scuring to a great degree the nearly constant 

 but large effect of inherent earth-radiation. 



The earth's desert areas are increasing and 

 the glaciers are retreating not because the 

 sun's influence has seemingly become predom- 

 inant, but because the earth has, even during 

 known historical time, grown sensibly colder. 



At any given time in the history of the 

 earth, an ice age was inaugurated at a given 

 place whenever the snow-fall during the colder 

 months of the year was so great that the snow 

 could not all be melted during the warmer 

 months of the year. But since the earth is 

 continually growing colder the supply of mois- 

 ture, through evaporation from the water sur- 

 face of the earth, is continually growing less, 

 60 that finally even the land areas in the polar 

 regions will he completely hare, and the upper 

 limit of the atmosphere will then practically 

 coincide with the surface of a solidly frozen 

 ocean. 



With the modifications, resulting from the 



