earth's climate in paleozoic times. ' 323 



u to a special kind of periodically recurring variations, which 

 " follow recognizable laws, depend upon the position of the sun 

 " in the ecliptic, and upon the daily rotation of the earth round 

 " its axis, and further ought no longer to be designated as irreg- 

 " ular, since we may distinguish in them, in addition to a special 

 «' local type, processes which affect the whole earth."* The 

 hypothesis of a metallic centre would seem to be capable of form- 

 ing the connecting link between the magnetic and astronomical 

 phenomena here referred to. The relation of the sun to the earth, 

 and the revolution of the latter on its axis, would naturally effect 

 a change in the position of the. metallic globe in the centre of the 

 earth, which change might alter the direction of the electric cur- 

 rents through the earth's crust, and these again the position of the 

 needle. Thus it seems not at all unreasonable to adopt the theory 

 of a metallic centre, since it alone is capable of affording a solution 

 of many problems in geology, magnetism and astronomy, and 

 since it is capable of uniting harmoniously and explaining the 

 most varied natural pnenoraena. 



Art. XXIII. — On the Earth's Climate in Paleozoic Times ; by 

 T. Sterrt Hunt, M.A., F.R.S. 



The late researches of Tyndall on the relation of gases and 

 vapors to radiant heat are important in their bearing upon the 

 temperature of the earth's surface in former geological periods. 

 He has shown that heat, from whatever source, passes through 

 hydrogen, oxygen and nitrogen gases, or through dry air, with 

 nearly the same facility as through a vacuum. These gases are 

 thus to radiant heat what rock-salt is among solids. Glass and 

 some other solid substances, which are readily permeable to light 

 and to solar heat, offer, as is well known, great obstacles to the 

 passage of radiant heat from non-luminous bodies ; and Tyndall 

 has recently shown that many colorless vapors and gases have a 

 similar effect, intercepting the heat from such sources, by which 

 they become warmed, and in their turn radiate heat. Thus while 

 for a vacuum the absorption of heat from a body at 212° F. is re- 

 presented by 0, and that for dry air is 1, the absorption by an at- 

 mosphere of carbonic acid gas equals 90, by marsh gas 403. by 



* Humboldts, Cosmos, v, 138. 



