April i8. 1895J 



NATURE 



5«; 



snow-caps are seen to melt in summer. True, they may 

 be solid carbonic acid, but I have recently read that the 

 green colour of vegetation had been observed to appear 

 and disappear regularly on the planet. If there is little 

 water on the surface of Mars, I should imagine that this 

 is rather due to its having soaked into the crust, which is 

 probably colder underground than ours. I'rof. Newcomb 

 has evidently not thought of Mars in this connection, for 

 elsewhere he says : " If there are any astronomers on 

 Mars . . ." On this question I venture to quote Lord 

 Kelvin, who said, in 1887 (" Pop. Lect," vol. i. p. 376), 

 that "the intensity of the solar radiation to the earth is 

 6i per cent, greater in January than in July ; and neither 

 at the equator nor in the northern or southern hemi- 

 spheres has this difference been discovered by experience 

 or general observation of any kind." It is difficult to 

 imagine that if the effect of 6i per cent, cannot be de- 

 tected, 25 per cent, should convert all the water to ice 

 and destroy all life. 



Even if a small diminution of the solar radia- 

 tion produced a very cold climate on our present 



heat convectively from considerable depths, this heat 

 again being carried about convectively by the earth's 

 atmosphere, keeping the solid parts of the earth's sur- 

 face in a fit state for the e.xistence of low forms of animal 

 life. 1 1 is possible that at the present time the surface 

 of Jupiter, which receives a very small intensity of solar 

 radiation, may have solid parts surrounding watery lakes 

 and oceans capable of supporting life because of the 

 existence of many lakes of melted lava. 



To sum up, we can find no published record of any 

 lower maximum age of life on the earth as calculated by 

 physicists (I leave out the estimates based upon the 

 assumption of uniform density in the sun, and also 

 that of Mr. Clarence King; than 400 million 

 years. From the three physical arguments, Lord 

 Kelvin's higher limits are loco, 400, and 500 million 

 years. I have shown that we have reasons for believing 



I that the age, from all three, may be very considerably 

 underestimated. It is to be observed that if we exclude 



i everything but the arguments from mere physics, the 

 probable age of life on the earth is much less than any of 



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The Tokio Seismologicai Observatory. 



earth, we must remember that the earth's atmosphere 

 may have been very different in the past ; the earth may 

 have been very greatly blanketed, and the surface may 

 have been actually warmer, although there was much 

 less solar radiation. That the atmosphere is far more 

 important in this connection than the amount of solar 

 radiation, is evident if we consider Langley's determina- 

 tion that in the tropics, if there were no atmosphere, the 

 temperature of the surface of the earth would be - 200' C. 

 Any addition to the quantity of air in our present atmo- 

 sphere means an increase of the temperature of the rocky 

 surface. But in the past, not only may there have been 

 more atmosphere, but there may have been a very dif- 

 ferent kind of atmosphere. Again, we must consider a 

 possible great amelioration of climate due to the earth's 

 internal heat. It could not occur by mere conduction, 

 but it is quite possible that for many millions of years 

 there was great blanketing by clouds of watery vapour, 

 and that underneath these blankets half the surface of 

 the globe may have been a lake, or a number of lakes, of 

 melted lava, which may have carried large amounts of 



NO. 1329, VOL. 51] 



the above estimates ; but if the palaeontologists have 

 good reasons for demanding much greater times, I see 

 nothing from the physicist's point of view which denies 

 them four times the greatest of these estimates. 



John Perry. 



THE SEISMOLOGICAL OBSERVATORY 

 DESTROYED AT TOKIO. 



THE destruction by fire of the Seismologicai Observa- 

 tory and Library, at Tokio, Japan, has already 

 been referred to in these columns (p. 533). The valuable 

 work which Prof. Milne has accomplished during his 

 long stay in Japan is well known to our readers ; and it 

 is to be hoped that means for its continuance will be fully 

 provided. By the kindness of Japanese friends. Prof. 

 Milne has been able to make observations in a temporary 

 home since the fire, and it will not be for lack of en- 

 thusiasm and activity if a new observatory is not soon in 

 working order. We print below extracts from Prof. Milne's 



