September 1, 1892.] 



KNOWLEDGE 



161 



^^ AN ILLUSTRATED "^^ 



MAGAZINE OF SCIENCE 



i SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: SEPTEMBER 1, 1892. 



CONTENTS. 



PAGE 



How Old is the World ? By the Eer. H. N. Hutchinson, 



B.A., F.G.S 161 



Bee Parasites— III. By E. A. Btttleb 16i 



The Climate of Mars. By E. W. irAU.NHER. F.K.A.S. ... 167 



On some Recent Advances In the Study of Solar 



Prominences and Faculae. Br A. C. Rantakd ... l69 



Letters :— T. E. Espin; .T. L. Bozwaed; S. E. Pkai, ... 170 

 The Oldest Fishes and their Fins. By E. Ltdeksee, 



B.A.Cantab 173 



Radiant Matter. By A. Jamesox 176 



The Face of the Sky for September. By Heeeekt 



Sadler, F.K.A.S 178 



Chess Column. By C. D. Locock, B.A.Oxon 179 



HOW OLD IS THE WORLD? 



By the Eev. H. N. Hutchinson, B.A., F.G.S. 



GEOLOGY is not one of the exact sciences with a 

 mathematical basis, like chemistry or electricity. 

 Nevertheless, problems arise now and then which 

 are capable of mathematical investigation. The 

 problem of the earth's antiquity, or rather, that 

 of the dm-ation of (jeiiloijk-al time, which is not the same 

 thing, is one that has attracted much attention, and has 

 led to a long controversy between certain physicists on the 

 one hand, and geologists on the other. According to the 

 " nebular hypothesis " now generally accepted, our planet 

 had cooled down from a molten and somewhat viscous 

 state long before geological time began — that is, before a 

 watery ocean settled down by condensation from a heated 

 atmosphere, and left our air as it now is, mostly composed 

 of the incombustible element nitrogen, with a little oxygen, 

 a variable amount of aqueous vapom-, and a trace of 

 carbonic acid. How many aeons passed away before this 

 state of things was arrived at, no one can say. Such times 

 were pre-geological. But at last an ocean fonned, then, 

 perhaps later on, dry land appeared ; the wind blew and 

 the rains fell, as they do now, and the earth reached a 

 phase which geologists believe to have been, generally 

 speaking, not very unlike that of the present day. The 

 question of geological time is the question of the duration 

 of this phase. The gi-eat series of stratified rocks (including 

 lava flows and intrusive igneous rocks, such as " dykes ") 

 were formed during geological time ; and these are the 



pages on which the earth has recorded her history. 

 Naturally, therefore, the geologist endeavours to seek for 

 some means of calculating the length of time requhed by 

 Mother Earth to write her autobiography. 



Now the earlier modem geologists, Hutton and his 

 followers, who, by teaching the great principle of uni- 

 formity in geological actions, placed the science on a sound 

 and reasonable basis, and gave it an enormous impetus, 

 were, unfortunately, so greatly impressed with this idea 

 that the}" could see no trace of a beginning or sign of an 

 end. Sir Archibald Geikie, in his recent address as Pre- 

 sident of the British Association, assembled in Edinburgh, 

 has thus eloquently described then- state of mind : " When 

 the curtain was then first raised that had veiled the 

 history of the earth, and men, looking beyond the brief 

 span within which they had supposed that history to have 

 beei) transacted," beheld the records of a long vista of ages, 

 stretching far away into a dim, ilhmitable past, the 

 prospect vividly impressed their imagination. Thus the 

 idea arose and gained universal acceptance, that, just as 

 no boundary could be set to the astronomer in his free 

 range through space, so the whole of bygone eternity lay 

 open to the requirements of the geologist. . . . This 

 doctrine was naturally espoused with warmth by the 

 extreme imiformitariau school, which required an un- 

 limited duration of time for the accomplishment of such 

 slow and quiet cycles of change as they conceived to be 

 alone recognizable in the records of the earth's past 

 history." 



This extreme teaching, in itself a reaction against the 

 old-fashioned previous teaching, produced another reaction, 

 and the pendulum of opinion swung back to some extent ; 

 only slightly, but still sufficiently to raise a controversy. 

 The physicists, led by Lord Kelvin (Sir William Thomson), 

 began to look about for some means of checking these 

 enormous demands. Lord Kelvin considered the question 

 of the world's antiquity from the physical standpoint. 

 His arguments, or rather calculations, were based on three 

 important considerations. These we must notice ; but 

 as our object in this paper is to consider purely (leolonk-al 

 measures of time, and his methods can only be judged by 

 the mathematician and astronomer, we must content our- 

 selves with a very brief account of his conclusions. 

 Lord Keh'in arrived at a very different conclusion, and 

 this was derived from three distinct lines of reasoning, 

 or rather calculation. First, he considered the internal 

 heat, and rate of coohng of the earth ; secondly, the tidal 

 retardation of the earth's rotation ; and thirdly, the origin 

 and age of the sun's heat. 



With regard to the earth's heat : the rate of increase of 

 temperature downward from the surface is known, for a 

 certain distance, by observations in mines. As many of 

 our readers are akeady aware, it is about 1° F. for 

 every 50 or 60 feet. But this rate is not maintained, and 

 becomes less at great depths. Then with regard to the 

 earth's present temperature — about 36^ F. at the bottom 

 of the oceans. From such available data he calculated 

 that the earth could not have consolidated, from its former 

 molten state, less than 20 millions of years ago, nor more 

 than 400 millions. In the one case the undergroimd heat 

 would have been greater than it actually is ; in the other 

 there would have been no sensible increase in temperature 

 downwards. He afterwards iuchned towards the lower 

 limit rather than the higher one, and said that we ought to 

 be quite satisfied with 1 00 millions of years for the duration 

 of geological time. Professor Tait would even limit the 



* Viz., the 4tKXl years of Archbishop Ussher's Chronology, a mere 

 "pious opiniou" nowhere expressed in Scriptme. 



