346 POPULAR SCIENCE MONTHLY. 



had approximately the same temperature; and that as the surface after- 

 ward cooled hy outward radiation there was a flow of heat to the sur- 

 face by conduction from below. The rate of this flow has diminished 

 from that epoch to the present time according to a definite law, and 

 the present rate, being known from observation, affords a measure of 

 the age of the crust. The strength of this computation lies in its 

 definiteness and the simplicity of its data; its weakness in the fact that 

 it postulates a knowledge of certain properties of rock — namely, its 

 fusibility, conductivity and viscosity — when subjected to pressures and 

 temperatures far greater than have ever been investigated experiment- 

 ally. 



A parallel line of discussion pertains to the sun. Great as is the 

 quantity of heat which that incandescent globe yields to the earth, it is 

 but a minute fraction of the whole amount with which it continually 

 parts, for its radiation is equal in all directions, and the earth is but a 

 speck in the solar sky. On the assumption that this immense loss of 

 heat is accompanied by a corresponding loss of volume, the sun is 

 shrinking at a definite rate, and a computation based on this rate has 

 told how many millions of years ago the sun's diameter should have 

 been equal to the present diameter of the earth's orbit. Manifestly the 

 earth can not have been ready for habitation before the passage of that 

 epoch, and so the computation yields a superior limit to the extent of 

 geologic time. 



Before passing to the next division of the subject — the computa- 

 tions based on rhythms — a few words may be given to the results which 

 have been obtained from the study of continuous processes. Eealizing 

 that your patience may have been strained by the kaleidoscopic charac- 

 ter of the rapid review which has seemed unavoidable, I shall spare you 

 the recitation of numerical details and merely state in general terms 

 that the geologists, or those who have reasoned from the rocks and fos- 

 sils, have deduced values for the earth's age very much larger than have 

 been obtained by the physicists, or those who have reasoned from earth 

 cooling, sun cooling and tidal friction. In order to express their results 

 in millions of years the geologists must employ from three to five digits, 

 while the physicists need but one or two. When these enormous dis- 

 crepancies were first realized it was seen that serious errors must exist in 

 some of the observational data or else in some of the theories employed; 

 and geologists undertook with zeal the revision of their computations, 

 making as earnest an effort for reconciliation as had been made a gen- 

 eration earlier to adjust the elements of the Hebrew cosmogony to the 

 facts of geology. But after rediscussing the measurements and readjust- 

 ing the assumptions so as to reduce the time estimates in every reason- 

 able way — and perhaps in some that were not so reasonable — they were 

 still unable to compress the chapters of geologic history between the 



