332 



EECEEATIVE SCIENCE. 



three periods — it has for at least 2000 years 

 designated an equal space of time, and of 

 course during the same period the velocity of 

 the earth's rotation has remained constant. 



But observe with what precision this fact 

 may be measured by the true philosopher. 

 Let us suppose, says Arago, the mean tem- 

 perature of each radius of the globe to have 

 increased I'G" Pahr. in 2000 years, and assume 

 the average dilatation by heat of the substances 

 composing the mass to be the same as that of 

 glass, that is, about \omsoo for each degree ; 

 a diminution of 1° would consequently have 

 occasioned one in the dimensions of the sphe- 

 roid of ro5o75o, which, according to dynamical 

 theory, would produce an acceleration in its 

 velocity of rotation of so^o, which would 

 shorten the sidereal day by 1'7". But since 

 the time of Hipparchus, the sidereal day has 

 not varied the xh^th of a second, a quantity 170 

 times less than 1'7". Hence the change of 

 temperature assumed above is 170 times 

 greater than the observations on the length 

 of the sidereal day admit of its having been ; 

 and we may conclude that the mean tem- 

 perature of the general mass of the earth has 

 not varied tfoth of a degree in 2000 years. 



If the cooling process takes place so slowly, 

 how long has it been in action to have pro- 

 duced its present result P The reason leaves 

 the conjecturfi to imagination, and imagina- 

 tion, unequal to the conception of periods for 

 which ordinary units of years, or even millions 

 of years, afford the elements, abandons the spe- 

 culation in despair. If we cannot look back, 

 dare we look forward for a period when, how- 

 ever slow the loss of heat, it shall escape at 

 last, and leave the earth frozen, arid, lifeless P 

 or, shall we seek for some law of compensa- 

 tion, such as the astronomer finds in the mo- 

 tions of the planets, and which shall preserve 

 the constancy of its perennial warmth and 

 the continuance of its gay circle of seasonal 

 changes, and flowery forms, and living and 

 happy creatures P Will the earth powder 

 ^vray under the pulverizing fingers of Time, 



or maintain for ever the glory of its warm 

 seasons and the freshness of its belt of 

 flowers P Will the fire expend its force, and 

 earthquakes cease for ever ? or will the same 

 round of changes go on eternally, swallowing 

 cities in the embrace of hills, and swamping 

 villages in burning seas of lava P The fact is 

 not isolated but universal. The moon is essen- 

 tially a volcanic body as regards the con- 

 figuration of its surface. The lunar craters 

 are vastly larger than those of the earth. 

 Tycho, as cited by Mrs. Ward in " Telescope 

 Teachings," having a diameter of fifty miles, 

 while others exceed even these great dimen- 

 sions. Sir John Hersehel and other ob- 

 servers have noted what appeared to be evi- 

 dences of existing igneous action; but Mr. 

 Nasmyth's view is now generally adopted, 

 that volcanic action has ceased ou the moon 

 for thousands of years. Suifice it, that the 

 evidences of fire are there to give it an 

 immediate physical relation to the earth 

 we inhabit. The planets are the same, 

 bvilged at the equator, as though their 

 inner parts were mobile, and yielded to 

 the whirling force which spins them round 

 and round. The heat is there, preserving 

 summer verdure, and when the sun shines 

 not, keeping them rotund and fertile ever ; 

 retarding the tendency of matter to slide 

 back into a crystalline form, lifeless, shape- 

 less, and dejected. What power the sun has 

 in warming the earth's crust ; how the forces 

 of heat within and sunshine without balance 

 each other on the surface ; how modern geo- 

 logical changes, not of a volcanic nature, de- 

 pend on internal expansions ; how the tempe- 

 rature of space operates in disturbing or pre- 

 serving an equilibrium, and a hundred other 

 speculations, spring out of these conclusions 

 on the existence of an elevated temperature 

 in the interior of the globe. As science per- 

 severes in her daily work of recording and 

 comparing facts, no doubt many of these pro- 

 blems will be elucidated, and if material 

 Nature is constant in its operations during 



