168 COSMOS 



influence of the seasons, scarcely amount to half a degree. In 

 tropical climates this invariable stratum is only one foot beluw 

 the surface, and this fact has been ingeniously made use of by 

 Boussingault to obtain a convenient and, as he believes, certain 

 determination of the mean temperature of the air of different 

 places.* This mean temperature of the air at a fixed point, or 

 at a group of contiguous points on the surface, is to a certain 

 degree the fundamental element of the climate and agricultural 

 relations of a district ; but the mean temperature of the whole 

 surface is very different from that of the globe itself. The 

 questions so often agitated, whether the mean temperature 

 has experienced any considerable differences in the course of 

 centuries, whether the climate of a country has deteriorated, 

 and whether the winters have not become milder and the 

 summers cooler can only be answered by means of the ther- 

 mometer ; this instrument has however scarcely been invented 

 more than two centuries and a half, and its scientific applica- 

 tion hardly dates back 120 years. The nature and novelty 

 of the means interpose, therefore, very narrow limits to our 

 investigation regarding the temperature of the air. It is 

 quite otherwise however with the solution of the great problem 

 of the internal heat of the whole Earth. As we may judge of 

 uniformity of temperature from the unaltered time of vibra- 

 tion of a pendulum, so we may also learn from the unal- 

 tered rotatory velocity of the Earth the amount of stability in 

 the mean temperature of our globe. This insight into the 

 relations between the length of the day and the heat of the 

 Earth is the result of one of the most brilliant applications of 

 the knowledge we had long possessed of the movement of the 

 heavens to the thermic condition of our planet. The rotatory 

 velocity of the Earth depends on its volume : and, since by 

 the gradual cooling of the mass by radiation, the axis of rota- 

 tion would become shorter, the rotatory velocity would neces- 

 sarily increase, and the length of the day diminish, with a 

 decrease of the temperature. From the comparison of the 

 secular inequalities in the motions of the Moon with the 

 eclipses observed in ancient times, it follows that since the 

 time of Hipparchus, that is, for full 2000 years, the length of 



* Boussingault, Sur la Profondeur a laquelle se trouve la couche de 

 Temperature invariable entre les Tropiques, in the Annalen ie Chimit 

 tt de Physique, t. liii., 1833, p. 225-247. 



