MEAN TEMPERATURE OE THE EARTH. 165 



diminished to less than 1 of Fahrenheit. In tropical cli- 

 mates the invariable stratum is only one foot below the sur- 

 face ; and Boussingault has ingeniously availed himself of this 

 fact to obtain a very convenient, and, as he thinks, certain 

 mode of determining the mean temperature of the air at a 

 station ( 139 ). This mean temperature of the air, either at a 

 fixed point, or at a group of points not far removed from each 

 other on the surface of the Earth, is, to a certain degree, the 

 fundamental element of the relations which determine the 

 climate, and the appropriate cultivation of a district; but 

 the mean temperature of the whole surface of the Earth is 

 very different from that of the Earth itself. The often 

 repeated questions, whether the superficial temperature has 

 undergone any considerable change in the course of cen- 

 turies, whether the climate of a country has deteriorated, 

 whether the winter may not have become milder, and the 

 summer at the same time cooler, are all inquiries which 

 can only be Decided by means of the thermometer, an instru- 

 ment only invented about two centuries and a half ago, 

 and of which the intelligent scientific employment scarcely 

 dates back to 120 years. The nature and the novelty of the 

 means, therefore, restrict within very narrow limits cur 

 inquiries concerning the temperature of the air ; but it is 

 quite otherwise with the solution of the larger problem 

 regarding the internal temperature of the whole globe. A s 

 from the unaltered time of vibration of a pendulum we are 

 able to conclude that the equality of its temperature has 

 been maintained, so the unchanged velocity of the Earth's 

 rotation furnishes a measure of the stability of its mean 

 temperature. This insight into the relation between the 

 length of the day and the heat of the globe, leads to a 



