PNEUMATICS. 303 



46. The cause of this extraordinary cold is by no 

 means sufficiently understood. 



413. The varieties of temperature to be met 

 with on the surface of the earth, appear to be con- 

 fined between the limits of + 100 and 40. 



No degree of natural cold much below 40* has ever 

 been observed, even when thermometers have been, 

 employed containing a fluid not liable to congelation. 

 Sir CHARLES BLAGDEN on the Congelation of Mer- 

 cury, p. 61. 



The heat of 100 is rare, but a heat approaching to 

 90, is found in the summer of most countries within 

 the limits of the temperate zones. 



There is hardly any climate, even in the frigid zone, 

 where a temperature between 60 and 50 is not oc- 

 casionally experienced. 



The greatest heat is much less above the mean tem- 

 perature than the greatest cold is below it. The 

 mean temperature for the whole surface may be ta- 

 ken at 58 ; the greatest summer heat is only 4* 

 above this; the greatest winter cold is 98 un- 

 der it. 



414. There is reason to think that the climates 

 of Europe were more severe in ancient times than 

 they are at present, and that the change which has 

 taken place may with great probability be ascribed 



to 



