20 Prof. Leslie on Heat and Climate. [Julyj 



times lighter than mercury, and has only one-eighth of its spe- 

 cific attraction for heat. Hence the changes of temperature 

 which equal additions of heat will produce on mercury and the 

 ferruginous oxide are as eight to three. I exposed to the rays of 

 the sun when at the altitude of 60° the blackened bulb of a 

 mercurial thermometer with a very large scale, and observed the 

 mercury to rise at the rate of a degree in 14 seconds. If the 

 rays had reached the bulb without being impaired in their pas- 

 sage through the atmosphere, the thermometer would have risen 

 a degree in 11". Wherefore had an equal ball of the rust of iron 

 been substituted, the same effect would require •§- x 1 1", or half 

 a minute for its production. But the quantities of light which 

 spheres receive in like exposures are proportional to their sur- 

 faces, and consequently the changes produced in their tempera- 

 ture will be reciprocally as their diameters. The diameter of the 

 earth, allowing for the atmosphere, may be estimated in round 

 numbers at 8000 miles, and that of the bulb of the thermometer 

 was six-tenths of an inch. Wherefore the time elapsed before 

 the temperature of the earth increases one degree, must be 



*| x 5280 x 8000 or 844,800,000 half minutes, which amounts 



•6 



to 803 years and 238 days. The result will be the same whether 

 the particles of light reach the surface, or are spent among the 

 vapours and clouds of the atmosphere, as they will ultimately 

 be communicated to the mass of the globe. If we reckon the 

 distance of the absolute zero of the thermometric scale to be 

 2000 degrees, and suppose the earth to have received all its 

 heat from the sun, we shall find that the enormous period of 

 1,600,000 years has elapsed since the primeval chaos. But the 

 earth consists of materials of such a slow conducting quality 

 that probably the heat is not yet equally diffused. No difference 

 of temperature indeed is perceived in descending our deepest 

 mines. These perforations, however, reach not beyond the 

 external crust ; far below them the cold may commence ; and 

 perhaps the genial influence of the sun has never yet penetrated 1 

 to the centre. At any rate, we may reasonably presume that the 

 superficial parts of the -globe will be heated quicker than the 

 vast internal mass. The increase of temperature may, therefore, 

 exceed a degree in eight centuries ; and as the same principles 

 reduce the distance of the thermometric zero, they will likewise 

 diminish somewhat the period we have assigned for the antiquity 

 of the primeval chaos.* 



These views will, I am afraid, be treated by some as chimeri- 

 cal. What relates to the age of the world, I freely give up ; but 

 the more I reflect on the proposition, that climates are growing 

 gradually warmer, the more am I convinced of its reality. The 

 relations of the historians and the descriptions of the poets of 



* More correct calculation?, on this subject, will be found in the " Experimental 

 Inquiry into the Nature and Propagation of Heal.'V-A. 



