SKOT. XXVI. INTERNAL HEAT OF THE EARTH. 241 



The cold endured by Sir Edward Parry one day in 

 Melville Island was 55 below zero ; and that suffered 

 by Captain Back on the 17th of January, 1834, in 62 

 46^' of north latitude, was no less than 70 below the 

 same point. However, M. Poisson attributes this to ac- 

 cidental circumstances, and by a recent computation, he 

 makes the temperature of space to be 8 above the zero 

 of Fahrenheit. This he considers greatly to exceed the 

 temperature of the exterior strata of the atmosphere, 

 which he conceives to be deprived of their elasticity by 

 intense cold, and he thus accounts for the decrease of 

 temperature at great elevations, and for the limited ex- 

 tent of the atmosphere. 



Doubtless, the radiation of all the bodies in the uni- 

 verse maintains the ethereal medium at a higher tem- 

 perature than it would otherwise have, and must event- 

 ually increase it, but by a quantity so evanescent that it 

 is hardly possible to conceive a time when a change will 

 become perceptible. 



The temperature of space being so low, it becomes a 

 matter of no small interest to ascertain whether the earth 

 may not be ultimately reduced by radiation to the tem- 

 perature of the surrounding medium ; what the sources 

 of heat are ; and whether they be sufficient to compen- 

 sate the loss, and to maintain the earth in a state fit for 

 the support of animal and vegetable life in time to come. 

 All observations that have been made under the surface 

 of the ground concur in proving that there is a stratum 

 at the depth of from 40 to 100 feet throughout the whole 

 earth, where the temperature is invariable at all times 

 and seasons, and which differs but little from the mean 

 annual temperature of the country above. According to 

 M. Boussingault, indeed, that stratum at the equator is 

 at the depth of little more than a foot in places sheltered 

 from the direct rays of the sun ; but in our climates it 

 is at a much greater depth. In the course of more than 

 half a century, the temperature of the earth at the 

 depth of 90 feet in the caves of the Observatory at Paris 

 has never been above or below 53 of Fahrenheit's ther- 

 mometer, which is only 2 above the mean annual tem- 

 perature at Paris. This zone, unaffected by the sun's 

 rays from above, or by the internal heat from below, 

 16 X 



