8 POPULAR LECTURES AND ADDKKSSKS. 



across the whole earth's surface per annum will 



/A 

 be '--. The bulk of a sphere being its surface 



multiplied by -J of its radius, the thermal capacity 

 of a mass of rock equal in bulk to the earth, and of 

 specific heat s per unit of bulk is J Ars. Hence 



.">/ 



=r is the elevation of temperature which a 



J )r.s- 



quantity of heat equal to that lost from the earth 

 in a year, would produce in a mass of rock equal 

 in bulk to the whole earth. The laboratory ex- 

 periments of Peclet ; Observations on Under- 

 ground Temperature in three kinds of rock in and 

 near Edinburgh, by Forbes ; in two Swedish strata, 

 by Angstrom, and at the Royal Observatory, 

 Greenwich, give values of the conductivity in 

 gramme-water units of heat per square centi- 

 metre, per i per centimetre of variation of tem- 

 perature, per second, from X)O2 (marble, Peclet) to 

 0107 (sandstone of Craiglcith quarry, Forbes) ; and 

 005 may be taken as a rough average. Hence, as 

 there arc 31,557,000 seconds in a year, we have 

 / = -005 x 31,557,000, or approximately i6xio 4 . 

 The thermal capacity of surface rock is somewhere 

 about half that of equal bulk of water ; so that 

 we may take ^ = '5- And the increase of tempera- 

 ture downwards may be taken as roughly averag- 

 ing i Cent, per 30 metres; so, that, = 3000 

 centimetres. Lastly, the earth's quadrant being 

 according to the first foundation of the French 



