xlviii NOTES. 



and 521 524 (see also the extract which de la Rive has made in the " Bib- 

 liotheque universelle de Geneve," T. k. p. 415), has developed an hypothesis 

 wholly different from Fourier's views ; " Theorie analytique de la Chaleur." 

 Poisson denies the actual fluidity of the interior of the Earth ; he thinks " that 

 in the process of cooling by radiation to the medium surrounding the earth, the 

 particles which were first solidified at the surface sunk, and that by a double 

 current, descending and ascending, the great inequality was diminished which 

 would otherwise take place in a solid body cooling from its surface." It 

 seemed to this great geometer more probable that the solidification should 

 have begun in the strata nearer the center ; " that the phenomenon of the 

 temperature increasing with increasing depth does not extend to the whole 

 mass of the Earth, and is merely a consequence of the movement of our 

 planetary system in space, of which some parts are of very different tempera- 

 ture from others, by reason of stellar heat (chaleur stehaire)." Thus, accord- 

 ing to Poisson's views, the warmth of the waters in our Artesian wells would 

 be merely a warmth which had penetrated into the Earth from without ; and 

 " the earth itself might be compared to a mass of rock conveyed from the 

 equator to the pole so rapidly as not to have entirely cooled. The increase of 

 temperature in such a block from the surface inwards would not extend to its 

 center." The physical doubts which have been justly raised against this sin- 

 gular cosmical view (attributing to the regions of space that which is better 

 explained by the transition from a primitive gaseous to a solid state) will be 

 found collected in Poggendorff's Annalen, Bd. xxxix. S. 93 100. 



( 13S ) p. 163. The increase of temperature has been found in the Puits de 

 Crenelle at Paris to be 1 C. for 98.4 (104.9 Eng.) feet ; in the mine at Neu- 

 Salzwerk, near Minden in Prussia, for nearly 91 (97.0 Eng.) feet ; and, ac- 

 cording to Auguste de la Rive and Marcet, the same (viz. 1 C. for 91 French feet 

 at Pregny near Geneva, although the mouth is situated 1510 (1609 Eng.) feet 

 above the level of the sea. The agreement between the results derived by 

 a method first proposed by Arago, in 1821 (Annuaire, 1835, p. 234), from 

 mines that are severally 1683 (1794 Eng.) feet, 2094 (2232 Eng.) feet, and 

 680 (725 Eng.) feet, in absolute depth, is remarkable. It is probable that 

 if there are two points on the Earth's surface situated at a small vertical 

 distance above each other, whose annual mean temperatures are exactly deter- 

 mined, they are to be found at the Paris Observatory, where the mean tem- 

 perature of the external air is 10.822 C. (51.48 F.), and of the Caves de 

 T0bservatoirell.834 C. (53.3 F.) ; the difference being l c .012 (1.8 F.) for 

 28 metres (91.9 English feet). Poisson, Theorie matn.d la Clialeur, p. 15 



