348 COSMOS. 



have to mention : the influence of an enormous pressure 

 upon fusibility, the different conduction of heat by hetero- 

 geneous rocks, the remarkable enfeebling of conductibility 

 with a great increase of temperature, treated of by Edward 

 Forbes, the unequal depth of the oceanic basin, and the 

 local accidents in the connection and nature of the fissures, 

 which lead down to the fluid interior ! If the greater vici- 

 nity of the upper limit of the fluid interior in particular 

 regions of the earth may explain the frequency of volcanoes 

 and the greater multiplicity of communication between the 

 depths and the atmosphere, this vicinity again may depend 

 either upon the relative average differences of elevation of 

 the sea-bottom and the continents, or upon the unequal 

 perpendicular depth at which the surface of the molten fluid 

 mass occurs, in various geographical longitudes and latitudes. 

 But where does such a surface commence ? Are there not 

 intermediate degrees between perfect solidity and perfect 

 mobility of the parts ? states of transition which have 

 frequently been referred to in the discussions relative to 

 the plasticity of some Plutonic and volcanic rocks which 

 have been elevated to the surface, and also with regard to 

 the movement of glaciers. Such intermediate states abstract 

 themselves from mathematical considerations, just as much 

 as the condition of the so-called fluid interior under an 

 enormous pressure. If it be not even very probable that 

 the temperature everywhere continues to increase with the 

 depth in arithmetical progression, local intermediate dis- 

 turbances may also occur, for example, by subterranean 

 basins (cavities in the hard mass), which are from time to 

 time partially-filled from below with fluid lava and vapours 

 resting upon it 41 . Even the immortal author of the Pro- 

 togcea allows these cavities to play a part in the theory of 

 the diminishing ceotral heat : " Postremo credibile est con- 

 trahentem se refrigeratione crustam bullas reliquisse, ingentes 

 pro rei magnitudine id est sub vastis fornicibus cavitates" 1 * 



41 Cosmos, vol. v, p. 168. See also with regard to the unequal dis- 

 tribution of the icy soil, and the depth at which it commences, inde- 

 pendently of geographical latitude, the remarkable observations of 

 Captain Franklin, Errimn, Kupffer, and especially of Middendorff (loc, 

 eit. sup, s. 42, 47 and 167). 



42 Leibnitz in the Protugcea, 4. 



