466 SUMMARY OP THE CONTENTS. 



ideas, and the progress of physical geology. Influence of the form 

 of the Mediterranean on the earliest ideas respecting volcanic phe- 

 nomena. Comparative geology of volcanos. Periodical recurrence 

 of certain natural changes or revolutions which have their origin 

 in the interior of the globe. Relative proportion of the height of 

 volcanos to that of their cones of ashes in Pichincha, the Peak of 

 Teneriffe, and Vesuvius. Changes in the height of the summit of 

 volcanos. Measurements of the height of the margins of the crater 

 of Vesuvius, from 1773 to 1822 : the author's measurements com- 

 prise the period from 1805 to 1522 &jSSte$ '->r .".< 375 386 



Particular description of the eruption in the night of 23-24th October, 

 1822. Falling, in of a cone of cinders 426 English feet in height, 

 which previously stood in the interior of the crater. The eruption 

 of ashes, from the 24th to the 28th of October, is the most remark- 

 able of which we possess any certain knowledge since the death 

 of the elder Pliny 386390 



Difference between volcanos with permanent craters ; and the pheno- 

 mena (very rarely observed within historic times) in which trachy- 

 tic mountains open suddenly, emit lava and ashes, and reclose again 

 perhaps for ever. , The latter class of phenomena are particularly 

 instructive to the geologist, because they recall the earliest revolu- 

 tions of the oscillating, upheaved, and fissured surface of the 

 globe. They led, in classical antiquity, to the view of the Pyri- 

 phlegethon. Voleanos are intermitting earth springs, indicating a 

 communication (permanent or transient) between the interior and 

 the exterior of our planet ; they are the result of a reaction of the 

 still fluid interior against the crust of the earth ; it is therefore 

 needless to ask what chemical substance burns, or supplies materials 

 for combustion, in volcanos. ' . . . . . 390 393 



The primitive cause of subterranean heat is, as in all planets, the pro- 

 cess of formation itself, i. e. the forming of the aggregating mass 

 from a cosmical gaseous fluid. Power and influence of the radia- 

 tion of heat from numerous open fissures and unfilled veins in the 

 Ancient World. Climate (or atmospheric temperature) atthat period 

 very independent of the geographical latitude, or of the position of 

 the planet in respect to the central body, the sun. Organic forms 

 of the present tropical world buried in the icy regions of the north. 



393394 



Scientific Elucidations and Additions p. 395 to p. 399. 

 Barometric measurements of Vesuvius. Comparison of the height of 

 different points of the crater of Vesuvius . . . 395398 



Increase of temperature with depth, 1 Reaumur for every 113 Parisian 

 feet, or 1 of Fahrenheit for every 53.5 English feet. Temperature 

 of the Artesian well at Oeynhausen's Bad (New Salzwerk, near 

 Minden), the greatest depth yet reached below the level of the sea. 

 The hot springs near Carthage led Patricius, Bishop of Pertusa, in 



