BABBAGE ON THE TEMPLE OF SERAPIS. 205 



The difficulties of this theory are, that some part of the surface at 

 the piers of Caligula's Bridge is at present raised above its former 

 level, and other parts, as the temple of the Nymphs and of Nep- 

 tune, are still below that level ; whilst the temple of Serapis appears 

 to have returned nearly to its former state. The answer to this is, 

 that the thickness of the expanding beds may differ in different parts, 

 or may have a different power of conducting heat — or it may be re- 

 marked, if the conducting power and the thickness be the same, the 

 distance from the source of heat may be different, and consequently 

 the full eff'eets may have reached the piers of the bridge, and yet not 

 have attained the other points. 



Another objection is, that the columns of the temple are nearly 

 vertical*, whilst the inclination of the strata, as proved by the perfo- 

 rations in the 6th and 12th piers of the bridge, shows an inclination 

 which would be sensible. To this it may be replied, that during the 

 transit of the wave of heat through the strata under the temple, the 

 columns may have been slightly inclined and yet have retained their 

 position, even if they had not been supported by being imbedded 

 in the tuff" and sand which then filled the temple. Or it may happen 

 that the beds on which the temple stands are separated by faults or 

 rents from their continuation under the other buildings ; and if they 

 be thus isolated, the effect of heating and cooling them would be to 

 raise and lower the temple in a vertical position. 



On the whole this explanation is the most tenable, because it is 

 founded on facts — viz. that matter expands by heating ; that great 

 accessions of heat have at various times taken place in the neigh- 

 bourhood of the temple ; that it is sufficient to account for the phse- 

 nomena by supposing a moderate depth of the beds below it heated 

 to a degree which it is not unreasonable to presume must have taken 

 place ; that such changes of level would on the whole occur gradually, 

 although they might be accompanied with earthquakes and occa- 

 sionally by sudden changes of level — facts of which we have historical 

 evidence as having happened on this spot. 



83. In reflecting on the preceding explanation of the causes which 

 produced the changes of level of the ground in the neighbourhood 

 of Pozzuoli, I was led to consider whether they might not be ex- 

 tended to other instances, and whether there are not other natural 

 causes, constantly exerting their influence, which, concurring with 

 the known properties of matter, must necessarily produce those 

 alterations of sea and land, those elevations of continents and moun- 

 tains, and those vast cycles of which geology gives such incontro- 

 vertible proofs. 



84. The small depth at which melted lava may exist below the 

 surface of bad conductors of heat, had forcibly struck me in an ex- 

 pedition I made to Vesuvius during the interval between my visits 

 to the temple of Serapis. Having descended into the great crater 

 of Vesuvius to examine a little crater within it, which was then in a 



* The late Capt. Basil Hall subsequently ascertained, by comparing these 

 columns with their image reflected in the water, that they are very sensibly out of 

 the perpendicular. 



