BABBAGE ON THE TEMPLE OF SERAPIS. 207 



necessary to assume the truth of the first, although it is not neces- 

 sary that we be acquainted with the law of its variation ; nor is it 

 absolutely essential that we should suppose the heat to increase to 

 such an extent, as to render the whole of the central parts of the 

 earth fluid. 



86. W we imagine at every point of the earth's surface a line 

 drawn to its centre, then if a point be taken in any one line at a 

 given temperature, there will be contiguous points of exactly the 

 same temperature in all the adjacent lines ; and if we conceive a sur- 

 face to pass through all these points, it will constitute a surface of 

 uniform temperature, or an isothermal surface. This therefore will 

 not be parallel to that of the earth, but will be irregular, descending 

 more towards the centre of the earth, where it passes under deep 

 oceans. 



An increase of 1° of Fahrenheit's thermometer, for every fifty or 

 sixty feet we penetrate below the earth's surface, seems nearly the 

 average result of observations. If the rate continue, it is obvious 

 that, at a small distance below the surface, we shall arrive at a 

 heat which will keep all the substances with which we are ac- 

 quainted in a state of fusion. Without however assuming the fluidity 

 of the central nucleus — a question yet unsettled, and which rests on 

 very inferior evidence* to that by which the principles here em- 

 ploj'^ed are supported — we may yet arrive at important conclusions ; 

 and these may be applied to the case of central fluidity, according 

 to the opinions of the several inquirers. 



87. If we consider the temperature of any point — for example, 

 G, situated two miles below the surface of an elevated table-land A, 

 in the annexed woodcut fig. 4; and if we imagine a surface passing 

 through all the points of equal temperature within the globe, then, as 

 this surface passes under the adjacent ocean, which we may suppose, 

 on an average, to be two miles deep, it is evident that the surface of 

 equal heat will descend towards the earth's centre ; because, if it did 

 not, we should have great heat nearly in contact with the bottom of 

 the sea. In number one, B is the surface of the ocean ; A D the 

 surface of the land, and of the bed of the ocean. The broken line 

 G F is the isothermal line. Let us now suppose, by the continual 

 wearing down of the continents and islands adjoining the ocean, that 

 it becomes nearly filled up. The broken line C, in number two 

 of the woodcut, indicates the new bottom. The former bottom of 

 the ocean being now covered with a bad conductor of heat, instead 

 of with a fluid which rapidly conveyed it away, the surface of uniform 

 temperature will rise slowly but considerably, as is shown at G E, in 

 number three. In number four, the first bed of the ocean A D, 

 and its isothermal line G F, as well as the new bed, A C, of the 

 ocean, and its corresponding isothermal line G E, are all shown at 

 one view. 



88. The newly-formed strata will be consolidated by the applica- 

 tion of heat ; they may, perhaps, contract in bulk, and thus give space 



* The reader will find this question fully discussed in the 32nd and 33rd chapters 

 of Lyell's Principles of Geology, 7th edit. 



