OUTLINES OF GEOLOGY. 



7 



in contact with it. This superficial temperature is therefore 

 subject to continual variations during the alternations of day 

 and night, and the vicissitudes of the seasons. But when we 

 penetrate beneath the surface of the earth in digging pits, in 

 sinking mines, or in natural caverns, we find that the extremes 

 of temperature gradually approach, until a depth is reached 

 at which the thermometer retains a constant height. This 

 constant temperature is, as nearly as can be ascertained, the 

 same with the mean annual temperature of the atmosphere 

 at the surface above. The thermometer first assumes a con- 

 stant indication, in extreme climates, such as our own, at 

 depths of from 30 to 40 feet ; but in climates where the va- 

 riation of temperature is less, the depth at which a constant 

 temperature first occurs is less also, and, in equatorial 

 islands, does not exceed two or three feet. 



9. At depths greater than that at which a constant tem- 

 perature first occurs, the temperature at a given point is 

 constant also, but this constant temperature of each different 

 depth increases as the depth increases. It has hence been 

 calculated, that at a depth of about 3000 yards, the tempera- 

 ture will be as great as that at which water boils. 



10. It is therefore obvious, that the mean temperature of 

 the body of the earth is greater than that of its surface, and 

 as it is situated in a space much colder than itself heat must 

 be constantly escaping by radiation in all directions. The 

 earth would therefore be continually growing colder, were 

 there not some extraneous source of heat. 



11. We find such an extraneous source in the rays of the 

 sun, and it is obvious that these two causes mwst, after some 

 period, less or greater, exactly balance each other, and the 

 mean temperature of the earth become constant. We know, 

 from astronomic facts, that this state of constant mean tem- 

 perature in the earth has been very nearly reached at the 

 present day, and that no perceptible decrease of mean tern- 



