1823.] Mr. Moyle on the Temperature qfMi?ies. 39 



third, 24 inches deep, stood at 68°. The last two thermometers 

 preserved the same temperature, both day and night, till the end 

 of the month, and then fell to 61°, the earth obstinately retaining 

 its heat at that depth, though the temperature of the air fre- 

 quently varied. On the 26th of October, a thermometer exposed 

 to the air stood at 35'5° ; but one sunk two inches below the 

 surface was heated to 43'85° ; another, at the depth of 16 

 inches, stood at 48*8° ; and another, 24 inches deep, showed 

 50° ; and from the 1st to the 12th of November, when the tem- 

 perature of the external air was 27°, a thermometer placed at 

 the depth of 24 inches, stood at 43-8° ; but from the month of 

 March to that of September in the following year, the external 

 air was constantly warmer than the earth at the depth of 16 or 

 24 inches. 



From these experiments, it may be inferred that the surface of 

 the earth is much heated during summer, but that the heat 

 descends very slowly, a great part of it being communicated to 

 the air ; that during the winter the earth gives out to the air 

 the heat which it had received during the summer ; and that 

 wet summers are generally succeeded by cold winters. 



Marriotte's experiments furnish nearly similar results to those 

 of Dr. Hales. Hence it appears that at the distance of about 80 

 or 90 feet below the surface, provided that there be a communi- 

 cation with the external air, or at a less depth if there be no 

 communication, the temperature of the earth admits of very 

 little variation, and generally approaches to the mean annual 

 heat. 



M. Van Swinden has observed, that the greatest cold, and 

 even that which is below 0° of Fahr. if it lasts no more than a 

 few days, penetrates no deeper than 20 inches, when the earth 

 is covered with snow, and not above 10 inches, if no snow lies on 

 the surface. 



Such facts tend to prove that the heat of the earth does not 

 increase as we descend into it ; but at the greatest depths, it is 

 nearly the same as the mean annual temperature of the latitude. 



The following table of temperatures, taken by myself, will be 

 convenient for a general reference, distinguishing the mines at 

 work from those long since abandoned ; and the medium in 

 which the temperatures were taken : 



