Observations on Subterraneous Heat, 321 



the terrestrial globe. The thermometer I used was a mer- 

 curial one, and graduated into twenty-four parts, from the 

 freezing point to that of boiling water. It was inclosed in 

 a glass tube. I ascertained by experiment, that when it In- 

 dicated a certain degree of heat, and when it was removed 

 about twelve degrees therefrom, three or four minutes were 

 requisite if it was dipped in water at the freezing tempera- 

 ture, and eleven or twelve minutes when held in the air. 

 According to these data, at all times when I wished to take 

 the temperature of a mass of water in mines', I plunged the 

 thermometer entirely into it and kept it there five minutes: 

 when a mass of air was to be examined, I helcf the thermo- 

 meter a quarter of an hour. All these observations were re- 

 duced to the centigrade thermometer. However great the 

 care and patience I bestowed, I could never answer pre- 

 cisely within a quarter of a degree. 



Observations made at Poullaouen. 

 I shall begin by describing the position of the place. 

 The mine of Poullaouen is situated in 48° 17' 49" of lati- 

 tude, and 5° 55* 57" longitude west from Paris.: its orifice 

 (St. George's pits) is 106 metres above the level of the sea.- 

 Tt is four myriametres from the north extremity of Britany, 

 and six from the south and east extremities. The country 

 in which it is situated forms part of the tongue of land 

 which, in the form of a roof, the ridge of which is 260 metres 

 above the level of the sea, advances into the ocean, and con- 

 stitutes the country called Britany. The district in which 

 the mine is situated is about 150 metres above the level of 

 the sea: this country is broken up in every direction by 

 valleys ; one of them resembles an almost circular basin 

 about a thousand metres in diameter, and it is under the 

 Soil of this basin (which is 106 metres above the level of the 

 sea) that the mine of Poullaouen is wrought. 



According to the law followed by the heat of the equator 

 at the pole, the mean temperature of the surface* of the 

 earth at Poullaouen ought to be 12*4°*. The elevation of 

 • I have been led both from theory and observation to use an extremely- 

 simple expression describing thethermometrical temperature of any place, the 

 latitude of which is known. This expression is, 30-7° Coss. a "-5 latitude ; or 

 with a sufficient exactitude in the temperate zone, 28° Coss. 2 latitude. 



Vol. 33. No. 132. April 1809. X the 



