NATURAL PHILOSOPHY. 171 



bar of metal used. This bar is 6 centims. long, and 1 centim. square, and is 

 so arranged in experiment that 1 cub. ^cent. is in the larger vessel; 1 cub. 

 cent, in the smaller vesselj 3 cub. cent, are covered by the sides of the boxes 

 through which it passes; and the last 1 cub. cent, is covered with a piece of 

 vulcanized India-rubber tubing, and the whole made secure from any leakage 

 by lining the sides of the holes through which the bar passes with a varnish 

 made of caoutchouc dissolved in benzoine. All being ready for the experi- 

 ment, 50 cub. cent, of water, at the temperature of the room, are poured into 

 the smaller vessel, the boxes covered, and each provided with a very sensi- 

 tive thermometer, and 200 cub. cent, of boiling water poured into the larger 

 vessel by means of a funnel ; the temperature of the liquid falls to 86 or 

 88, but is again raised to 00, by a small jet of steam generated in a flask, 

 the water in which is kept boiling during the whole experiment. The con- 

 ducting power of the metal being tested is noted with the greatest care. For 

 mercury and sodium they employed a very thin sheet-iron box, the internal 

 dimensions of which were exactly those of the square metallic bars they 

 usually employed, and of the conducting power calculated, but the figures 

 are very near the truth. 



THE INTERNAL TEMPERATURE OF THE EARTH. 



The following is an abstract of a paper recently read before the Royal 

 Institution, London, by William Hopkins, F. R. S., " On the internal tem- 

 perature of the earth and the thickness of its solid crust : " If we descend 

 beneath the surface of the earth, and observe the temperature at different 

 depths, it is found that within a depth ranging from 50 to 80 feet, the tem- 

 perature changes periodically, being affected to that depth by the heat which 

 the earth receives from the sun at different seasons of the year. The annual 

 variation, however, becomes less as the depth increases, till at the depth 

 above mentioned it becomes insensible. At greater depths the temperature 

 is invariable at each point, but increases with the depth at the rate, on an 

 average, of 1 Fall, for a depth of between GO and 70 feet. The best obser- 

 vations which have been made on this subject are those in deep mining 

 shafts and deep artesian wells ; the greater the depth, the more completely 

 do anomalous influences counterbalance each other. The greatest depths at 

 which such observations have been made in Western Europe, are at Monk- 

 wearmouth and Dukinfield in England; the Puit de Grenelle, at Paris; 

 Mondorff, in the Duchy of Luxemburg; New Seltzwerk, in Westphalia; 

 and at Geneva. At the first two places the observations were made in ver- 

 tical shafts of coal mires, the depth of the one at Monkwearmouth being 

 upwards of 1800 feet, and that at Dunkinfield upwards of 2000 feet, and in 

 both cases the observations were made while the workmen were sinking the 

 shafts, and with every precaution against the influence of any extraneous 

 causes which might affect the observations. The former gave an increase 

 of 1 Fah. for every 60 feet of depth, the latter for about every 72 or 73 

 feet. The sinking of the Puit de Grenelle was superintended by Arago. 

 The mean increase of temperature was 1 for every 60 feet. At Mondorff 

 the bore was 2100, being that of an artesian well; the increase was 1 for 57 

 feet. At New Seltzwerk the artesian well, penetrating to the depth of 2100 

 feet, giving an increase of 1 Fah. for 55 feet. The average of these is very , 

 nearly 1 for sixty feet. Numerous other observations are confirmatory of 

 those results, though observations at smaller depths present many anom- 



