Things not generally Known. 



by Professor Airy, in the Harton coal-pit, near South Shields :* 

 the oscillations of the pendulum at the bottom of the pit were 

 compared with those of a clock above ; the beats of the clock 

 were transferred below for comparison by an electric wire ; and 

 it was thus determined that a pendulum vibrating seconds at the 

 mouth of the pit would gain 2 seconds per day at its bottom. 

 The final result of the calculations depending on this experiment, 

 which were published in the Philosophical Transactions of 1856, 

 gives 6*565 for the mean density of the earth. The celebrated 

 Cavendish experiment, by means of which the density of the 

 earth was determined by observing the attraction of leaden 

 balls on each other, has been repeated in a manner exhibiting 

 an astonishing amount of skill and patience by the late Mr. F. 

 Baily.f The reoult of these experiments, combined with those 

 previously made, gives as a mean result 5 '441 as the earth's 

 density, when compared with water ; thus confirming one of 

 Newton's astonishing divinations, that the mean density of the 

 earth would be found to be between five and six times that of 

 water. 



Humboldt is, however, of opinion that " we know only the mass of 

 the whole earth and its mean density by comparing it with the open 

 strata, which alone are accessible to us. In the interior of the earth, 

 where all knowledge of its chemical and mineralogical character fails, 

 we are limited to as pure conjecture as in the remotest bodies that 

 revolve round the sun. We can determine nothing with certainty re- 

 garding the depth at which the geological strata must be supposed to 

 be in a state of softening or of liquid fusion, of the condition of fluids 

 when heated under an enormous pressure, or of the law of the increase 

 of density from the upper surface to the centre of the earth." Cosmos, 

 vol. i. 



In M. Foucault's beautiful experiment, by means of the 

 vibration of a long pendulum, consisting of a heavy mass of 

 metal suspended by a long wire from a strong fixed support, is 

 demonstrated to the eye the rotation of the earth. The Gyro- 

 scope of the same philosopher is regarded not as a mere philo- 

 sophical toy ; but the principles of dynamics, by means of 

 which it is made to demonstrate the earth's rotation on its own 

 axis, are explained with the greatest clearness. Thus the inge- 

 nuity of M. Foucault, combined with a profound knowledge of 

 mechanics, has obtained proofs of one of the most interesting 

 problems of astronomy from an unsuspected source. 



THE EARTH AND MAN COMPARED. 



The Earth speaking roundly is 8000 miles in diameter ; 



* The result of these experiments for ascertaining the variation of the gra- 

 vity at great depths, has proved beyond doubt that the attraction of gravitation 

 is increased at the depth of 1250 fret by -^-^ part. 



t See the account of Mr. Baily's researches (with two illustrations) in Things 

 not generally Known, p. vii., and " Weight of the Earth," p. 16. 



