26 NECESSITY OF GENERAL 



tained in modern times, as well as in the middle ages, are, 

 therefore, wholly beyond the range of our actual experimental 

 knowledge. 



I may also borrow from physical astronomy other examples, 

 of which the grandeur and the interest cannot be felt without 

 some general knowledge of the forces which animate the 

 universe, and may adduce the elliptic revolutions of many 

 thousands of double stars, or suns, around each other, or 

 rather around their common centre of gravity, revealing 

 the existence of the Newtonian attraction in those distant 

 worlds; the periodical abundance or paucity of spots on the 

 eun (openings in the opaque but luminous envelope of the 

 solid nucleus) ; and the periodic appearance, observed for 

 some years past about the 12th or 13th of November and the 

 10th or llth of August, of countless multitudes of shooting 

 stars, moving with planetary swiftness, which probably form 

 a belt of asteroids intersecting the orbit of the earth. 



Descending from the skies to the earth, we may notice 

 how the oscillations of a pendulum in air (the theory of 

 which has been perfected by BesseFs acuteness) have thrown 

 light on the internal density, I might say on the degree of 

 solidification, of our planet ; they have also served, in a cer- 

 tain sense, to sound terrestrial depths, conveying informa- 

 tion respecting the geological nature of strata otherwise 

 inaccessible. In this manner, as well as in others, we are 

 enabled to trace a striking analogy between the production 

 of granular rocks in lava currents which have flowed down 

 the slopes of active volcanoes, and those granites, porphyries, 

 and serpentines, which, issuing from the interior of the earth, 

 have broken, as eruptive rocks, through the secondary 

 strata, modifying them by contact, hardening them by the 



