24 COSMOS. 



There are variations, it is true, which in obedience to the 

 laws of universal gravitation, affect the form of the earth's 

 orbit, and the inclination of the ecliptic, that is, the angle 

 which the axis of the earth makes with the plane of its orbit ; 

 but these periodical variations are so slow, and are restricted 

 within such narrow limits, that their thermic effects would 

 hardly be appreciable by our instruments in many thousands 

 of years. The astronomical causes of a refrigeration of our 

 globe, and of the diminution of moisture at its surface, and 

 the nature and frequency of certain epidemics phenomena 

 which are often discussed in the present day according to the 

 benighted views of the middle ages ought to be considered 

 as beyond the range of our experience in physics and chemistiy . 

 Physical astronomy presents us with other phenomena, 

 which cannot be fully comprehended in all their vastness 

 without a previous acquirement of general views regarding 

 the forces that govern the universe. Such, for instance, are 

 the innumerable double stars, or rather suns, which revolve 

 round one common centre of gravity, and thus reveal in 

 distant worlds the existence of the Newtonian law ; the 

 larger or smaller number of spots upon the sun, that is to 

 say, the openings formed through the luminous and opaque 

 atmosphere surrounding the solid nucleus ; and the regular 

 appearance, about the 13th of November, and the llth of 

 August, of shooting stars, which probably form part of a belt 

 of asteroids, intersecting the earth's orbit, and moving with 

 planetary velocity. 



Descending from the celestial regions to the earth, we 

 would fain inquire into the relations that exist between the 

 oscillations of the pendulum in air (the theory of which has 

 been perfected by Bessel), and the density of our planet ; and 

 how the pendulum, acting the part of a plummet, can, to a 

 certain extent, throw light upon the geological constitution 

 of strata at great depths ? By means of this instrument we 

 are enabled to trace the striking analogy which exists between 

 the formation of the granular rocks composing the lava cur- 

 rents ejected from active volcanoes, and those endogenous 

 masses of granite, porphyry, and serpentine, which, issuing 

 from the interior of the earth have broken, as eruptive rocks, 

 through the secondary strata, and modified them by contact, 

 either in rendering them harder by the introduction of silex, 



