44 COSMOS. 



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

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

 bit and the inclination of the ecliptic, that is, the angle M^hich 

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

 these periodical variations are so slow, and are restricted with- 

 in 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 4ay according to the benighted 

 views of the Middle Ages — ought to be considered as beyond 

 the range of our experience in physics and chemistry. 



Physical astronomy presents us with other phenomena, 

 which can not 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 center of gravity, and thus reveal in dis- 

 tant 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 1 1th of August, of shoot- 

 ing stars, which probably form part of a belt of asteroids, in- 

 tersecting the earth's orbit, and moving with planetary ve- 

 locity. 



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 ha.s 

 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 1 By means of this instrument we 

 are enabled to trace the striking analogy which exists be- 

 tween the formation of the granular rocks composing the 

 lava currents ejected from active volcanoes, and those endog- 

 enous misses of granite, porphyry, and serpentine, which, is- 

 suing from the interior of the earth, have broken, as erup- 

 tive rocks, through the secondary strata, and modified them 

 by contact, either in rendering them harder by the introduc- 

 tion of silex, or reducing them into dolomite, or, finally, by 

 inducing within them the formation of crystals of the most 

 varied composition. The elevation of sporadic islands, of , 



