INTRODUCTION. 43 



the simullaiieous occurrence of perturbations, and the frequen- 

 cy and duration of tnagnetic storms. 



Let me be permitted here to touch upon a few points con- 

 nected with discoveries, whose importance can only be esti- 

 mated by those who have devoted themselves to the study 

 of the physical sciences generally. Examples chosen from 

 among the phenomena to which special attention has been 

 directed in recent times, will throw additional light upon the 

 preceding considerations. Without a preHminary knowledge 

 of the orbits of comets, we should be unable duly to appre- 

 ciate the importance attached to the discovery of one of these 

 bodies, whose elliptical orbit is included in the narrow limits 

 of our solar system, and which has revealed the existence of 

 an ethereal fluid, tending to diminish its centrifugal force and 

 the period of its revolution. 



The superficial half-knowledge, so characteristic of the 

 present day, which leads to the introduction of vaguely com- 

 prehended scientific views into general conversation, also gives 

 rise, under various forms, to the expression of alarm at the 

 supposed danger of a collision between the celestial bodies, or 

 of disturbance in the climatic relations of our globe. These 

 phantoms of the imagination are so much the more injurious 

 as they derive their source from dogmatic pretensions to true 

 science. The history of the atmosphere, and of the annual 

 variations of its temperature, extends already sufficiently far 

 back to show the recurrence of shglit disturbances in the 

 mean temperature of any given place, and thus afibrds suffi- 

 cient guarantee against the exaggerated apprehension of a 

 general and progressive deterioration of the climates of Eu- 

 rope. Encke's comet, which is one of the three interior 

 comets, completes its course in 1200 days, but from the form 

 and position of its orbit it is as little dangerous to the earth 

 as Halley's great comet, whose revolution is not completed in 

 less than seventy-six years (and which appeared less brilliant 

 in 1835 than it had done in 1759) : the interior comet of 

 Biela intersects the earth's orbit, it is true, but it can only 

 approach our globe when its proximity to the sun coincides 

 with our winter solstice. 



The quantity of heat received by a planet, and whose un- 

 equal distribution determines the meteorological variations 

 of its atmosphere, depends alike upon the light-engendering 

 force of the sun ; that is to say, upon the conditio-i of its 

 gaseous coverings, and upon the relative position of the planet 

 and the central body. 



