INTRODUCTION. 28 



Let me be permitted here to touch upon a few points 

 connected with discoveries, whose importance can only be 

 estimated 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 preliminary 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 

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

 variations of its temperature, extends already sufficiently far 

 back to show the recurrence of slight disturbances in the mean 

 temperature of any given place, and thus affords sufficient gua- 

 rantee against the exaggerated apprehension of a general and 

 progressive deterioration of the climates of Europe. Encke's 

 comet, which is one of the three interior comets, completes 

 its course in 1,200 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 

 unequal distribution determines the meteorological variations 

 of its atmosphere, depends alike upon the light-engendering 

 force of the sun, that is to say, upon the condition of its 

 gaseous coverings, and upon the relative position of the planet 

 and the central body. 



