PART III 



LECTURE XLI. 



ON THE FIXED STARS. 



THE departments of natural philosophy, which are to be the subjects of 

 the third and last division of these lectures, are included in the description 

 implied by the term physics, or the history of the particular phenomena of 

 nature ; and the account which will be given of these phenomena, will be 

 accompanied by as much of mechanical theory and analogical reasoning, 

 as can be applied to them with sufficient certainty, and without too great 

 intricacy of calculation. 



The science of astronomy might, without any great impropriety, have 

 been considered as a part of mechanics ; but there are circumstances 

 intimately connected with it, for the complete investigation of which, a 

 knowledge of the motions of fluids in general, and also of optics, is abso- 

 lutely necessary. It could not, therefore, hold any other place in a strict 

 order of arrangement, than that which is here allotted to it ; and, since it 

 will not be in our power to enter completely into a mathematical examina- 

 tion of all the motions of the heavenly bodies, although we shall be able to 

 pursue the detail of the most remarkable appearances which they exhibit, 

 we may for this reason more properly consider such a view of astronomy 

 as belonging to descriptive than to theoretical philosophy. This method of 

 treating the subject is sometimes denominated plain astronomy, in contra- 

 distinction to the mechanical theory of the science, which is called physical 

 astronomy ; but it is obvious that in the sense which we are at present 

 annexing to the word physics, that which is commonly called plain 

 astronomy must be termed physical or descriptive, and what is usually 

 called physical, must be denominated mathematical astronomy. We shall, 

 therefore, confine ourselves in great measure to descriptive astronomy, 

 and shall take only a general view of the laws of gravitation, as an illus- 

 tration of the phenomena previously described. After having considered 

 the magnificent objects of astronomy, which are scattered throughout the 

 universe, we descend to geography, or the particular history of the terra- 

 queous globe, and to the tides, produced by the influence of the celestial 

 bodies on the ocean ; and then, quitting the affections of the larger features 



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