488 LECTURE XLI. 



to enter completely into a mathematical examination 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 some- 

 times denominated plain astronomy, in contradistinction 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 de- 

 scriptive, and what is usually called physical, must be denominated mathe- 

 matical astronomy. We shall, therefore, confine ourselves in great measure 

 to descriptive, astronomy, and shall take only a general view of the laws of 

 gravjitation, as an illustration 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 terraqueous globe, and to the tides, produced by the influence of the 

 celestial bodies on the ocean: and then, quitting the aft'ections of the larger 

 features of the matter, that constitutes the earth, we come naturally to the 

 properties and powers of its individual particles, and to the phenomena 

 of heat, electricity and magnetism, which are either qualities of matter, or 

 dependent on substances differing in some respects from -common matter; 

 and in the next place, to the combination of all these substances and actions 

 in meteorology, and in the phenomena of vegetable and animal life, a general 

 view of which will complete our discussions on the subject of physics. The 

 science of chemistry, or the doctrine of the qualities of particular kinds of 

 matter, might be said to belong to the investigation of the properties of 

 matteu in general; but this science is of too great extent and importance to 

 occupy a subordinate place in a system of natural philosophy, and must, 

 therefore, be considered as requiring a separate course of study. 



In our astronomical inquiries, we shall first examine the phenomena of the 

 heavens and earth in their simplest form, not as they immediately appear to 

 our observation, but as they are shown by unexceptionable proofs to be na- 

 turally arranged. The stars and sun, the planets and their satellites, and 

 lastly the comets, will be severally described; the causes of the motions of 



