BOOK VIII 



INTRODUCTION. 

 The Secondary Mechanical Sciences. 



IN the sciences of Mechanics and Physical Astronomy, Motion and 

 Force are the direct and primary objects of our attention. But 

 there is another class of sciences in which we endeavor to reduce 

 phenomena, not evidently mechanical, to a known dependence upon 

 mechanical properties and laws. -In the cases to which I refer, the 

 facts do not present themselves to the senses as modifications of posi- 

 tion and motion, but as secondary qualities, which are found to be in 

 some way derived from those primary attributes. Also, in these cases 

 the phenomena are reduced to their mechanical laws and causes in a 

 secondary manner ; namely, by treating them as the operation of a 

 medium interposed between the object and the organ of sense. Tin-. . 

 then, we may call Secondary Mechanical Sciences. The sciences of 

 this kind which require our notice are those which treat of the sensi- 

 ble qualities, Sound, Light, and Heat; that is, Acoustics, Optics, and 

 Thermotics. 



It will be recollected that our object is not by any means to give a 

 full statement of all the additions which have been successively made 

 to our knowledge on the subjects under review, or a complete list of 

 'the persons by whom such additions have been made ; but to present 

 a view of the progress of each of those branches of knowledge as 

 a theoretical science ; to point out the Epochs of the discovery of 

 those general principles which reduce many facts to one theory ; and 

 to note all that is most characteristic and instructive in the circum- 

 stances and persons which bear upon such Epochs. A history of 

 any science, written with such objects, will not need to be long; but 

 :t will fail in its purpose altogether, if it do not distinctly exhibit some 

 well-marked and prominent features. 



