16 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



whicn have the greatest value, and of which the use is most obvious ; 

 or, again, with those which are more specially interestiDg from a 

 purely scientific point of view. Some of these applications have 

 come down to us from a high antiquity, others are of recent date, 

 but we shall endeavour to give the most recent developments. 



We shall find in many cases that the discovery of a physical law 

 has been the consequence of an entirely empirical inquiry, having for 

 its object the perfection of a certain branch of industry ; and, again, 

 in other cases that a discovery of great commercial importance has 

 been brought about by an experimental or mathematical demonstra- 

 tion of a truth of the most abstract order. These are considerations 

 on which we most strongly insist : because, in our opinion, they 

 have a real philosophical importance. They seem to us, in fact, 

 to be well qualified to warn our readers against two opposite ten- 

 dencies, both unfortunate. On the one hand, we find persons 

 conscious of their practical skill disdaining scientific theory ; while 

 on the other, some men of science who consider themselves to be 

 great philosophers look down upon knowledge acquired in the opera- 

 tions of industry, though the knowledge is often of a very real kind, 

 and far removed from the so-called " rule of thumb." 



