State of the Useful Arts. 263 



sound which it occasions, while the microscope reveals it in all 

 its realities, though minute in the breadth of the waves. 



Now simple, mechanical, as the management of the slide-rest 

 may aj)pear, it requires no small progress in a knowledge of the 

 laws of mechanics, and no little ability in the application of these 

 laws, to guard against these minute, yet complicated and im- 

 portant sources of error ; and therefore, so far from considering 

 the slide-rest an instrument likely to lull asleep the mental 

 powers of the workman, I regard it as a powerful incentive to 

 the exercise of mind. And of this indeed I am assured, that 

 he who has mastered the difficulties of slide-rest turning, and has 

 learned to shape his tools for the work they have to perform, 

 will thence pass easily and rapidly to the art of manipulating 

 hand-tools. Accustomed to study the principles of cutting, it 

 will be to him a simple matter to put these principles in prac- 

 tice. 



AH our knowledge of physical phenomena is derived from ex- 

 perience. Mere experience indeed will collect but a small quan- 

 tity of information ; the intellect must seek out the results of 

 experience, must compare, arrange, and classify them, before 

 any thing worth calling knowledge is attained to ; yet still expe- 

 rience lays the foundation of science, and affords the materials 

 on which the mind operates. That such is the case with all 

 our knowledge of material phenomena is, I think, evident on an 

 inspection of any one of or all the branches of physics. The al- 

 gebraist knows of the existence and properties of prime numbers 

 from experience alone ; he cannot shew why they are or ought 

 to be. As little can the geometer tell why the three angles of 

 a trigon make together half a revolution ; or the mechanician 

 deduce o priori the law of equilibrium, or of accumulated mo- 

 tion. So much, indeed, are our ideas tied to the course of our 

 experience, that even those poets who have ventured into the 

 unknown land of spirits ; have been utterly unable to shake off 

 the impressions of materiality — 



Gli occhi in giuvolse, 

 says Tasso, as if the Author of all must have the eyes of a man. 



If, then, knowledge be so essentially experimental ; and if, for 



VOL. XXIII. NO. XLVI. — OCTOBEB 1837. T 



