96 LECTURE II. 



to the degree and direction in which the 

 force has been applied. If, therefore, I 

 push a bullet contained in a tube, in a hori- 

 zontal direction, I infer that it moves for 

 the same reason, or from the same kind of 

 cause. But if I put the tube in an oblique 

 or perpendicular direction, I find the bullet 

 move with different degrees of velocity and 

 torce from another cause, gravitation. If I 

 explode gunpowder in the lower part of a 

 tube held in a perpendicular direction, con- 

 taining also a bullet, I find the ball forced 

 upwards for a time by one cause, and then 

 it decends in consequence of another. My 

 senses may indeed inform me of the ap- 

 pearances of light and shade, in the sur- 

 faces of spherical and many-sided figures ; 

 but if, after having studied all that human 

 ingenuity and industry has discovered rela- 

 tive to these subjects, I am able, in the 

 absence of the object, to represent upon 

 paper a spherical or many-sided figure, do 

 I not manifest a knowledge of the causes 

 of light and shade beyond that which my 

 perceptions alone would have produced? 

 If also, I can at will present the angle of a 



