74 GRAVITATION. 



der, and the flight of the ball, are the circumstances 

 intervening, which were unknown to the inquirer until 

 the explanation brought them to light. 



Now take another case. A child sees a bar of steel, 

 balanced upon a pivot, in a peculiar case with a glass 

 top. He finds upon moving the case that the bar turns 

 upon its pivot, so as to point in all cases in one direction. 

 He too asks an explanation, and is told, that a certain 

 kind of steel bars, when freely suspended, always arrange 

 themselves in a North and South direction. 



This is another kind of explanation. It will be per- 

 ceived that it is totally different from the other. It men- 

 tions no intervening circumstances between the cause 

 and the effect. It merely points out other phenomena of 

 the same kind; shows what is essential in the arrange- 

 ment which produces this effect, and which will insure 

 its repetition, and thus, in fine, describes in general 

 terms the class of phenomena to which this belongs. 



JNfow it will be evident, by a moment's reflection, that 

 such an explanation as this is nearly if not quite as vaj li- 

 able as the other. Without it the inquirer might be led 

 into a great many errors. He might suppose that the 

 glass, - the shape of the room, the form of the bar, 

 the skill of the operator, were concerned in the result. 

 The explanation has dispelled all these, pointed out 

 to him what is really essential, and given him all the 

 knowledge which can be of any practical benefit. There 

 are many phenomena which admit of no other kind of 

 explanation than this; for this simple reason that in re- 

 gard to them, we know of no links connecting the cause 

 and effect, and we must therefore only ascertain what 

 are the circumstances essential for the production of the 

 effect, and give to all the phenomena of the same class 

 one general name. This is what we design to do with 

 the subject before us. 



Sir Isaac Newton was the individual whose discoveries 

 brought to view and established the law of gravitation. 

 When first led to think of the subject by seeing an apple 

 fall to the ground, he did not attempt to find an explana- 

 tion of the kind like the first mentioned above ; i. c. to 

 discover something intervening between the earth and 



