4 LIFE ON THE EARTH. 



of force and time are obtained by observation of 

 the earth's constant force of attraction toward its 

 centre, and unchanging velocity of rotation on its 

 axis. By comparing phenomena with uniform mea- 

 sures of force, and uniform measures of time, we 

 obtain, or strive to obtain, for each natural effect, 

 a correct numerical valuation in terms of the force 

 employed and the time consumed in the production 

 of the effect. 



There are, or appear to be, different kinds of 

 force, producing different kinds of effects as magnet- 

 ism and gravitation ; but they are often comparable, 

 and capable of valuation under the one comprehensive 

 idea of relative magnitude. Thus all natural effects 

 known to us are measured or conceived to be 

 measurable, by units of force, operating through 

 units of space in units of time. And nature appears 

 to us to be the sum of these effects combined 

 into a ' System,' harmonious, mutually dependent, 

 and preserved entire amidst an endless succession 

 of limited vicissitudes. In expressing our concep- 

 tions of this well-adjusted 'System of Nature' we 

 employ the term 'Laws;' and the more compre- 

 hensive these are, the higher the abstractions 

 which they represent the less do we conceive them 

 to be variable ; so that the most general laws which 

 we reach or strive to reach, are conceived to be, 



