GRAVITATION. 



269 



apple, which may be true or not, but which at all events bears 

 on the present subject. No matter on what portion of the 

 spherical earth a tree may be, every fruit becoming disengaged 

 from it is attracted to the earth, the line which it takes, unless 

 disturbed by external forces (such as wind, &c), being that 

 which passes from the zenith to the centre of the earth. 



This imaginary line is a perfect perpendicular, and the 

 visible line which is formed by the delicate wire of the tunnel- 

 boring engineering instrument, or the comparatively coarse 

 string of the plumb-rule and level, are approximations suffi- 

 ciently close for practical purposes. So it is in a mathematical 



FALLING FRUIT. 



PLUMB -RULE. 



proposition. As mathematical lines have no breadth, they are 

 simply indicated or represented by the lines of the figure, the 

 bodily eye. being incapable of seeing what is perfectly visible 

 to the mental eye, namely, length without width. So the 

 wire and string perform in practical work exactly the same 

 office which is fulfilled by the lines of a mathematical proposi- 

 tion drawn on paper. 



We have already, when treating of the Fall-trap, seen how 

 this principle is brought into operation by those who are 

 utterly incapable of discerning the physical principle, though 

 they can apply it materially with wonderful effect. 



It is, perhaps, needless to mention the value of the Measure 

 to any handicraftsman. 



I well remember that when, some twenty-four years ago, I 

 was taking lessons from a carpenter in the art of making 

 ladders, gates, fences, hurdles, and other rough-and-ready work, 

 my quaint old tutor related an anecdote of and against himself. 



