10 GRAVITY, AND ITS EFFECTS. 



we certainly discover, in the first instance, only 

 the strength of the earth's attraction at different 

 points of the surface. But it is clear that, from 

 the proportion which the decrease of this force 

 bears to the increase of the upright height, the 

 law must come out, according to which gravitation 

 lessens as the distance of the points which attract 

 each other becomes greater. And this law being 

 once known, the distance of any point on the sur- 

 face from the centre of the earth may be at once 

 found, by reckoning from the strength of gravi- 

 tation as observed at that point. This problem 

 being solved, due regard being had to the effect of 

 the centrifugal force, the amount of flattening is 

 found. The curvature, as discovered by this means, 

 agrees very closely with that found by direct mea- 

 surements of arcs of the meridian, a process, to treat 

 of which would be to enter the domain of astronomy. 



I must not leave the subject of the pendulum, 

 without reminding you that pendulums, differing 

 in bulk and in material, swing in equal times, if 

 their length is exactly the same ; that is, they are 

 equally subject to the force of gravitation. 



Thus, then, the same plumb-line, which in com- 

 mon life we employ to find a horizontal level, or the 

 true upright line for the walls of a house, and 

 which, swinging as the pendulum, rules the going- 

 rate of our clocks, becomes in the hands of the 

 physical philosopher a most precious instrument. 



