436 TftE YARD, PENDULUM, AND METRE. 



^ w ^ 



and rrtlis of the earth's mean density that is by a 

 simple calculation rmiJi, or rather less than one 1800th 

 part of the whole attraction of the earth a fraction far 

 too large, as well as far too uncertain in its amount 

 either at any given spot or in general, not to vitiate irre- 

 mediably any conclusion as to the ultimate result of the 

 operation. 



(16.) Similarly, if we look to the reductions to the 

 sea level necessary for stations in the interior of conti- 

 nents, we shall find that they depend, partly on the 

 diminution of gravity due to the Jieight alove the sea- 

 level, or to the increase of distance from the earth's 

 centre, which always tells in diminution of gravity ; and 

 partly on the protuberant matter, be it mountain or 

 elevated table-lard immediately beneath and around the 

 pendulum, which always tells in favour of ///^^r^i-^v/ gravi- 

 tation. The former portion is rigorously calculable, and 

 therefore need not trouble us, but the latter is in an ex- 

 treme degree uncertain in particular localities, and in a 

 general estimate falls very short of compensating for the 

 sea-deficiency. For the mean height of the F_^uropean 

 continent is only 1342 feet; of Asia 2274; of North 

 America 1496; and of South America, 2302. The 

 mean is 1840 feet, or rather more than a third of a 

 mile, which, on the same principle of reckoning, would 

 be equivalent to about T?.0(Toth part only of the total 

 gravity, which has to be reduced to one-third of its 

 amount, or to 1-45 000th, inasmuch as the proportion of 

 land to v/ater over the whole globe is only that of 51 to 

 146, or about i to 3. This is the mean effect of the 



