378 



them all to one given surface. For many years it was 

 the custom to reduce all observations to the level of the 

 sea, taking account only of the height of the station. But 

 in the Phil. Trans, for 1819, Dr. Young pointed out that 

 this correction was too great, as it entirely neglected the 

 attraction of the intervening strata. Supposing the obser 

 vation to have been made on table land of an altitude h 

 above the level of the sea, the attraction of this stratum 

 will clearly differ but little from that of an infinite plane of 

 the same thickness and density. Let &amp;lt;r be the density of 

 the land, then this attraction is well known to be 2 TT a- h. 



Let o be the mean density of the earth and a its radius, 



4. 

 then g, the force of gravity, will be - TT p a = g ; hence 



G 



the correction for the attraction of the intervening land 



Q T* 



will be - . - . - g. But the old correction for distance 

 is clearly -- g\ hence the total correction will be 



The quantity -fo r most rocks on the earth s surface is 



2*5 

 nearly - ; hence the old reduction must be multiplied by 



O O 



66. This corrected result gives the force of gravity at 

 the level of the sea, if all the land above this level were 

 cut off, and the sea constrained to keep its present level. 

 As the sea would tend in such a case to change its level, it 

 has been pointed out within the last three or four years 

 that a further correction is necessary, if we wished to 

 reduce the result the surface of that spheroid which most 

 nearly represents the earth. But the above is sufficient 

 for tabulating the results. 



V. Another important use of the pendulum is to deter 

 mine the law of the resistance of different fluids to bodies 



