THE 



PHILOSOPHICAL MAGAZINE 

 AND JOURNAL. 



30''' NOVEMBER 1825. 



LIII, On the Variation of Density and Pressure in the in- 

 terior Parts of the Earth. By J. Ivory, Esq. M.A. F.R.S. 



TPWO different methods have been employed for obtaining 

 ""■ a knowledge of the figure of the earth. One is to mea- 

 sure certain lines upon its surface ; the other is to investigate 

 the variations of the force of gravitation as we advance from 

 the equator to the pole. The first of these methods has been 

 pretty extensively carried into execution. We are in posses- 

 sion of measurements made in England, in France, in Italy, 

 in America, in India, at the Cape of Good Hope, at the equa- 

 tor, and in Lapland. The results obtained from so many 

 operations do not agree well with one another, and it is diffi- 

 cult to deduce from them a mean ellipticity in which much 

 confidence can be placed. Nor has the reason of this discre- 

 pancy escaped detection. The accuracy of the measurement 

 of terrestrial arcs depends chiefly upon the direction of gravity, 

 which is much more sensibly affected than its absolute force 

 by irregularities on the surface, or by the unequal distribution 

 of density in the interior parts. An inequality on the surface 

 of the earth, producing no sensible change in the total amount 

 of gi-avity, may nevertheless, by acting laterally upon the 

 plumb-line, cause a considerable variation in estimating the 

 curvature. 



We become acquainted with the relative force of gravity at 

 different points of the earth's surface by observing the length 

 of the pendulum that performs its vibrations in a given por- 

 tion of time. Of late years, this mode of experunenting has 

 been greatly improved, or rather it has been carried to the 

 utmost degree of perfection of which it seems capable. Ex- 

 periments with the pendulum are also less expensive, and less 

 laborious in the execution, than the measurement of terrestrial 

 arcs. They are well calculated for investigating the mean 

 figure of the earth ; since the time of a pendulum's vibration 

 depends upon the absolute force of gravity, and is little af- 

 fected by minute changes in its direction, which is the cause 

 of so much irregularity in the lengths of a degree of the me- 

 Vol. G6-. No. asi. jVor. 1825. Ss ridian. 



