450 K D. Preston — Study of the Earttis 



Caroline Island. At all of these gravity is more than theory 

 demands, and they are generally islands situated in a deep sea 

 and considerably removed from continental masses. This ex- 

 cess has in many cases been shown to be more than can be due 

 to the extra attraction of the water, so that there must be a 

 real increase in density under the ocean bed. We may there- 

 fore look to pendulum observations for information in regard 

 to the internal structure of the earth. Deviations from the 

 law T of uniform density may be greater than has hitherto been 

 supposed. Two well known authorities on the subject have 

 already expressed the belief that the center of gravity of the 

 earth does not lie at the center of figure, but is to be found 

 inside the hemisphere that is under the Pacific Ocean. 

 We have an example of this excessive seaward attraction in 

 India. When a chain of triangles was thrown across the pen- 

 insula of Hindoostan from the Bay of Bengal to the Arabian 

 Sea, it was expected that the astronomical amplitude of the 

 arc would exceed the geodetic amplitude : in other words that 

 that the plumb-line would be deflected towards the high table 

 lands over which the arc was measured. The contrary was 

 found to be the case. The seaward attraction was more than 

 that from the continent, and an identical result followed from 

 two independent arcs. We may therefore accept the fact as 

 proven, that the attraction of continental masses is in some 

 way partially compensated by a deficiency of density in the 

 immediately underlying strata. It may be asked what rela- 

 tion this defect of gravity on high table lands, as revealed by 

 the pendulum, bears to the horizontal attraction as brought out 

 by a comparison of astronomical and geodetic latitudes. The 

 Himalayan attraction on the plumb line at Dehra, a point less 

 than 50 miles distant and with an elevation of 2000 feet, is 

 l/5000th part of the total force of gravity at the earth's surface, 

 whereas the defect of gravity at or near the summit is l/200oth 

 part of the total force. This somewhat strange result may be 

 explained in two ways. First the deflection at Dehra may be 

 produced by matter lying between this place and the summit ; 

 or the great plateau of Thibet with an average elevation of 

 15,000 feet may exert the attractive influence on Dehra, and 

 the strata immediately under the pendulum station at the sum- 

 mit may possess a very small density. This last view would 

 seem to be supported by the fact that there is a deflection in 

 azimuth as well as in latitude at Dehra, 



The condensation theory assumes that all pyramids of matter 

 having their vertices at the center of the earth and having 

 equal bases, contain equal amounts of matter, and that the ver- 

 tical attraction at any point on an elevated plateau is equal to 

 that obtaining at a point on the sea-level immediately under it,. 



