due to Elasticity of the Earth's Surface. 415 



ination, it might be interesting to make a comparison between 

 the computed height of tide and the observed height, in con- 

 nexion with the distribution of barometric pressure. It is 

 probable that India would be the best field for such an attempt, 

 because the knowledge of Indian tides is more complete than 

 that for any other part of the world. On the other hand, we 

 shall see in the following section that tidal observations on 

 coast-lines of continents are liable to disturbance, so that an 

 oceanic island would be a more favourable site. 



It has already been shown that the maximum apparent 

 deflection of the plumb-line, consequent on the elastic com- 

 pression of the earth, amounts to //# 0117 ; and this is aug- 

 mented to 0' / '0146, when we include the true deflection due 

 to the attraction of the air. It is worthy Of remark that this 

 result is independent of the wave-length of the barometric 

 inequality, and thus we get rid of one of the conjectural data. 

 Thus, if we consider the two cases of high pressure to right 

 and low to left, and of low pressure to right and high to left, 

 we see that there will be a difference in the position of the 

 plumb-line relatively to the earth's surface of //, 0292. Even 

 if the rigidity of the upper strata of the earth were as great 

 as that of steel, there would still be a change of CK'-Oll. 



A deflection of magnitude such as (K /- 03 or //- 01 would 

 have been easily observable with our instrument of last year ; 

 for we concluded that a change of ^q of a second could be 

 detected when the change occurred rapidly. 



It was stated in our previous Report that at Cambridge the 

 calculated amplitude of oscillation of the plumb-line due 

 directly to lunar disturbance of gravity amounts to //- 0216. 

 Now, as this is less than the amplitude due jointly to elastic 

 compression and attraction, with the assumed rigidity (300 

 millions) of the earth's strata, and only twice the result if the 

 rigidity be as great as that of steel, it follows almost certainly 

 that from this cause alone the measurement of the lunar dis- 

 turbance of gravity mnst be impossible with any instrument 

 on the earth's surface. 



Moreover the removal of the instrument to the bottom of 

 the deepest known mine would scarcely sensibly affect the 

 result, because the flexure of the strata at a depth so small, 

 compared with the wave-length of barometric inequalities, is 

 scarcely different from the flexure of the surface. 



The diurnal and periodic oscillations of the vertical observed 

 by us were many times as great as those which have just been 

 computed; and therefore it must not be supposed that more 

 than a fraction, say perhaps a tenth, of those oscillations was 

 due to elastic compression of the earth. 



