110 REPORT— 1882. 



We thus see that the ground is 9 cm, higher under the barometric 

 depression than under the elevation. 



If the sea had time to attain its equilibrium slope, it would stand 

 5 X Id 6, or 68 cm. lower under the high pressure than under the low. 

 But as the land is itself depressed 9 cm., the sea would apparently only 

 be depressed 59 cm. under the high barometer. 



It is probable that, in reality, the larger barometric inequalities do not 

 linger quite long enough over particular areas to permit the sea to attain 

 everywhere its due slope, and therefore the full difference of water-level 

 can only be attained occasionally. 



On the other hand, the elastic compression of the ground must take 

 place without any sensible delay. Thus it seems probable that the elastic 

 compression of the ground must exercise a very sensible effect in modifying 

 the apparent depression or elevation of the sea under high and low baro- 

 meter. 



It does not appear absolutely chimerical that, at some future time 

 when both tidal and barometric observations have attained to great 

 accuracy, an estimate might thus be made of the average modulus of 

 rigidity of the upper 500 miles of the earth's mass. 



Even in the present condition of barometric and tidal information, it 

 might be interesting to make a comparison between the computed height 

 of tide and the observed height, in connection 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 com- 

 plete 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 compression of the earth, 

 amounts to 0""0117, and this is augmented 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 0"'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 

 0"-011. 



A deflection of magnitude such as 0""03 or 0"*01 would have been 

 easily observable with our instrument of last year, for we concluded 

 that a change of o^ro^h 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 0"*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 disturbance of gravity 

 must be impossible with any instrument on the earth's surface. 



Moreover the removal of the instrument to the bottom of the deepest 



