296 REID— CONSTITUTION OF THE [April 24, 



the attraction of the moon. If the earth yielded Hke a fluid, its 

 surface would always remain at right angles to the vertical, and a 

 pendulum would remain relatively stationary for all positions of the 

 moon ; if the earth were absolutely rigid, the moon's attraction would 

 deflect the pendulum an extremely small amount, but an amount 

 capable of being measured. The Darwins did not obtain definite 

 results because the disturbances of their pendulum were greater 

 than the deflections they attempted to determine. 



A little later von Rebeur-Paschwitz attacked the same problem 

 with better success, using a horizontal pendulum. 



Hecker, in Potsdam, and OrlofT, in Dorpat, have repeated von 

 Rebeur-Paschwitz's experiment; and both found values for the 

 average rigidity of the earth comparable with that of steel. But, 

 what was most remarkable and what is still unexplained, the rigidity 

 was apparently greater in an east-west than in a north-south direc- 

 tion. Orloff, experimenting at a greater distance from the ocean, 

 found a smaller difference than Hecker did, and it has been sug- 

 gested that the tides of the ocean are the cause of the difference. 

 The International Seismological Association, at its Manchester 

 meeting in 191 1, made plans to repeat the experiments in Paris, in 

 central Canada, in the middle of Southern Africa and in the middle 

 of Russia ; but no reports have yet come from these stations. 



In the autumn of 1913, Michelson attacked the same problem by 

 a new method, which seems capable of yielding more accurate 

 results than the horizontal pendulum. He measured the deflection 

 of the vertical under the influence of the moon by what was prac- 

 tically a water level 500 feet long, sunk six feet in the earth. ^^ 

 Michelson's results for the E-W rigidity do not differ greatly from 

 those of Orloff ; but his N-S rigidity is somewhat less than Orloff's. 

 Michelson's experiments also show that the viscosity of the earth 

 must be as great as that of steel. These experiments are of great 

 interest; they should be repeated at various places, and especially 

 at places symmetrically situated with respect to the great oceans, 

 and on mid-oceanic islands, in order to determine how far they are 

 affected by the oceanic tides. 



10 " Preliminary Results of Measurements of the Rigidity of the Earth," 

 The Astrophysical Journal, 1914, Vol. XXXIX., p. 97. 



