122 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



Ay = 2. 7. Water which had stood in the same room for days, alongside of 

 the apparatus, was now poured in and observations of y made in half- 

 hour periods throughout the afternoon, August 18. The readings as given in 

 figure 156 b indicate a drifting needle, modified according as the drift to 

 larger numbers was with the gravitational pull (5) or against it (N). This 

 force is wholly secondary and almost masked. Its mean effectiveness is 

 but Ay = 0.73 cm. 



The same experiment (half-hour periods) was continued throughout the 

 whole of the ensuing day, August 19, the greatest attention being paid to 

 carefully sitrring the water. The results are given in figure is6cand show 

 only slight improvement. There is still an excess of drift, but alternations of 

 gravitational pull are now usually apparent. The mean excursion is but 

 Ay = 0.65 in the morning and Ay =0.42 in the afternoon. The change of 



temperature of the water-bath could not have exceeded a few tenths degree; 

 but to give greater definiteness to this statement, the experiments were con- 

 tinued in the same way during a third day, August 20, and parallel observa- 

 tions made on the temperature of the water-bath by a tenth-degree thermom- 

 eter. The results also given in figure 156 d are quite as erratic as the pre- 

 ceding. In the morning the gravitational and radiant (repulsive) forces are all 

 but equal. In the afternoon, with a different adjustment, the effect of grav- 

 itation gradually emerges, frequently only as an acceleration or an impedi- 

 ment on the drift. In fact, the needle is in continual motion and the results 

 would have been more rational if the periods instead of being 30 minutes, 

 had been shorter. As it is, the mean (p. m.) gravitational excess is but Ay= 

 0.42 cm. The gradual increase of the temperature of the water-bath scarcely 

 amounted to one-fifth of a degree for the day. Now the actual temperature 



