66 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



As the area of the disk is 324 sq. cm., the corresponding average pressure 

 observed is therefore here 



. = 3Xio- 7 cm. Hg. 



in the maximum case, or about one-third of this in the minimum case. These 

 differences are to be referred to the distance apart of the plates which was not 

 always measured in the earlier experiments. 



The maximum of gravitational attraction of the air within the crevice by 

 either disk is 727r? where 7 is the gravitational constant and ? the mass of 

 the disk per square centimeter or 78.5 X 1.4, so that 2^9 = 78.8 dynes per gram 



TABLE 6. Equilibrium of brass disks after long waiting. Constants as in table i. 



of air attracted. If the air-film is i mm. thick, the mass per square centi- 

 meter is thus 0.1X0.0013 = io~ 4 nearly, and hence the pressure increment can 

 not exceed = 78.8Xio~ 4 . The smallest observed pressure is thus 1/7 times 

 in excess of the largest computed values. The forces in the fissure must thus 

 be of a different kind from increased gravitational attraction if the excess of 

 air-pressure in the field is to account for the observed phenomenon. 



