PHYSICS. 375 



corresponding change of this incHnation would be less than3xl0~^ 

 seconds of arc. The surroundings of an ordinary laboratory are not of 

 a kind to warrant results to this degree of precision; but continuous 

 observations were nevertheless carried out with two instruments 

 (one attached to a pier, the other to the concreted sub-basement) 

 for the purpose of studying their behavior. In one casethe pendulum 

 was all but supported on a cylindrical float. If this is placed sym- 

 metrically to the axis, it does not change the sensitiveness, but it 

 relieves the pivots of friction, on the one hand, and serves as an ade- 

 quate damper on the other. A variety of interesting results were 

 obtained with this apparatus. To increase the sensitiveness the float 

 would have to be attached at the center of gravity. In many of the 

 measurements the ruled-glass grating was replaced by a film grating. 

 In this case the initial adjustment is difficult; but once obtained, the 

 subsequent work proceeds as well as with the ruled grating. 



In view of the small rotations measurable in this way, the apparatus 

 is available for measuring correspondingly small variations of torque, 

 while on the other hand it has sufficient strength to support relatively 

 large loads. It was therefore intended to measure the gravitational 

 attraction of two disks, since, if they are all but in contact, the attrac- 

 tion, for a constant mass, may be estunated to increase roughly as the 

 fourth power of their radii. On trial, however, the disks were found 

 actually to repel each other, with forces much in excess of their gravi- 

 tational attraction. Plates of brass, 20 centimeters in diameter and 

 about 1 millimeter apart, showed repulsions of the order of 0.5 dyne, 

 which is equivalent to a pressure excess of about 10~^ atmosphere. 

 It is much larger than such electrical repulsion as might be ascribed to 

 the absolute voltaic potential of the disks. It persists after the accu- 

 mulation of air (due to the viscosity of the film of air between the 

 disks) should have been dissipated. The forces in question, moreover, 

 may be determined by charging the plates to definite differences of 

 potential. So far as can be seen at present, the repulsion is due to the 

 condensation of air at the surface of the plates, by molecular attraction, 

 so that the investigation of these forces, for different distances, should 

 be of considerable interest. 



In a different class of experiments, and with the object of testing the 

 limits of the equations assumed, the refraction of glass columns 25 

 centimeters or more in length was investigated for different colors of 

 the spectrum. It was at first expected that when such long columns 

 are inserted into one of the interfering beams the change of the index 

 of refraction with wave-length would be measurable with exceptional 

 precision; but this anticipation was not realized in full, as the amount of 

 shifting of the ellipses per unit of length of the compensating microme- 

 ter-screw decreases with the length of the column. The equations 

 were found to reproduce the observations accurately. It was necessary 



