116 DISPLACEMENT INTERFEROMETRY APPLIED TO 



dwindled in importance. The plenum effect, however, refuses to be so easily 

 dissipated. The circles in figure 148 are higher than in figure 147, owing to the 

 attraction of M. 



Figure 148 has the same characteristics as figure 147, except that the ball 

 M was left in front, thus tending, if warmer, to produce deflections toward 

 larger numbers. Consequently figure 148 should be an inversion of the other, 

 as it is . All these results were very definite and the figures might have been 

 drawn to a larger scale with advantage. 



91. Tendency of needle to stick to glass window. This very annoying 

 phenomenon, frequently mentioned in these experiments, now finds a rational 

 explanation. If the glass plates of the case are different in temperature from 

 the needle, there is a radiant force on one side of it (the side depending on the 

 exhaustion), which increases as the needle approaches the glass window, suf- 

 ficiently to hold the needle there against the torque of the quartz fiber. In the 

 damp laboratory room electrical forces are, I think, out of the question. 

 Radium close at hand does not change the phenomenon. I may cite a marked 

 example here. In order to locate a leak in apparatus II, the whole case was 

 submerged in a water-bath. After replacing the apparatus, well-dried, etc., at 

 about i p. m. , the east end of the needle first stuck tothe front window. If shaken 

 off by tapping the case, it vibrated, but finally returned to the same position. 

 Later, from 2 to 5 p. m. it similarly adhered to the rear. Still later it stuck to 

 the front again, after tapping. It was found so the next morning; i. e. after 

 more than twelve hours. Now, however, after tapping, it soon took its position 

 in the middle of the case and was free. The Ly was a normal value, but both 

 readings y were still high, showing that the window attraction was not quite 

 spent. In all this work the case was kept exhausted to a pressure less than 5 cm. 



It is such results as these that lead one to question, even granting the law of 

 cooling, which associates long times of cooling with small temperature dif- 

 ferences, whether something more than mere temperature is not in question. 

 It is probable, also, that the continuous drift of the zero-point, in case of the 

 daily readings of apparatus /, is attributable to a slight quasi-temperature 

 excess of the front plate (east) as compared with the rear plate. 



92. Needle excursions under increasing pressure. The excursions of the 

 needle of apparatus II in a plenum of air were similar to those in apparatus J, 

 but less variable, as has been already stated. Thus in figure 142 the mean 

 data for July 30 to August 2 are 



July 30. July 31. Aug. i. Aug. 2. 

 A. M., Ay = 4.oo 4.14 2.83 2.86 



P. M., 4.40 3.90 



which is a drop in Ay corresponding to the case of apparatus /, notwithstanding 



the east- west needle in apparatus II. The details in figure 142 are also similar. 



As this apparatus may be evacuated, it was thought well to observe the 



