126 DISPLACEMENT INTERFEROMETRY APPLIED TO 



fore thought interesting to pursue the records of these three instruments for a 

 reasonable time, to determine their differences, and in particular to compare I 

 and II at atmospheric pressure with the contemporaneous behavior of III 

 under exhaustion. 



Figure 159 shows the mean results of experiments of this kind, made in 

 half-hour periods at intervals of over 2 hours apart. Apparatus I and II were 

 at full atmospheric pressure and in the location stated. The day (August 31) 

 was overcast, with rain. Apparatus /// was kept exhausted to different 

 degrees at the pressures marked on the curve. Although the three instruments 

 differ in sensitiveness, they at first tell the same story, even as to No. II in 

 its dark niche. Moreover, the high or moderate exhaustions in No. 777 have 

 seemingly produced no marked difference, such as appears for instance in 

 figures 149 and 157. One would thus be obliged to conclude, in the latter cases, 

 that the air absorbed by the wood, in its issue or reentrance, was responsible 

 for the effect in by obtained. 



During the course of the experiments (fig. 159) the apparatus developed a 

 slight leak. Though this did not apparently influence the results, as the figure 

 shows, it was thought well to remove it. In the long search which was necessary 

 to find it, the fiber of the needle was unfortunately broken and in the trouble- 

 some repairs one-half of it had to be sacrificed. This reduces the sensitiveness 

 of 777, from its high value above, to an intermediate grade between apparatus 

 7 and 77, the latter being least sensitive. Though 777 was at first quite 

 tight, it again after usage developed a leak of about 3.5 cm. per hour at 

 complete exhaustion. As this on trial made no difference with the behavior of 

 the metallic case 777, it was disregarded. In the work with the wooden case, 

 however, even the smaller leak of 2 cm. per hour at the highest exhaustion was 

 held accountable for the startling effect of figures 149 and 157. Such an infer- 

 ence is therefore no longer tenable. One is left to surmise that the non- 

 conducting wood retains the heat almost indefinitely, while the thin metal 

 frame (half -inch tube of square section) loses it relatively soon. 



A comparison of the records of apparatus 7, 77, and 777 in their several 

 locations will now be given (fig. 160), in order to discriminate between a possible 

 absolute temperature effect, if it exists, and the relative effect which would 

 naturally follow from changes of atmospheric temperature. 



The comparisons in figures 160 and 161 begin with the cloudy days on Sep- 

 tember 2 (observations taken at the same time are in the same vertical), when 

 the three instruments still show some similarity of behavior, though it is very 

 meager. With the appearance of sunshine in the atmosphere without, the 

 three instruments part company. Initially No. 777 was observed at different 

 pressure (numbers, cm. of mercury on the curve) . At first the excursions under 

 p = j6 and = 3 cm. were about the same, so that the final high values at 

 p = ?6 cm. or at lower pressures must obviously be referred to other causes 

 in addition to pressure. For this reason I temporarily abandoned the pressure 

 work and made some of the subsequent observations for No. 777 in a plenum. 

 They soon became extremely erratic. On September 4 the needle adhered to 



