92 



THE INTERFEROMETRY OF 



suppose that the cylindrical tube as a whole is gradually progressing toward 

 an ultimate spherical form, but this seems far-fetched. It is more reasonable 

 to suppose that the viscosity is simply flexural. The tube is curved slightly 

 convex upward and therefore the end mirrors N and H, figure 54, rotate con- 

 tinually towards each other on a horizontal axis, under the end-thrusts of the 

 internal pressure. The component beams and HNH and HMH are thereby 

 each modified in length. Though it is difficult to specify why the former 

 should be shortened relative to the latter, such a result is easily conceived. 



46. Thin steel tube. The steel tube in 43 and 44 was adapted for high 

 pressures only, showing but 0.2 interference ring per atmosphere. In con- 

 trast with this a thin steel tube was now inserted, adapted for lower pressures. 

 This was more sensitive than the Bourdon gage, the other tube being on the 

 whole less so. The dimensions were: length, L=i6i cm.; diameter inside, 

 00 = 0.799 cm.; diameter outside, 01 = 0.951 cm. The outside diameter was 

 calipered. A short length was then cut off and slit open and the wall thick- 

 ness similarly found (0.076 cm.). The tube was not quite straight. Snugly 

 fitting brass plugs were carefully soldered into the ends, and these were then 

 tapped to receive the tubes conveying pressure. 



The observations shown in table 21 were recorded, the steps of pressure 

 being 50 atmospheres in the first two and 100 atmospheres in the last two 



series. 



TABLE 21. 



At 400 atmospheres the tube developed a slight leak at the ends. At 750 

 atmospheres one of the soldered end-plugs was blown out. It is remarkable 

 that the plugs held so well. 



An example of the individual data may be given for the second series. In 

 figure 6 1 these data are shown, positively. 



Pressure 50 100 150 200 250 300 300 250 200 150 100 50 atm. 



Micrometer reading 260 210 164 115 67 23 24 67 120 163 212 266 cm./io 4 



Very little viscosity is, therefore, in evidence, but there is some displace- 

 ment or irregularity, probably in the reading of the Bourdon gage. 



Pressure increments and decrements slightly rotated the mirrors in opposite 

 directions around both a vertical and a horizontal axis. These were compen- 

 sated by adjustment at the mirror M before each reading. As the mirrors 

 inclined towards each other for pressure increments the tube must have been 

 slightly convex upward, and therefore successively straightened as pressures 



