ACOUSTICS AND GRAVITATION. 



n- 



IO~ 4 X2.23 



= 32 fringes per turn of screw hs 



6Xio- a Xo.ii5S' 



In the experiments fringes of one-half scale-part were installed. In sep- 

 arate experiments, immediately after closing the cock C, a half turn of the 

 screw produced a displacement of 8.3, 8.0, 8.0, 8.5, 8.5 scale-parts; as the 

 average, therefore, 16.4 scale-parts per turn or about 33 fringes per turn. This 

 agrees as closely as may be expected with the number computed. 



The pressure increment per turn of screw is dp = n\/2 cm. of mercury or 

 per turn of screw about io~ 3 cm. ; per fringe, therefore, 3 X io~ 6 cm. of mercury 

 as anticipated. A range of about 2 or 3 turns of screw was possible with each 

 fringe, i.e., the range of pressure measurement should be from 3Xio~ 5 to 

 3 X io~ 3 cm. of mercury. 



Experiments of the same kind were made in great variety. There is no 

 difficulty is using much larger fringes, so that 3 X icr 6 cm. of mercury should be 

 appreciable. By exhausting both sides of the U-tube the apparatus becomes 

 a vacuum gage. I did not, however, attempt such work, as the present appara- 

 tus was not well adapted for the purpose. 



\ . \ 



v "A. 



a _=_ 



.^"^ C^/T~^ t 



I 



4. Air-thermometer. If the cock C is permanently closed, the air-space A 

 becomes the bulb of an air-thermometer of approximately constant volume. 

 In this way the heat produced by the rays of light L' may be measured. In a 

 variety of experiments of the kind, the mean result was about 10 scale-parts or 

 20 fringes in a lapse of 210 seconds. If r denotes absolute temperature, the 

 intrinsic equation may now be written 



which reduces as above to 



dp , = 

 ' V~~ = T 



dh dh^dr 

 76 ~'' 



Thus, if T = 300, 



dr =rn ( 7 H 7+ I = 



2 \76 zHJ 



and for n=20 in 210 seconds, 



= 0.042 C. 



