REVERSED AND NON-REVERSED SPECTRA. 87 



even though the tube was not jacketed with water. The data, as was to be 

 expected, show an expansion less than was computed for the case of traction, 

 being only about half as large, or equivalent to about a quarter of an inter- 

 ference ring per atmosphere. An arc lamp was used as a source of light and 

 its nickering was very annoying. 



Tait* in the Challenger reports justifies the expression 



2 



dx ai 2 a 



where d%/dx is the elongation due to the internal pressure II, in case of a 

 tube of sectional diameters a and a\ and bulk modulus k. This merely re- 

 places Young's modulus by the linear equivalent of the bulk modulus. Hence 

 for the tube of length x=i6o.S cm. and 00 = 0.515 cm., 01=1.278 cm., if for 

 tool steel k = i.84X io 6 and pressure is given in atmospheres (io 6 dynes/cm. 2 ) 

 the elongation per atmosphere is 



160.8X0.265 



~ cm. 



1.368X5. 52Xio 6 



This is smaller than the value found, probably because the tube is made of 

 mild steel. If k be computed for the tube, since the elongation per atmosphere 

 is 7-5Xio" 6 



160.8X0.265 



-=i3.9Xio 6 



which is about the order of value given by Everett t for wrought iron. Voigt 

 gives i.46Xio 6 for steel (see Landolt and Boernstein's Tables, 1905, p. 45). 



44. Further experiments. The washer of the screw of the compressor, in 

 the default of marine glue, was a perforated disk of pitch. This proved to 

 be quite inadmissible for further work. The behavior of pitch was very 

 peculiar and in itself interesting. The perforated disk was found to adhere 

 without slip to the screw at the inner edge r, figure 57, and 

 to the wall of the stuffing-box at its outer edge R. On 

 turning the screw a smaller coaxial disk r' of pitch broke 

 out of the larger disk, and the turning proceeded on this 

 surface, r', without leakage. It was found nearly impos- 

 sible to force the screw with the adhering pitch into the 

 socket without serious injury to the screw. The sharp 

 edges of the threads were in fact planed off flat and it was only by leaving 

 fore-and-aft room for play in the stuffing-box that the compressor could be 

 used at all. The part of the screw turning in pitch was ruined. 



* Tait: Challenger Reports, n, 1882, App. A, p. 26. 



t See Everett tables, 1879, p. 53, containing original experiments of the author. 



