Isometrics of Liquid Matter. 2>il 



It follows, in general, that to obtain trustworthy data 

 pressure observations must be recorded during the pressure- 

 increasing phase of work only. In most cases (see next 

 section) this condition is easily fulfilled, since the pressures 

 sought are reached from zero. 



20. From the dimensions a , a l3 L, p, (Table I.), the in- 

 crease of the external volume of the steel gauge-tube is 47/1 6 

 per unit of external volume, per centimetre of displacement of 

 the thread in the capillary tube ss (fig. 4). This corresponds 

 to a volume increment 



v/Y = -0000008 (1) 



per unit of external volume, per atmosphere. 



Tait's formula (L c.) modified to suit the present case is 

 v'/V = Tla 2 (llk-k-lln)/(a l 2 — a 2 ), where II is the internal 

 pressure, and 1/k and n the compressibility and rigidity, re- 

 spectively, of the steel employed. Taking k and n from 

 Everett's* tables, viz., k=VSix 10 12 and n = S'2x 10 11 , and 

 putting II = 10 6 dynes, 



v'/Y = -0000007 (2) 



This result is of the same order as (1), § 19. 



Similar results could be obtained for Tables II. and III. 

 I think this method can be made sharp enough for the ex- 

 perimental measurement of 1/k-i 1/n. 



21. In addition to the work done with Tait gauges, I spent 

 some time in constructing a direct-reading helical Bourdon. 



© ©7 



The results are not of interest here. 



22. Instead of having a screw enter the barrel, similar re- 

 sults must be obtainable by forcing a cylindrical shaft into 

 the barrel. The device shown within C C, fig. 1, proves that 

 the gasket of marine glue is quite as serviceable for smooth 

 cylinders as for screws. 



The labour necessary in producing the above pressures 

 decreases nearly as the fourth power of the diameter of the 

 screw, co3t. par. Similar advantages are gained by increasing 

 the number of threads to the inch. Hence, supposing the 

 initial pressures produced by a thick screw (diameter 1 inch 

 say) at one end of the barrel, and the final pressures (above 

 2000 atmospheres, when the enclosed liquids have become 

 much less compressible) produced by a thinner screw (diame- 

 ter \ inch say) at the other end of the barrel, the efficiency 

 of the screw-compressor would be increased. In such a case 

 the piezometer tube must be vertical, § 12. My purpose, 



* J. D. Everett, ' Units and Pnys. Constants ' (Macmillan, 1873), 

 p. 53. 



