342 Mr. Carl Barus on the 



flows must surround U U, between the barrel and the vapour- 

 bath, and between this and the end-screw V. It is essential 

 that vapour be directly in contact with the piezometer. For 

 fine temperature-work (§ 30, next section) and below 200°, I 

 use the drum //merely for the reception of vapour, leading it 

 in at one end of the bottom, and suitably withdrawing it at 

 the other, Low temperatures are obtained by a jacket of 

 circulating water. 



10. In figure 4, P Q indicates the method of filling the 

 barrel with oil. It is quite as convenient, however, to simply 

 pour it in, after removing one of the screws M, fig. 1. 



11. A case of two-inch plank closely surrounding the 

 machine is easily devised. A barricade should be placed 

 beyond the end V of the piezometer tube. 



12. A vertical piezometer similar to C C U U, but smaller 

 in dimensions, and ending below in a finely perforated steel 

 screw, may be attached at M (§ 23). 



Pressure Measurement. 



13. Pressures are measured in terms of the volume increase 

 of a long, cold-drawn steel tube, closed at one end, while the 

 other end communicates with the interior of the barrel B B, 

 fig. 4. It may be called a Tait gauge, since the elastics of 

 this case were specifically investigated by Prof. Tait*. This 

 gauge is inserted at N, and is shown at q q, tt, s s. To 

 measure the expansion of the steel tube q q, it is surrounded 

 by a close-fitting glass tube tttt, one end of which is joined 

 to q q by an internal layer of marine glue, v v, surmounted by 

 a tight cap of fusible metal. This joint is rigid as well as 

 reliably hermetic. The other end, of 1 1, communicates with 

 the capillary tube s s, by which the volume increase is 

 measured. In fig. 4 ss is shown vertically ; but it is ex- 

 pedient to bend it horizontally, parallel and close to the water- 

 jacket uu, to which ss is rigidly fastened directly over a 

 glass millimetre-scale. Pressure on the outside of tt appre- 

 ciably displaces the meniscus in s s, an error to be guarded 

 against. 



The shell-like space between steel tube q q and glass tube 

 1 1 is quite filled with a liquid of small coefficient of expansion. 

 Neither mercury nor alcohol were found satisfactory. I 

 therefore used pure water, coloured with a little fuchsine. 

 This is introduced through s s, by repeatedly evacuating 1 1 

 and boiling out all traces of air. 



I obtained the requisite constancy of temperature by sur- 



* Tait, ' Challenger Keports/ II. 1885, App. A, p. 26. 



