298 Dr Searle, Some methods of measuring 
so 
. 
the atmosphere over the same difference of level*. Hence p, the 
pressure excess within the bubble, is given by 
= 92 (pi — pa) Gy Me\C.97). 2 eee (11), 
where On = pol ty ee potty 
Dol Pole 
When the radius, r cm., of the bubble is known, the surface 
tension is calculated by 
IN igo 
§ 13. Practical details. The bends at B and C are formed as 
in Fig. 10. The end of the long tube is soldered into a block of 
ee 
IZ 272777 
Fig. 10. 
brass. The short side tube screws into the block, the joint being — 
made tight by a flange and a leather washer. The short tube 
must be stout to withstand the strain involved in making the 
joint tight. The side tubes are screwed into the blocks after the 
long tube has been placed within the steam jacket. Steam is 
supplied by a small boiler; the waste steam from the jacket should 
be led away clear of the apparatus. The inner tube is about 08 
cm. in diameter and the tube forming the steam jacket about 
2°5cm. in diameter. 
The bubble is formed on the ring and the observations for its 
radius are made just as in §§7,8. The temperature of the sur- 
rounding air is observed by a thermometer. 
If, as suggested by Mr J. D. Fry, the tube ABCD be turned 
about CD as a horizontal axis, the difference of level, z, between 
D and A can be changed and with it the pressure excess in the 
bubble. The difference of level can be calculated in terms of the 
distance AD if the tube be mounted so that the inclination of the 
plane ACD to the vertical can be measured. 
* It will be easily seen that no care need be taken to ensure that those parts of 
the tube DE which are at atmospheric temperature shall be in the same horizontal 
plane as CD. 
