296 Dr Searle, Some methods of measuring 
=3°307 x 10°cm.=3 
Hence 4 9.817 x 105cm.~3 and i 
Ap a3 
Thus, by (9), § 7, 
es 4 | 
Bele (c+ )=1946 x 10-4 x 6-124 x 105= 94°66, : 
Pp Gi Ad a3 | 
1 
H LI Sa ea Pee 
ence P= 9566 1:045 x 10-2, 
Radius of rim of cup=c=2:90 cm. | 
A | 
The table gives the results of a number of observations for various values ; 
of the difference of level, Mcm., of the water in the limbs of the manometer. — 
| 
Difference of Height of vertex | Radius of bubble 
water levels above rim pie: I? Mr 
h 2h 
2°53 cm. 1:28 cm. 3°92 cm. 9:93 em.2 
2-64 1°42 3°67 9-69 
2°93 1°73 3°30 9:66 
2°99 1°68 3°34 9:99 
311 1:90 3°16 9°84 
3°29 2°38 2-96 9°73 
3°36 Oeil 2-97 10:00 
Mean 9°83 
Since P=981.pM, where p=density of water=1 germ. cm.~*, we have — 
Pr=981Mr, and hence, by (10), 
T=Yrp=} 5. Pr=} x 1045 x 10-2 x 983 x 981 =25'19 dynes per em. 
It will be seen that there are some inconsistencies among the readings. 
They probably arise from small variations of pressure in the gasometer due 
to friction in the pulley or capillary action: these variations do not instanta- 
neously cause changes of the radius of the bubble. Better results would 
probably be obtained with a larger gasometer. Greater steadiness would be 
obtained by inserting a piece of small bore tube between the gasometer and 
the manometer joint and then using a higher pressure in the gasometer. 
A petroleum gauge would be better than a water gauge as it is less affected — 
by capillarity. 
$12. Buoyancy method. This method was suggested by the 
plan adopted by Mr J. D. Fry in the calibration of his new Micro-— 
manometer*; it depends upon the difference of density between 
cold and hot air at the same pressure. The method is instructive 
as it helps the student to realise the presence of the atmosphere. 
A metal tube ABCD (Fig. 9) has the two portions AB, CD, each a 
* J.D. Fry, Philosophical Magazine, April 1913, p. 494. 
