﻿4:28 .Mr. A. Ferguson on Pliotograpluc 



evenly distributed with regard to sign, 7 being positive and 

 9 negative, while in Table I. 5 are positive and 11 negative. 

 To compute a value for T, taking as coordinates 



x = 1-515, y='2690, 



= •3526, 

 .-. l+y 1 2 =l-1243. 

 ij 2 = 2b + 12cx 2 

 = -2252, 



1 -2352 7-166 



Rj ~~ (1-1243)1 '4459 

 1 -3526 7-166 



= 3-172, 



= 3-527, 



E 2 ~ 1-515(1*1243)4 '4459 

 ,. £ + £« 6-699. 



T , e -2690 x -4459 m a7 



Irue value or ?/= ,_ ,, n „ ='0167, 



rloo 



7i-y=-5186--0167 = -5019. 



^, ,, m <7px'5019 ^ -dynes 



Finally, T = -~ r = / 3'5 - J . 



b'bLty cm. 



The mean value for the surface-tension of water at 11° C. 



„ dynes 

 is therefore 73*4 — . The value as given bv other 



cm. ° J 



experimenters varies with the method of experiment adopted, 

 but taking the formula in Poynting & Thomson's * Properties 

 of Matter'' as a standard of reference, viz. : 



T,= 75-8-*152f, 



this gives T n as 74*1 dyne-cm. -1 , and the two numbers are 

 in agreement to within one per cent. 



The above results have been selected as typical from the 

 examination of a number of photographs, all of which gave 

 results very similar to those detailed above. Amongst the 

 photographs measured it was, as might be expected, found 

 that the parabolic formula could be best applied when the 

 drop was fairly, but not too flat in outline ; as a rough 

 criterion, the ratio of the extreme breadth to the extreme 

 depth of the drop (a'b'.-z-c'd' in fig. 3, A) should be about 

 4 : 1 to give convenient measurements. 



