﻿Photographic Measurements of Pendent Drops. 417 



Now Harlow bases his calculation on the mean value 

 182'05 . 10" 6 for the thermal expansion coefficient of mercury 

 between 0° and 100° as found by Oallendar and Moss. If, 

 however, the mean value 182*57 . 10~ 6 obtained from the 

 measurements of the Bureau international and the Reichs- 

 anstalt as given in Table II. is used instead, then the value 

 1*52 . 10 " 6 will be obtained for the mean cubical expansion 

 of fused silica between 0° and 100°, and from this through 

 division by 3 the linear coefficient as 0*51 . 10'" 6 . This value 

 is, however, in complete agreement with the direct measure- 

 ments on very different specimens. 



The observations of Harlow therefore do not force the 

 assumption of the seolotropy of fused silica, but, on the other 

 hand, indicate rather the presence of error in the measure- 

 ments of Callendar and Moss. 

 Charlottenburg, January 1912. 



XXXVIII. Photographic Measurements of Pendent Props. 

 By Allan Ferguson, B.Sc. (Lond.), Assistant Lecturer 

 in Physics in the University College of North Wales, 

 Bangor*. 



[Plate VIII.] 



IN two interesting papers on Pendent Drops f , Mr. A. M. 

 Worthington has shown how the surface tension of a 

 fluid may be evaluated by tracing on paper the magnified 

 image of a pendent drop of the fluid ; if this outline be cut 

 out of paper of uniform texture, the volume, position of the 

 C.Gr., and the area of a meridional section of any given 

 portion of the drop can be estimated, and from a knowledge 

 of these quantities the surface tension of the fluid concerned 

 may be calculated. 



It seemed to the writer that this result might be obtained 

 in a more convenient manner by photographing the pendent 

 drop in such a way that the photograph should not only 

 show the contour of the drop, but should also enable the 

 distance from the " free" surface of the liquid emploved to 

 any point on the surface of the drop, to be measured ; for 

 the pressure-excess at any point P in the interior of the drop 

 is given by 



*= T (E + ff)«W*. 



* Communicated by Prof. E. Taylor Jones. 



t Proc. Roy. Soc. June 1881 ; Phil. Mag. vol. xx, (1885). 



Phil. Mao. S. 6. Vol. 23. No. 135, Manh 1911. 2 



