1890.] recently formed Liquid Surfaces, 283 



phase of vibration depends upon the time elapsed, it is always the 

 same at the same point in space, and thus the motion is steady in the 

 hydrodynamical sense, and the boundary of the jet is a fixed surface. 

 Measurements of X under a given head, or velocity, determine the 

 time of vibration, and from this, when the density of the liquid and the 

 diameter of the column are known, follows in its turn the value of the 

 capillary tension (T) to which the vibrations are due. Ceteris 'paribus, 

 T oc \ -2 ; and this relation, which is very easily proved, is all that 

 is needed for our present purpose. If we wish to see whether a 

 moderate addition of soap alters the capillary tension of water, we 

 have only to compare the wave-leugths X in the two cases, using the 

 same aperture and head. By this method the liquid surface may 

 be tested before it is T ^ second old. 



Since it was necessary to be able to work with moderate quantities 

 of liquid, the elliptical aperture had to be rather fine, about 2 mm. 

 by 1 mm. The reservoir was an ordinary flask, 8 cm. in diameter, to 

 which was sealed below as a prolongation a (1 cm.) tube bent at 

 right angles (figs. 1, 2). The aperture was perforated in thin sheet 

 brass, attached to the tube by cement. It was about 15 cm. below the 

 mark, near the middle of the flask, which defined the position of the 

 free surface at the time of observation. 



The arrangement for bringing the apparatus to a fixed position, 

 designed upon the principles laid down by Sir W. Thomson, was 

 simple and effective. The body of the flask rested on three pro- 

 tuberances from the ring of a retort stand, while the neck was held 

 by an india-rubber band into a \J -groove attached to an upper ring. 

 This provided five contacts. The necessary sixth contact was 

 effected by rotating the apparatus about its vertical axis until the 

 delivery tube bore against a stop situated near its free end. The 

 flask could thus be removed for cleaning without interfering with 

 the comparability of various experiments. 



The measurements, which usually embraced two complete periods, 

 could be taken pretty accurately by a pair of compasses with the 

 assistance of a magnifying glass. But the double period was some- 

 what small (16 mm.), and the little latitude admissible in respect to 

 the time of observation was rather embarrassing. It was thus a 

 great improvement to take magnified photographs of the jet, upon 

 which measurements could afterwards be made at leisure. In some 

 preliminary experiments the image upon the ground glass of the 

 camera was utilised without actual photography. Even thus a 

 decided advantage was realised in comparison with the direct 

 measurements. 



Sufficient illumination was afforded by a candle flame situated a 

 few inches behind the jet. This was diffused by the interposition of 

 a piece of ground glass. The lens was a rapid portrait lens of large 



