SURFACE-TENSION OF WATER BY THE METHOD OF JET VIBRATION. 303 



same time photographed instantaneously. Let the distance between two cuts, 

 measured by help of the photographic plate, be a, and the time-interval be t, we have 

 c = aft, c being the velocity of the jet. 



Fig. 2 shows the arrangement seen from above and from the side. 



The rotation -apparatus ABCD executes the cutting of the jet and the opening and 

 closing of the light. A is a metal disk, to the edge of which the knives were fastened 

 in radial direction. The knives, made from ground needles, measured about 0'4 mm. 

 in width and were about 0'03 mm. thick. The axis of the rotation-apparatus was not 

 parallel to the jet, but formed a small angle with it, so that the knife cutting through 

 the jet had the same velocity parallel to the axis of the jet as the water-particles. 



D is a metal disk which has a radial slit close to the edge, which once at every 

 revolution passes a corresponding slit in the screen E. The apparatus was driven by 

 an electric motor, the speed of which could be regulated by means of an adjustable 

 resistance, and, in order to make the velocity steady, the axis of the rotation- 

 apparatus was provided with a small fly-wheel B. Further, to count the revolutions, 

 the axis of the apparatus carried a contact C, which, completing the circuit of an 

 electric current once at every revolution, marked a kymograph by help of an electro- 

 magnet. The kymograph was also marked every second by another electromagnet. 



abcdefg provided for the illumination of the jet. By help of a powerful lens- 

 system b an image of the horizontal linear filament of a Nernst lamp a was formed on 

 the slit of the screen E. The mirrors c, e, f, and the lenses d and g, thereupon formed 

 a magnified image of the slit on the jet, and from the lens g all the light was finally 

 directed into the camera K. In the figure the dotted lines show the limitation of the 

 beam of light. 



Every photograph was taken during about 12 seconds, which corresponded with 

 about 600 revolutions of the apparatus with the following exposures of the plate- 

 Some photographs are shown in the accompanying figure. (The direction of the jet is 



