74 



Lord Rayleigh on 



[May 15, 



distance between consecutive corresponding points of the recurrent 

 figure, or, as it may be called, the wave-length of the figure, is directly 

 proportional to the velocity of the jet, i.e., to the square root of the head 

 of water. This elongation of wave-length with increasing pressure 

 was observed by Bidone and by Magnus, but no definite law was 

 arrived at. As a jet falls under the action of gravity, its velocity 

 increases, and thus an augmentation of wave-length might be ex- 

 pected ; but, as will appear later, most of this augmentation is com- 

 pensated by a change in the frequency of vibration due to the 

 attenuation which is the necessary concomitant of the increased velo- 

 city. Consequently but little variation in the magnitudes of successive 

 wave-lengths is to be noticed, even in the case of jets falling vertically 

 with small initial velocity. In the following experiments the jets 

 issued horizontally from orifices in thin plates, usually adapted to a 

 large stoneware bottle, which served as reservoir or cistern. The 

 plates were of tin, soldered to the ends of short brass tabes rather 

 more than an inch in diameter, by the aid of which they could be 

 conveniently fitted to a tubulure in the lower part of the bottle. The 

 pressure at any moment of the outflow could be measured by a water 

 manometer read with a scale of millimetres. Some little uncertainty 

 necessarily attended the determination of the zero point ; it was 

 usually taken to be the reading of the scale at which the jet ceased to 

 clear itself from the plate on the running out of the water. At the 

 beginning of an experiment, the orifice was plugged with a small roll 

 of clean paper, and the bottle was filled from an india-rubber tube in 

 connexion with a tap. After a sufficient time had elapsed for the 

 water in the bottle to come sensibly to rest, the plug was withdrawn, 

 and the observations were commenced. The jet is exceedingly sensi- 

 tive to disturbances in the reservoir, and no arrangement hitherto 

 tried for maintaining the level of the water has been successful. The 

 measurements of wave-length (A,) were made with the aid of a pair of 

 dividers adjusted so as to include one or more wave-lengths ; and as 

 nearly as possible at the same moment the manometer was read, The 

 distance between the points of the dividers was afterwards taken from 

 a scale of millimetres. The facility, and in some cases the success, of 

 the operation of observing the wave-length depends very much upon 

 the suitability of the illumination. 



The first set of observations here given refers to a somewhat elon- 

 gated orifice of rectangular form. The pressures and wave-lengths 

 are measured in millimetres. The third column contains numbers 

 proportional to the square roots of the pressures. 



