produced by a Fluid in Motion. 109 



It is usual to employ a gaseous body (air or steam) in motion 

 for lecture illustrations of lateral action. As examples take, (a) 

 the suspension of a thin glass bulb in a vertically effluent jet of 

 steam ; (j3) the approach of two disks facing each other by direct- 

 ing a jet of air through the centre of one of them so that it 

 strikes the other ; and (7) the collapse of a partially open sheet 

 of paper by blowing a current of air between the opposite leaves. 

 But the lateral action of a liquid may be quite as readily shown 

 as a lecture experiment. Thus, if a tube A, fig. 8, 3 millims. in 

 internal diameter, be caused to deliver a stream of water (flow- 

 ing from a cistern with a waterhead of at least a metre), parallel 

 to, and immediately beneath the surface of w r ater, air will force 

 itself through the water, as shown at B, and will be carried along 

 by the jet until it finally escapes from the water some distance 

 from the point at which it entered. Or construct a small mer- 

 cury gauge of the form shown in fig. 9, and let the extremity A, 

 drawn out to a line point, be introduced into a liquid mass, while 

 a magnified image of the limb B is thrown upon a screen by 

 means of the electric lamp : on now causing a jet of water to 

 impinge upon A at right angles to it, the diminution of pressure 

 will be very apparent in the magnified image. Or, once again, 

 let a jet be allowed to flow near a sphere of oil floating in static 

 equilibrium in a medium of its own density, and the sphere will 

 be observed to lengthen itself into an ellipsoid, with the major 

 axis at right angles to the direction of the influent jet. 



3. Note on the constitution of a descending liquid jet. 



It is well known that a descending liquid jet presents a per- 

 fectly smooth continuous appearance above the vena contractu ; 

 whereas below it a series of swellings and contractions appear, 

 and the previously transparent jet becomes opake. 



By looking at a descending jet placed in front of a rapidly 

 rotating strap furnished with alternate light and dark bands, 

 Savart* found that its opake portion was composed of detached 

 masses of liquid ; moreover he observed that the swellings of the 

 jet were produced by drops extended horizontally, and the con- 

 tractions by drops extended vertically — each drop passing through 

 a series of oscillations, being alternately extended vertically and 

 horizontally, and (of necessity) midway between the two posi- 

 tions assuming the form of a perfect sphere. He further detected 

 very small spherical drops between the larger drops. Plateau, 

 in a series of admirable and elegant researches (" Sur les figures 

 d'equilibre d'une masse liquide sans pesanteur >, ) } examined with 



* " Memoire sur la constitution des veines liquides, lancees par des 

 orifices circulaires en mince paroi," Annates de Chimie et de Physique, 

 vol. liii. (1833). 



