170 Prof. Magnus's Hydraulic Researches. 



broader, having nearly the form of fig. 7. Frequently no inter- 

 vals at all occur between them. They then form a luminous 

 band which appears notched above, but irregularly bounded 

 below. 



147. It follows from the above, that when swellings are pre- 

 sent in the stream, even that portion of it which otherwise ap- 

 pears smooth to the eye has alternating greater and smaller 

 diameters. But, as stated, this is only the case when swellings 

 occur in the lower portion of the stream. When these are absent, 

 the upper continuous part of the jet next the efflux-opening is 

 perfectly smooth ; that is, its diameters continually decrease to 

 the point where it begins to be turbid. 



Savart's method of observing the jet. 



148. In order to investigate the different parts of the jet, 

 Savart, as is well known, made use of the endless strap described 

 by him*, provided with alternate bright and dark bands. In 

 place of this strap we may employ, as Savart himself proposedf, 

 and as has since been done by many other experimenters, a wheel 

 provided with spokes, which is placed between the strongly- 

 illuminated jet and the observer, or between the source of 

 light and the stream, so that in the latter case the stream is 

 between the wheel and the observer. If, then, the wheel be put 

 into such quick rotation that the separate spokes can no longer 

 be distinguished, the part of the jet above the swellings ap- 

 pears quite smooth and continuous, presenting thus the same 

 appearance as it does to the naked eye, only somewhat more 

 faintly illuminated. But if the wheel with the spokes be placed 

 opposite a swelling, isolated masses separated from one another in 

 a vertical direction are recognized, if the proper velocity of rota- 

 tion be given to the wheel. The form of these masses, however, 

 cannot be recognized by means of the spoked wheel, even although 

 the stream be illuminated in a darkened room by the system of 

 lenses mentioned in § 142. 



149. A simple consideration is sufficient to inform us why 

 the form cannot be recognized. If, namely, two motions occur 

 in opposite directions, as is here the case in respect to the 

 stream of water and the spoked wheel, the outlines of the sepa- 

 rate moving masses cannot appear as they are in reality. We 

 need only remember that two wheels, having spokes as broad as 

 the spaces between them, and turning with equal velocity on a 

 common axis in opposite directions, give the appearance (if the 

 number of the spokes of the wheels is the same) of one wheel 

 with the double number of spokes. Each spoke appears then 

 half as broad as it is in reality. 



* hoc. ait. p. 349. t Loc. cit. p. 348. 



