Prof. Magnus's Hydraulic Researches. 173 



the form of the masses appears somewhat shortened^ but incon- 

 siderably so, because the slit is very narrow. In the same man- 

 ner the form appears somewhat, but inconsiderably, lengthened, 

 if the eye be so placed behind the disc that the observation takes 

 place while the slit is moving in the same direction as the stream. 

 Moreover the observation may be perforoaed at the moment when 

 the sht is parallel to the stream ; and then the breadth of the 

 latter appears almost the same as it is in reality, and it is also 

 seen unchanged in the direction in which the water moves. All 

 three kinds of observation may be employed in order to supple- 

 ment one another. But the last-mentioned method of observing 

 the jet is the surest, and it has also the advantage of allowing a 

 much greater portion of the stream to be seen at once. 



154. The discs which I used had a diameter of 250 millims. 

 As the breadth of the slit was at most 1 millim., it was about 

 y^^th of the surface of the disc. The light which reached the 

 eye from the jet on turning the disc, bore, consequently, the 

 proportion of 1 : 780 to that which was transmitted when no 

 disc was there. It was necessary therefore to illuminate the 

 stream very strongly if it had to be observed by means of the 

 disc. In order to see the stream uninterruptedly, it was neces- 

 sary to give 20 to 25 revolutions per second to the disc. Be- 

 tween each two of these feeble impressions of light an interval 

 of from ~Q to 27*'^ ^^ * second elapsed ; and each impression 

 endured for -^ to y^th of a second. 



155. If, instead of one fine slit, four be introduced into such 

 a disc, the observation cannot be properly performed. For if 

 the velocity of the disc remains unchanged, it is true that the 

 time of each separate impression of light remains also unchanged; 

 but the impressions follow one another at intervals of from ^jj 

 to x^o*^ ^^ * second ; and this interval of time is too short for 

 observation. If, on the other hand, the disc be turned more 

 slowly, so that the times between the observations become longer, 

 the slits pass more slowly before the eye, the outlines of the 

 separate masses undergo a more considerable alteration, and the 

 distinctness is thereby spoilt. 



Appearance of a jet without swellings when seen by means of the 

 narrow slit. 



15G. But if the apparatus as it is described in § 154 be em- 

 ployed, the jet aj)pcars, everything being tranquil and no swell- 

 ings present, as is represented in PI. II. rig. 11. This i-epresen- 

 tation is taken from a jet which issued from a hole 3 millims. 

 in diameter, under a pressure of 5 centims. But all jets which 

 issue from somewhat smaller, as well as from much larger cir- 



