172 Prof. Magnus's Hydraulic Researches. 



Observation by means of the electric spark. 



153. In order to observe the form of an object which moves 

 vcith great velocity, it is sufficient to illuminate it for a single 

 moment. Hence the electric spark has been proposed for in- 

 vestigating the jet. By its means, however, 1 have not been 

 able to recognize more than that the swellings consist of separate 

 masses. The form of these masses could not be determined, 

 either by those who were engaged with me or by myself. I 

 imagine that the cause of this is the following: — The rounded 

 masses of which the swellings consist reflect the light emanating 

 from a point in such a manner that the eye sees only the single 

 point of each which is principally illuminated. Hence, when 

 the stream is illuminated by the electric spark, the swellings 

 appear like a string of pearls ; but their form cannot be recog- 

 nized, because the intensity of the light reflected from the 

 remaining portions of the masses is too small to allow this, on 

 account of the velocity with which the impression is lost. 



Observations by means of a vei-y narroio rotating slit. 

 153. Inasmuch as the means mentioned were not sufficient to 

 recognize the form of the separated portions of the stream, I 

 have had to seek for other ones. The object has been com- 

 pletely attained by the employment of a disc having a single slit 

 in the direction of a radius, and not quite a millimetre in breadth. 

 This disc must revolve before the eye with such velocity that we 

 imagine the stream to remain continually visible, although its 

 light can only reach the eye after the completion of a revolution. 

 If the jet be viewed through the slit when it is moving from 

 below upwards, that is, in the opposite direction to the stream, 



ened upon its axis m (which pinion is not shown in the figure), in which the 

 toothed wheel R S works. The wheel R S is turned by a winch. 



A trifling arrangement is perhaps deserving of mention, by means of 

 which it is possible quickly to replace the pasteboard discs A B and C D 

 by other ones. In order to set up a new jjaper disc, it is generally neces- 

 sary to loosen completely the screw X by which the disc CD is fastened. 

 In order to avoid this, the paper discs have in the middle a hole of the 

 form e e, fig. 10. Upon the axis c d there is a small brass screw which has 

 a projecting rim. In fig. 9, y represents this screw with its rim u. The 

 circular part of the hole f e in the pasteboard disc fits exactly upon the rim 

 «, which is a very little conical. There is also a piece of brass, /3, upon the 

 axis c d. The anterior form of this piece of brass has the form r) rj, which 

 is shown in dotted lines in fig. 10. The piece of brass may be turned round 

 the axis cd, and may be pressed against y by adjusting the screw X. If a 

 pasteboard disc has to be set up, its hole, e e, is passed over X and /3 ; the 

 piece /3, which also serves the purpose of a washer, is tiu-ned and pressed 

 by the screw X against the pasteboard disc. In order that, in taking off 

 the disc, the piece /3 may be easily disengaged, it is advisable to introduce 

 a couple of turns of a spiral spring, n, in the rim ct. 



