188 Prof. Magnus's Hydraulic Researches. 



far smaller than the first, and by many it appears to have been 

 quite neglected, for they considered the jet to be cylindrical after 

 experiencing the first contraction; at least Bossut says*, "at 

 the point of contraction the jet assumes a prismatic form and 

 retains it for a short distance." It scarcely needs mention, how 

 difficult it is to determine where the part which we consider as 

 cylindrical begins, aud how little we are in a position to measure 

 its diameter with accuracy. We cannot, therefore, in a sti'ict 

 sense of the word, speak of a maximum of contraction in cir- 

 cular jets. 



Savarfs ventral segments. 



84. If, whilst a jet of water is issuing from a circular orifice 

 quite tranquilly and without any dilatations, an agitation lasting 

 only a short time be made, by stamping for instance on the 

 ground, the jet separates close to the orifice and carries an air- 

 bubble down with it. In thin jets this separation is not noticed, 

 but when the diameter of the jet is 12 millims. or more, it is 

 seen very distinctly. It arises evidently from the vibration which 

 the vessel assumes, and hence the liquid in the orifice assumes 

 for a moment a motion which is opposed to that with which the 

 liquid would have passed out of the orifice. If, instead of this 

 agitation, a tone of some duration be produced in the neighbour- 

 hood of the vessel, it will also be set in vibrations. They are 

 not so strong as those produced by agitation, so that the jet does 

 not separate ; but the liquid in it is moved by the vibration 

 partly in a direction opposed to that of its efflux, and partly in 

 various other directions. Hence the connexion of the parts is 

 smaller, the jet does not form in its entire length a continuous 

 mass, and Savart's ventral segments are produced at those 

 places where it begins to separate. 



85. If the jet issues tranquilly from a circular orifice, and if 

 all vibrations are avoided, no, or at any rate no perceptible ven- 

 tral segments are formed, even where the mass ceases to be con- 

 tinuous ; for the liquid moves in the jet with greater velocity the 

 longer it falls. Hence the jet becomes thiianer, until the velocity 

 in any one section is so much greater than that in the preceding, 

 that the difi"erence becomes too great for the force of cohesion 

 with which the strata are held together. When the lowest stra- 

 tum has separated, the separated mass moves with the velocity it 

 has acquired by the fall, and no ventral segments are foi'med. 



86. If vibrations are communicated to the jet after it has left 

 the orifice, the liquid in it separates, not so much on account of 

 the velocity which the lower sections have attained, as that a 



* Lekrbegriff der Hydrodynamik, translated by Langsdorff, vol. ii. § 446. 

 p. 19. 



