Prof. Magnus's Hydraulic Researches. 187 



motion which is communicated to the vessel causes a change in 

 the form of the jet. Sometimes the spiral motion in jets issuing 

 from circular orifices is so violent that they separate into two or 

 more jets, which then continue their way separately, just as with 

 the jet from an elongated quadrangular orifice, § 58. 



Maximum of contraction. 



81. In perfectly regular jets from circular orifices there is no 

 maximum of contraction. Although their diameter decreases 

 most rapidly in the neighbourhood of the orifice, it also continues 

 to decrease until the jet has lost its continuity. 



82. Newton* first maintained that the quantity of water flow- 

 ing out was regulated by the contraction of the jet, and he mea- 

 sured this contraction; but I could not find a decided explana- 

 tion of the contraction of a jet {contractio vena), either in New- 

 ton's work, or in any others on the same subject. 



Since it is a question of a determinate plane, we must pre- 

 suppose, and many have thus understood the expression, that a 

 section of the jet is meant, which is a minimum, i. e. smaller 

 than all other sections ; so that the jet, after it has contracted 

 to this minimum, assumes either larger sections, or at any rate 

 contracts no more. 



Such a smallest section is seen in all jets which present dila- 

 tations in their continuous part ; hence it is present in all which 

 do not issue from circular orifices ; and even in these, when the 

 afflux to the orifice does not occur with equal regularity from all 

 sides. But in circular jets, which issue regularly from a hori- 

 zontal aperture, there is no maximum of contraction perceptible, 

 but, as before remarked, § 78, their diameter continually de- 

 creases till they cease to form a continuous mass. F. Savartf 

 mentions this in his description of jets. 



83. It is remarkable that measurements have been made, 

 principally of circular jets, not only by Newton J, but also by 

 the greater number of those who have measured the contraction 

 of the jet. 



Probably they have never used quite x-egular circular jets, or by 

 contraction they have understood something quite difiierent ; for, 

 as already mentioned, § 78, the diameter of perfectly circular 

 jets diminishes most rapidly near the efflux orifice, obviously in 

 consequence of the hoi'izontal motion by which the particles of 

 liquid in the vessel reach the orifice. After this decrease has 

 commenced, the diameter is smaller, because the velocity of the 

 falling liquid is greater. This last decrease in the diameter ia 



* Principia Philos. Natur., Prop. XXXVI. 

 t Annates de Chimie et de Phyxique, 2iiil series, vol. liii. p. 338, 

 X Principia Philos. Nat., Prop. XXXVI. 

 02 



