Refearcbes inta the lateral Communication of Motion in Fluids. j« 



The mark f diftinguiflies the ftones, which, from their fpecific gravity, 1 think belong to 

 the genus of corundum. 



The generic name corundum, I am in the habit of giving to thofs forts which have a fparr}', 

 or a granulated fradure. When corundum has a vitreous crofs fra£ture, I call it fapphire; 

 and diftinguifli its varieties by their colours, white, red, blue, yellow, green ; and by the accit 

 dental refleftion of light from their lamins: when in one dire£Hon, I call the fapphire cha- 

 toyanti when the refleftion is compounded of rays which interfeft each other, and appear t© 

 diverge from a common centre, I call them ftar-ftones, as red, blue, or greyifli ftar-ftones, or 

 ftar fapphires. 



IV. 



Experimental Refearches concerning the Principle of the lateral Communication of Motion in 

 Fluids, applied to the Explanation of various Hydraulic Phenomena. By Citizen J. B. Ven- 

 7VRI, Prfejfor of Experimental Philofophy at Modena, Member of the Italian Society., of 

 the Injlitute of Bologna, the Agrarian Society of Turin, i^c. 



(Concluded from page 494, vol. II.) 



I PROPOSITION XI. 



If the water of a refervoir, which flows through an horizontal aperture, be influenced by 

 Any foreign motion, it will form an hollow whirl above the orifice itfelf. 



V-/ITIZEN Boflut, has given a very good defcription of this kind of eddy*. It is of a differ- 

 ent nature from thofe confidered in the foregoing propofition ; but the caufes of both are, in fomc 

 refpedts, fimilar, for which reafon I propofe to attend to them more particularly in this place. 



Let D Q.fig. 1 8 (plate XXII, vol. II), reprefent an horizontal plane near the orifice E F, 

 through which the fluid of the refervoir M N flows. A fluid particle D, fituated in this plane, 

 has a modon D B, inclined to the axis A B. This motion may be decompofed into two, D C, 

 C B ; let us fuppofe that plane D Q to defcend parallel to itfelf along the axis, with the motion 

 G B ; the motion D C of the particle D on the plane D Q, remains to be examined. This 

 motion impreffes upon all the particles, fituated in the plane D Qj a centripetal force, towards 

 the centre C. 



Let any other horizontal motion whatever, not coincident in direffion with D C, be im- 

 prefled upon the fame particles : Under the government of thefe two forces, the particles will 

 defcribe round the centre C areas proportional to the times, and by the equilibrium of thefe 

 motions, they may afiume an horizontal circular rotation. 



Let us imagine, that during this horizontal circulation, the particle D, in its approach to- 

 ward the centre C, as in a fpiral, fhall defcribe circular orbits, of which the diameter is fuccef- 

 fjvely diminiflied ; let us call the velocity of rotation of the particle V)^v ; its diflrance from the 

 centresr J the time of one revolution=? ; and fince the areas muft be as the times, we (hall 



* Hydrodyn, No. 432. 



have 



