Accounts of Booh, 525 



tance of one femi diameter of the orifice itfelf. If a fmall adjutage be adapted to the ori- 

 fice, having its internal cavity of the fame conoidal form as the fluid itfelf affedts in that 

 interval, the expenditure is the fame as by the fimple orifice. But if at the extremity of 

 this adjutage a cylindric tube be affixed, of a greater diameter than that of the contrafted 

 vein, or a divergent conical tube, the expence of fluid increafes, and may exceed the double 

 cf that which pafles through the aperture in the thin plate, though the adjutage pofTefs aa 

 horizontal or even afcending direflion. 



By the interpofition of a fmall adjutage, adapted to the form of the contra£ted vein. 

 Citizen Venturi afcertained, in the firft place, that there is an increafe of velocity in the 

 tubes he employed, though the velocity of emiffion itfelf be lefs than that of the ftream 

 which iflues from an hole in a thin plate. He afterwards proves, by the fadl, that the 

 interior velocity and expenditure of fluid, which is increafed through tubes, even in the 

 horizontal or afcending direftion, is owing to the preflure of the atmofphere. If the fmalleft 

 hole be made in the-fide of the tube near the place of contra£lion of the vein, the increafed 

 expenditure does not t;ike place ; and when a vertical tube is inferted in fuch a hole, 

 the lower end of which tube is immevfed in water or mercury, it is found that afpira- 

 tion takes place, and the water or mercury rifes ; and this afpiration in conical tubes is lefs 

 in proportion, as the place of infertlon of the upright tube is more remote from the fedlion 

 where the greateft contraiSion would have taken place. And, laflJy, the difference be- 

 tween the expenditure of fluid, through an orifice made in a thin plate, and that which is 

 cbferved through an additional tube, does not take place in vacuo. 



The influence of the weight of the atmofphere on the horizontal or afcending flux being 

 thus eftablifhed, the author confiders it as a fecondarycaufe, referable to, and explicable by, 

 his principle of the lateral communication of motion in fluids. In conical divergent tubes, 

 for example, the elTecS of this lateral communication is, that the central cylindrical jet, hav- 

 ing for its bafis the fecflion of the contra£led vein, carries with it the lateral fluid which' 

 would have remained flagnant in the enlarged part of the cone. Hence a vacuum tend,"! to 

 be produced in this enlarged part which furrounds the central cylindric ftream ; the prefTurc 

 of the atmofphere becomes aftive to fupply the void, and is exerted on the furface of the re- 

 fervoir, fo as to increafe the velocity of the fluid at the interior extremity of the tube. 



The author proves, that the velocity or totalexpenditure of fluid through an aper- 

 ture of given dimenfions, may be increafed by a proper adjutage in the proportion of 24 

 to 10: he applies this refult to the conflruflion of the funnels of chimneys. He 

 determines the lofs of emitted fluid, which may be fuftained by finuofity in pipes. He 

 flifws by experiment, that a pipe which is enlarged in any part, afl'ords a much lefs quan- 

 tity of fluid than if it were throughout of a diameter equal to that of its fmallelt 

 feftion. This, as he remarks, is a circumflance to which fufficient attention has not bcea 

 paid in the conflruftion of hydraulic machines. It is not enough to avoid elbows and 

 contraflions ; for it fomelimes happens, that by an intermediate enlargement the whole of 

 the advantage arifmg from other judicious difpofitions of the parts of the machine is lofb. 



There are two caufes of the increafe of expenditure through defcending pipes. The firft 

 is owing to the lateral communication of motion which takes place in defcending pipes, in 

 the fame manner as in thofc which poflefs an horizontal fituation ; the fecoiid arifes from 

 the acceleration by gravity which takes place in the fluid while it falls through tlic dcfccnd- 



