150 WELLS'S NATURAL PHILOSOPHY. 



of what is called the "contracted vein," tend very much to diminish the mo- 

 tion and discharge of water. 



"When water flows throiicrh a circular aperture 

 What 13 the . , , ■,. ", , . . 



" contracted m a vessel, the diameter of the issumg stream 



vain" 11 a (•>"■- j^ contracted, and attains its smallest dimensions 

 rent of water ? ' 



at a distance from the orifice equal to the diam- 

 eter of the orifice itself The section of the jet at tliis point. Fig. 

 141, 5 s', will be about two tliirds of tlie magnitude of the onfiue. 

 This point of greatest contraction is called tlie vena contracia, or con'cracted van. 



,„, , . „ This phenomenon arises from the circumstance that a liquid 



What IS the '■ ... 



cause of this contained in a vessel rushes from aU sides toward an orihce, 

 phenomenon? g^ ^^ ^^ j-^j.^^ ^ sj-stem of converging currents. Tliese issuing 

 out in oblique directions, cause tlie shape of the stream to change from the 

 cjdindrical form, and contract it in the manner described. 

 How may the ^^ *^'^ attaclunent of suitable tubes to the aperture, the 



effect of the effect of the contracted vein may be avoided, and the quan- 

 vei'n' be' avoid- tity of flowing water be very greatly increased. A short pipe 

 ed? xvill discharge one half more water in tlie same time, than 



a simple orifice of the same dimensions. The tube, liowever, must be 

 Fig 142 entirely without the vessel, 



as at B, Fig. 142, for if co' • 

 tinned inside, as at A, the 

 quantity of liquid discharged 

 will be diminished instead 

 of augmented. 



The rapidity of the discharge of the water will also depend much on the 

 figure of tlie tube, and that of the bottom of the vessel, since more water 

 will flow through a conical, or bell-shaped tube, as at C, Fig. 142, than 

 through a cylindrical tube. A still further advantage may be gained by hav- 

 ing tlie bottom of the vessel rounded, as at D, and the tube bell-shapod. 



An inch tube of 200 feet in length, placed horizontally, will discharge only 

 one fourth as much water as a tube of the same dimensions an inch in 

 length ; hence, in all cases where it is proposed to convey water to a distance 

 in pipes, there will be a great disappointment in respect to the quantity actu- 

 ally delivered, unless the engineer takes into account the friction, and the 

 turnings of the pipes, and makes large allowances for these circumstances. 

 If the quantity to be actually delivered ought to fill a two inch pipe, one of 

 three inches will not be too great an allowance, if the water is to be conveyed 

 to any considerable distance. 



In practice, it will be found that a pipe of two inches in diameter, one hun- 

 dred feet long, will discharge about five times as much water as one of one 

 inch in diameter of the same length, and under the same pressure. This dif- 

 ference is accounted for, bj' supposing that both tubes retard the motion of 

 the fluid, by friction, at equal distances from their inner surface.?, and conse- 

 quently, the effect of this cause is much greater in proportion, in the small 

 tube, than in the large one. 



W i,-J L;J 



