HYDRODYNAMICS. 



487 



a placed on one Ride, and through iM body passe* a 

 bent *yphon QP, the lower branch of which goes into 

 the compartment BG. 



n the two upper compartments are nearly filled 

 up to a little below K with water, through two aper- 

 tures for that purpose, and when these apertures are 

 shut, it U obvious that when the cock of the pipe LM 

 is opened, the water will descend through LM, and 

 occupying the compartment EC, will drive the air up 

 through the pipe IK, and compres the air contained in 

 the cavity HF. This condned air prewing on the sur- 

 face HK. of the water, will raise it in the tube NO, and 

 cause it to be projected upwards in a jet d'eau. The wa- 

 ter from the jet, after being carried to a height due to 

 thr pressure which it experiences, will fall down into the 

 veMel SR. But as the water etcapes from the compart- 

 ment BG, the air in that compartment will be rarefied, 

 and will therefore not be sufficient to balance the pres- 

 sure, if the exterior air upon the surface of the water 

 U SR. This unbalanced pressure will therefore force 



the water up the syphon QI', through which all the Hydraulics. 

 water in the vessel SR will be conveyed into the cavity ^Y 

 BG, as if it had been drank by the bird. 



For farther information respecting the subjects treat- 

 ed of under this Chapter, the reader is referred to the 

 following works: Pascal's Trail e: de I'Equilibre de* 

 Liqueur*, &c. &c. Paris, 1664 ; S. Gravesende's Phytices 

 Elementa Mathematica, lib. iii. Leid. 1742; Phil, front. 

 1 732, vol. xxxvii. p. 301 ; Desagulier's Courte of EJC- 

 perimental Phihtophy, vol. ii. lect. 7 and 8, Lond. 1763 ; 

 Ferguson's Lecture* an Select Subjects in Mechanics, 

 Hydrostatics, Hydraulic*, &c. vol. ii. Edin. 1806; Dr 

 Thomas Young's Course of Lecturet on Natural Philo- 

 sophy, vol. i. Description of Plates ; Leslie's Short Ac- 

 count of Experiments and Instruments depending on the 

 relations of Air to Heat and Moisture, Edin. 181S ; 

 Campbell's Travel* in the South of Africa ; and Ozanam's 

 Mathematical Recreations, edited by Montucla and Hut- 

 ton. See also Part III. of this article on HYBBAULIC 

 MACHINERY. 



PAHT II. ON HYDRAULICS. 



HVDII \cLics, from k ')( witter, and *<, which 

 tfanti signifies a lorrrnl, is that branch of the science of 

 hydrodynamics which treats of fluid* considered as in 

 motion. It therefore embraces the phenomena exhibi- 

 ted by witer issuing from orifices in reservoirs, project- 

 ad obliquely or perpendicularly in jet d'eaux, moving 

 *s, canals, and rivet*, oscillating in wave*, or op- 

 posing a resistance to the progress of solid bodies. 



(HAP. I. 



ON THE MOTION or FLUIDS issuivo FROM RESERVOIRS 

 T VERTICAL OR HOKUOXTAL Oairiiu. 



General Principle*. 



WHIN a vessel i* filled with a homogeneous fluid, and 

 ii in equilibrium, all the particles of fluid which it con- 

 tains are equally pmsed in every direction. But if a 

 small aperture is made in the bottom of the vessel, the 

 particles whirh rested upon the part of the bottom which 

 is removed being no lunger supported, will descend by 

 their own gravity. The particles immediately above 

 them will alko descend, and all the fluid in the vessel 

 will descend in line* nearly vertical ; and when they ar- 

 rive within 3 or 4 inches of the orifice, they will 

 ly turn into a direction more or IMS) oblique, and snake 



directly for the nnfice. The seme thing 

 the orifice i. made on the side of the vessel." The 

 ceding results were obtamed experimentally by M. 

 sut, who employed a glasa vessel kept constantly, 

 rendered the motion of the fluid particle* visible, 

 throwing into the water minute 

 heavier than it ; such as filing*, 

 of pounded slate. 



the vessel was allowed to empty itself by an 

 orifice in the bottom of the vessel, the surface of the 

 fluid preserved il horixontality during it* descent, and 

 when it came to within ', line*, or hall* an inch of the 

 orifice, a funnel-shaped hollow or C4vity appeared in the 



surface of the fluid. When the water issued from an 

 orifice in the side of the vessel, the water abo preserved principle*. 

 ha horisontality ; and when the surface reached the 

 upper edge of the orifice, the water inclined a little from 

 the orifice, forming an approximation to a hollow. 



As the various particles of fluid which rush to- 

 wards the orifice move in directions which converge 

 to a point w ithout the orifice, it is obvious that the column 

 of fluid which issues from the vessel ought to have 

 smaller diameter than the orifice itself. This diminu- 

 tion in the diameter of the column was first observed 

 by Sir Isaac Newton, and was called by him the vena 

 contraela, or the contraction of the fluid vein. The 

 distance from the orifice at which the greatest con- 

 traction take* place, is equal to the semidiameter of the 

 orifice, and the area of the section of the vein at this 

 piace was to the area of the orifice as 10 to 14.14 ac- 

 cording to Newton, or as 10 to 16 according to Bossut. 

 When the orifice, instead of being a mere aperture in 

 a thin plate, is a short cylindrical tube, Bossut found 

 that the area of the section of the vena coatmoU was to 

 that of the orifice M 10 t*> M.S. 



Jn iisnosirting to give a very short view of the theory 

 of fluids issuing from orifices, we. s*jst warn the reader 

 not to expect that strict coincidence between theory 

 and practice which is to be found in many other branches 



mul of science. In optic* and astronomy, and even in those 

 make parts of hydrostatic* which we have already considered, 

 when the theoretical rssnks scarcely differ at all from those 



which are obtained from accurate experiment* ; but in 

 every branch of hydraulic* the deduction* of theory are 

 so uncertain, that they are of no use whatever in any 

 of the important purpose* to which this science is ap- 

 plicable. It is only the general laws deduced from ex- 

 periment that can be safely employed in the various 

 operations of hydraulic architecture. 



We shall therefore pas* over very rapidly the theo- 

 retical part of the subject, and endeavour to lay be- 

 fore oar readers a full account of the practical parts 

 of hydraulic*, avail, g ourselves of the invaluable la- 

 bours of the AbM ' 



