HYDRODYNAMICS. 



411 



Dtcd 1*68. 



:...i- -. . 



DM 16SO. 



rally correct ; but he has committed a mistake in making 

 the" absolute velocity proportional to the declivity of 

 ial, or to the height of the water. * 



As toon as the curious results obtained by Toricelli 

 were known in France, the celebrated Pascal, who was 

 then residing at Rouen in Normandy, repeated them 

 with great care, and under various modifications, in the 

 year 16H> ; and in 1'ilT he published h 'nert 



Nouv'Ue* toneJunt le vuide, from which he concluded, 

 that the upjR-r part of the tube* of glass which he used 

 t contain air similar to that of the attroepbere, 

 nor any portion of water or mercury, and that it is en- 

 tirely void of every material substance with which we 

 are acquainted. He inferred also, that all bodies have 

 a repugnance to separate into parti, and that this re- 

 pugnance, which -doe* not vary with the magnitude of 

 ictiity, is equivalent to the weight of a column of 

 water 93 feet high. This little work was vehemently 

 attacked by Father Noel ; and Pascal was led, by the 

 views and experiments of which we hare given a detailed 

 account in our article on the BAKOMETFR, to demon- 

 strate the important principle in Hydrodynamics, that 

 the rise of water in pump* was owing to* the pressure 

 of the atsnoi|lhriT 



The research** of Pascal on the gravity of the air 

 natural I y led him to the examination of the laws which 

 regulate the equilibrium and pressure of fluids. Ste- 

 vinus had already observed, that the force exerted upon 

 the bottom of a vessel by the mperincumbent fluid was 

 equal to the weight of a column of fluid, whose base 

 was eOjii.il to that of the vessel, and whose height was 

 the height of the fluid, and the quaquaversus pressure 

 of fluid* had been generally known ; but it was reserved 

 ijcal to deduce from these principles the general 

 laws of the equilibrium of fluid bodies. He supposes. 

 that two apertures are made in a vessel full of rim 1. 

 and encloMrd on all sides, and that two pistons ap- 

 plied to these apertures, are pushed with farces pro- 

 portional to toe area* of the apertures ; and he demon- 

 strates, in two different ways, that, under these circum- 

 stances, the fluid will remain in equilibrium. In the 

 flnt demonstration he shews, that the pressure of the 

 piston is communicated to every particle of the fluid, 

 so that if the one piston advances through a certain 

 spate, the other inu.t retreat; and as the volume of wa- 

 ler continue* invariable, the spaces described by the 

 two pistons will be reciprocally proportional to the 

 area of their bases, or to the forces which impel them. 

 Hence it follows, from the principles of mechanics, that 

 the two pitton* are in qutHoriam. Tin. general theo- 

 rem conduct* it* ingenious author to the different cases 

 of the equilibrium and pressure of fluids, which flow 

 from it as so many corollaries. The*e fine results were 

 not published durn .f Pascal, but were found 



'ath in a MS. entitled HurrKipUilire 

 euevri, hirt, ippeared in 16f>S, a year after the death 

 of the ant' 



A It hi -I i-ii Descartes is not entitled to be considered 

 overer of the pressure of the atmosphere, yet 

 brious from eae et* his letten. which i da- 



the suspension of the mercury 



in a (t'as* "*" from the pressure of the su- 



perincumbent column of ir ; snd by the same cause 

 he accounts for the force with which a glass filled with 



air rarefied by heat adheres to the palm of the hand Huiory. 

 when it is quickly inverted upon it. In another of ~~Y"~ 

 Descartes' letters, of a date only a little posterior to the 

 publication of Galileo's Mechanics, he criticises this 

 work with unjust severity, and, rejecting the idea of a 

 vacuum as entertained by (inlileo, he ascribes the ad- 

 hesion of two polished surfaces to the pressure of the 

 atmosphere, and attributes to the game cause the ele- 

 ration of water in the sucking pump ; and in another 

 he maintains, that, in reservoirs kept full of war 

 tor by the superior aperture being shut, the fluid is not 

 suspended by the dread of a vacuum, but by the weight 

 of the air.t 



M. Mariotte, who was the first person that introduced M^otte. 

 experimental philosophy into France, contributed great- Died 168*. 

 ly to the progress of practical Hydrodynamics. Pos- 

 sessing the rare talent of contriving and performing 

 experiments, he embraced the opportunity which cir- 

 cumstances presented to him of executing a great num- 

 ber of experiments on fluids at the splendid water- works 

 .f Versailles and Chantilly. An account of the results 

 which he obtained was published in 1<>84 after his 

 death in his Traitt du viouvcmenl det Eaiuc. In this 

 work Mariotte employs the theorem of Toricelli ; and 

 though he treated some important points very superfi- 

 cially, and committed considerable errors in the dis- 

 cussion of others, yet it contains many valuable mate- 

 rial*. He was* unacquainted with the diminution of 

 efflux, which arises IVom the vma conlracta when the 

 adjutage is a perforation in a thin plate ; but he had the 

 honour of being the first who ascribed the discrepancies 

 between the theory and experiment to the effect of 

 ion. Having observed that water suffered consi- 

 derable retardation even when moving in the smooth- 

 est glass tubes, he supposed the retardation to arise 

 from the friction of the particles upon the sides c<: 

 tube, in the same way as the velocity of solid bodies is 

 dfaninuhnl by the friction ofthe surfaces over which they 

 particles or fllanents immediately 



to those which rub upon the sides ofthe tube, 

 them in VI-IIH ity, and nave their own velocity dm 

 d fa a bat degree ; so that the diminution os*aiMsi* 

 arising from friction grows less toward- the axis of the 

 tube. Hence the medium velocity of the fluid from 

 which the quantity of efflux ii determined, is much 

 mailer than it would have been had there been no 

 friction to retard its motion. 



The motion of riven, or of water in open pipes and pmgrt*. at 

 canal*, i perhaps one of the most interesting subjects in tudruihn 

 which science can lend her aid to relieve the wants and "> ll * 1 J r - 

 necessities of man. In Italy, where the fertility ofthe 

 soil is not more owing to her genial climate than to 

 the numerous canal* and river* with which it is tra- 

 versed, the attention of her philosophers was impe- 

 riously called to the - loving writer. To pro- 

 tect themselves from the inundation* with whii h tin y 

 were often threatened, it became neetssary to r 

 their rivers into new channels ; and the ravages wlnc-li 

 were thus accidentally made on the territories of their 

 neighbours gave rise to those fierce contentions whirli 

 never fail to spring from contending interests. The de- 

 fence of their persons and properties, and the necessity 

 of adjusting the opposing interest* of neighbouring 

 states, rendered the cultivation of Hydrodynamics a 



era! letter, from Galileo to C-.i.lD on OK 

 Ih* A'MM Attnto. torn. IT. 



t aM/hes^ **. As. M. D*******.^ ok act. 3. si *0t { ton. ii. ictt. 91 

 rrtnlt <m mMr, MI. MMC V / -msMrar W fmr, MSB. 0. tftt 94. 



4 



of fluid. but relating priadptlljr to Cuulli'i opinions hve bcrn published in 

 L'tf* me imtnrt ft* Jt*t la rauiam* fv Im 



