42-3 



HYDRODYNAMICS. 



Uydroit*. mask. Hence it is obvious, that the weight of the at- 

 ._^""' s ^_> mosphere, or 73 pounds avoirdupois, pressing on the 

 Canton's outside of the ball, and not on the inside, had squeezed 

 experiments it into less compass, and that, by this compression of 

 on the com. the ball, tlie mercury and the water would be equally 

 possibility rai^d i n the tube. But the water rose ^^ of an inch 

 ater< more than the mercury, and consequently the water 

 must have expanded so much more than the mercury 

 by removing the weight of the atmosphere. In order 

 to determine how much compression was produced, ei- 

 ther by the weight of the atmosphere, or by a greater 

 weight, he took a glass ball about 1.6 inch in diameter, 

 joined to a cylindrical tube 4.2 inches long, and T ^g- of 

 an inch in diameter, and, by weighing the quantity of 

 mercury that exactly filled the ball, and also the quan- 

 tity that exactly filled the whole length of the tube, he 

 found that the mercury in T ^. of an inch of the tube 

 was the 1 00,000th part of that contained in the ball, 

 and he divided the tube accordingly with the edge of 

 a file. When the ball and part of the tube was filled 

 with water exhausted of air, he placed it in the receiver 

 of an air pump, and also in the receiver of a condensing 

 engine, and he observed the degree of expansion of the 

 water that corresponded with any degree of rarefac- 

 tion, and the degree of compression that corresponded 

 with any degree of condensation. In this way he found, 

 from repeated trials, that, when the mercury was at a 

 mean height, and the temperature of the air 50 of 

 Fahrenheit, the water rose four divisions and 6-10ths, 

 or one part in 21740, by removing the weight of the 

 atmosphere; consequently the compression of water, 

 under twice the weight of the atmosphere, is one part 

 in 10870 of its own bulk. 



In combining these experiments, Mr Canton found, 

 that water was more compressible in winter than in 

 summer, while, on the contrary, alcohol and oil of olives 

 were more compressible when expanded by heat, and 

 less so when contracted by cold. The results were, as 

 expressed in the following Table, suited to the mean 

 weight of the atmosphere. 



Temperature in 

 Fahrenheit's scale. 



64 



Compression in millionth 



parts of their own bulk. 



Water. Alcohol. 



49 60 



44 71 



The following Table contains all the results which 

 Mr Canton obtained. It is suited to a temperature of 

 50 of Fahrenheit, and to 29^ inches of the barome- 

 ter. 



Compression in millionth Specific 



Names of parts of their own hulk gravities at 



fluids. by the weight of 294 the same tern- 



inches of mercury. perature. 



Alcohol 66 0.846 



Oil of olives 48 0.918 



Rain water 46 1.000 



Sea water 40 1.028 



Mercury . S 13.595 



From these results it appears, that the compressions 



ratios of the specific gravities. If the law of compres- 

 sion in water is the same as that in air, it would fol- 

 low, that, at a depth of 100 miles, the density of the 

 water would be doubled, and at the depth of 200 qua- 

 drupled. 



In the year 1774, the Ex-Jesuit Herbert published 



* Hauy's Elementary Treatiie on Natural 1 



at Vienna a treatise entitled De Ayiue Elasticitale, in 

 which he confirmed the general result of Canton's ex 

 periments, and in 1779 M. Zimmerman published an ac- 

 count of similar experiments at Leipsic, under the title 

 of Traite de tElaslicite de feau el d'autresfluides. He 

 found, that sea water, when inclosed in the cavity of a 

 strong iron cylinder, and pressed by a force equal to a 

 column of sea water 1 000 feet high, was compressed 

 T: J s th part of its own bulk, a result much greater than 

 we should have expected from the experiments of Can- 

 ton. A number of results similar to these were ob- 

 tained by the Abbe Mongez, who has printed an ac- 

 count of them in the 9th volume of Rozier's Journal. 



As the doctrine of the compressibility of water has 

 long been considered as a fact rigorously established, 

 we were surprised to find its incompressibility stated 

 by the Abbe Hauy, without the slightest reference to 

 any of the preceding experiments. " One of the expe- 

 riments," he observes, " which has served to shew the 

 incompressibility of water, consists in charging that li- 

 quid with a column of mercury, by employing a bent 

 tube in the form of a syphon, the shortest branch of 

 which is closed at its superior part, and contains water, 

 at the same time that the longest branch is occupied by 

 the mercury, which presses the surface of the water. 

 The column formed by this latter fluid was not short- 

 ened by the smallest perceptible quantity, even when 

 that of the mercury was 227 centimetres, or about seven 

 feet high, in which case it exerted upon the water an 

 effect triple of that of a column of water 33 feet high."* 

 In this experiment, which must have been carelessly 

 made, the compression ought to have been thrice as great 

 as in the experiments of Canton. 



Fluids have also been divided into perfect and imper- p er f cct an ,i 

 feet ; but this division is quite arbitrary, as there is no imperfect 

 body which possesses the character of perfect fluidity, fluids. 

 Boiling water approaches nearer to a state of perfect flu- 

 idity than water in any other state. As its temperature 

 diminishes, its viscidity increases, andits fluidity becomes 

 less perfect. In many of the oils, varnishes, and in 

 melted glass, the fluidity is extremely imperfect ; where- 

 as it may be considered as nearly perfect in water, al- 

 cohol, mercury, &c. 



. CHAP. I. 



ON THE PRESSURE AND EQUILIBRIUM OF FLUIDS. 

 FUNDAMENTAL PRINCIPLE. 



When a mass of fluid, in a state of equilibrium, is 



subjected to the action of any forces, every particle of the 



fluid mass is pressed equally in every direction, and vice 



versa if every particle of the fluid mass is pressed equally 



in cvi.j direction, the whole, mass will be hi equilibria. 



THIS principle, which has been adopted as the foun. 

 dation of hydrostatics by Euler, D'Alembert, Bossut, 

 anil Prony, is a necessary consequence of the definition 

 which we have already given of fluidity ; for, since the 

 parts of a fluid yield to the smallest pressure, any par- 

 ticle which is more pressed in one direction than ano- 

 ther, would move to the side where the pressure was 

 least, and consequently the equilibrium would be de- 

 stroved. If the particles are equally pressed in every 

 direction, it is equally evident, that the mass of which 

 they are composed must be in equilibrium. 



y, translated by Dr 0. Gregory, vol. i. IT*. 





