PHYSICAL SCIENCE. 



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depth. These two propositions constitute the 

 leading principles of hydrostatics. 



Hydraulics, or the motion of fluids, is a much 

 more difficult subject. The first step was made 

 by Torricelli, who proved that water issues from 

 a hole in the side or bottom of a vessel with the 

 velocity which a body would acquire by falling 

 from the level of the surface to the level of the 

 orifice. This proposition, which constitutes the 

 leading- principle of hydraulics, was published at 

 the end of Torricelli's book De Motu gravium 

 et projectorum. 



We owe to Torricelli another discovery no 

 less important. The question had been put to 

 Galileo, why water will rise in a pump to the 

 height of thirty-three feet, but no farther. This 

 question he was unable to answer though it 

 could not but be obvious from it that the reason 

 assigned by the ancients for the rise of water in 

 a pump, namely, nature's abhorrence of a vacuum, 

 could not be the true one. Torricelli reflecting 

 on the subject, conceived that if a heavier fluid 

 than water were used, a vacuum might be pro- 

 duced in a much shorter tube. He filled a glass 

 tube three feet long, open at one end, and shut 

 at the other with mercury, and covering the open 

 end with his finger, immersed it in a basin of 

 mercury. The result is universally known. He 

 found the mercury suspended in the tube to the 

 height of about thirty inches ; an effect which 

 he ascribed to the pressure of the atmosphere. 

 It was easy to see that the rise of water in pumps 

 was owing to the same cause. 



The objections made to this explanation were 

 removed by carrying a barometer to the top of 

 the Puy de Dome, at the suggestion of Pascal. 

 The mercury fell lower in the tube, because the 

 pressure of the atmosphere was less at the top 

 than at the bottom of the mountain. 



The invention of the air pump, by Otto 

 Guericke, burgomaster of Magdeburg, quickly 

 followed that of the barometer. To obtain a 

 space entirely void of air, he filled a barrel with 

 water, and having closed it exactly on all sides, 

 began to draw out the water by a sucking pump 

 applied to the lower part of the vessel. He had 

 proceeded but a little way, when the air burst 

 into the barrel with a loud noise. After various 

 failures, he at last procured a vacuum by employ- 

 ing a sphere of glass. This was about the year 

 165k The air pump was greatly improved by 

 Mr Boyle, assisted by Dr Hooke, who was his 

 operator. For the sphere of Otto Guericke he 

 substituted a more convenient receiver, and made 

 the pump to work more easily, and produced a 

 better vacuum. 



Thus the elasticity, as well as the weight of the 

 ii', became known. Boyle showed that water 



always contains air, and lie placed the weight 

 and elasticity of air in a variety of new lights. 



When we measure the quantity of water dis- 

 charged from a small hole in the bottom of a 

 cylinder filled with that liquid, the efflux amounts 

 only to about five-eighths of the first emission. 

 To explain this disagreement, Newton had re- 

 course to the supposition of a cataract or funnel- 

 shaped conoid, by which the various streamlets 

 bend their course to the orifice. The theory was 

 first investigated by Daniel Bernoulli, in his 

 treatise on hydrodynamics. It was discussed in 

 1744 and 1752 by D'Alembert, by Euler in 

 1755 and 1772, and by Lagrange in 1781. But 

 these admirable analytical investigations have 

 been found of very little utility in the practice of 

 the science. The science is nearly in the state 

 in which it was left by Newton; or at least for 

 the progress which it has made, it is rather in- 

 debted to the experimental investigations of 

 practical men than to the abstract reasoning of 

 mathematicians. 



The principles of hydrostatics constitute the 

 grounds of naval architecture, which embraces 

 the theory of the construction and sailing of 

 ships. Euler published an important work on 

 the subject in 1749. Bouguer, who from his 

 situation possessed much more experience, and 

 was not deficient in mathematical skill, examined 

 the subject in detail, and reduced the theory to 

 a great deal of simplicity. In 1746, he demon- 

 strated a very useful theorem. When a body 

 floats on the surface of water, it is held in equi. 

 librium by the action of two opposite and equal 

 perpendicular forces, passing through its centre 

 of gravity and its centre of buoyancy or that of 

 the fluid displaced by its immersion. Its weight 

 draws it downwards, while its buoyancy presses 

 it directly upwards in the line of support ; the 

 vertical position of those centres being essential 

 to floating, the centre of buoyancy does not re- 

 main stationary, but shifts with the inclination 

 of the floating body. The sustaining force may 

 be conceived to act where the line of support 

 crosses the axis. On this point Bouguer bestow- 

 ed the name of metacentre. It is the limiting 

 position, upon which the stability of floating de- 

 pends. Whenever it stands above the centre of 

 gravity there is stability ; but not so when it is 

 under that centre. 



The most complete set of experiments on the 

 resistance of water in narrow canals was made by 

 Bossut between the years 1766 and 1775. He 

 found that the celerity of water was reduced to 

 one-tenth part by flowing through a smooth 

 leaden pipe whose length is four thousand times 

 its diameter or that every particle of the fluid 

 has its motion extinguished and renewed ten 



