HYDRODYNAMICS. 



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board I., Figs. S. and 5. in which the upright wire M i< 

 fixed. Leaden weights N, X, with holes through their 

 centre, to the amount of 16 ponnds, are put upon this 

 wire, and press upon the board L. The crow bar p is 

 then put on, in order to keep the glass tube in an up- 

 right position ; and afterward* the bent piece EFG, for 

 keeping tb weights N, N horizontal, and the wire M 

 vertical. Fonr upright pin?, about an inch long, are 

 placed on the comers of the box, for the purpose of 

 supporting the board L, and preventing it from press- 

 ing the sides of the bladder together. When the ma- 

 chine is thus fitted op, poor water into the tube I, till 

 the bladder is filled up to the board L, Continue pour- 

 ing in more water, and the upward pressure which it 

 will excite in the bladder will raise the board with all 

 its weights, even though the tube should be so small as 

 to contain no more than an ounce of water. See Fer- 

 guson'* Lecturti, vol. ii. p. 104. 



4. Experiment* for illustrating the quaquaveniu pretntre 

 oj Fluids, and the Effect* produced at different Depth*. 



Exn. \. If we take a common wine gla*s AB, and, 

 holding it in a vertical direction, bring its motith in 

 contact with the surface of water in the vessel M, it will 

 be seen, that a small quantity of water has entered the 

 of flu- wine glass, and that the remainder of the glass is fiil- 

 iat, UMitbc ed with air. By depressing the glass, or (inking it in 

 fct pio. th e fluid, greater quantity of water will enter it ; 

 the included air being condensed into smaller space 

 as the pressure of the superincumbent column of wa- 

 ter is increased. By continuing to depress the glaM, it 

 ... will be seen that the pressure of the water increases 

 with the depth, and by holding the mouth of the glass 

 fn different directions, ai shewn at CD and KF, it will 

 appear, that the water prcMC* equally in every direction. 

 Exp. 2. If we insert into an empty vessel a number 

 of tubes of glass bent into various angle*, or if we hold 

 ihrisi in tht- hand, and introduce into their lower ori- 

 fices a quantity of mercury, so that the. surface of the 

 mercury may come to the very orifice of the shorter 

 , then, if water is poured into the vessel, it will 

 be seen, during the time of in filling, that the mercury 

 i* pressed gradually from it* lower orifices towards the 

 higher orifices, which are uppo*ed to rise to a greater 

 height than the surface of thr water. Now, as the lower 

 orifices of the glass tubes may be made to point in every 

 possible.! follows, that the pressure of the so- 



perincnmbrnt fluid is also propagated in every direction. 

 When it i* required that the lower orifice should point 

 exactly downwards, in order to shew the upward pres- 

 sure of fluids, a straight tube should be nvtl, ami the 

 mercury introduced into it mint be kept in by the ap- 

 plication of the extremity of the finger, till the height 

 of the water above the orifice is eqml to 14 time* the 

 length of the column of mercury introduced. Upon re- 

 moving the finger, and continuing to pour in water, 

 the mercury will ascend in the tube. If the finger 

 were removed before- it had risen to a height 1 1 times 

 greater than the length of the mercurial column, the 

 mercury would have fallen out of the tube, as it is 14 

 times heavier than water, and therefore requires co- 

 lumn 14 time* a* long to keep it in equilibrio. 



i. 5. The pressure of fluid* at different depths 

 may be very simply illustrated, by attaching a bag, 

 made of leather, and filled with mi-miry to the extne- 

 mhy of glaM tube, so that the mercury may just en- 

 ter the tube when the bag is held in air. By immer- 

 sing the bag in water, the pressure of the fluid upon 



the bag will force up the mercury, and the height to Hjrdrosuti. 

 which it rises will shew die magnitude of the pressure ^ ' nstru - 

 at different depths. 



Exp. 4. The propagation of pressure through fluids is 

 also illustrated by the amusing experiment of the Car- ^ v ^^ 

 teitan Devil, as it has been called, after Descartes, by PIATE 

 whom it was discovered. The figure of a man made CCCXVii. 

 of glass or enamel, is so constructed that it has the same F ' 6- 7> 

 specific gravity as water, and is therefore suspended in 

 a mass of fluid. A bubble of air, (similar to the air in 

 the glass of Exp. 1.) communicating with the water, is 

 placed in some part of the figure, sometimes in a small 

 globe, as shewn at m. At the bottom B of the vessel is 

 a dmphmjm of bladder, which can be pressed upward* 

 by apply iiiir the finder to the extremity e of a lever eo 

 moving round o as its centre of motion. The pre- 

 applied to a is communicated through the water to the 

 bubble of air, which is thus compressed : The specific 

 gravity of the figure is therefore increased, and it sinks 

 to the bottom. By removing this pressure, the figure 

 again rises, so that it may be made to oscillate or dance 

 in the vessel without any visible cause. Fishes made 

 of glass are sometimes substituted in place of the human 

 . and when a common jar is used for the experi- 

 ment, the pressure is applied to the upper surface of it 

 at A. The construction ol the apparatus shewn in Fig. 6. 

 it obviously the best, as the spectator does not ob- 

 serve the meaiufwhich are employed to alter the speci- 

 fic gravity of the figure. 



krp. 5. The pressure of fluid* at very great depths 

 n finely illustrated by an experiment which has often 

 been made at sea, of making an empty bottle well cork- 

 ed descend to a great depth. The pressure of the wa- 

 ter drives in the cork, and the bottle when brought up 

 is always filled with water. Mr Campbell, the respec- 

 table author of Travels in the South of Africa, tried this 

 experiment on his voyage home from the Cape of Good 

 Hope. He drove very tight into an empty bottle a 

 cone, which was so large that half of it remained above 

 the neck. A cord was then tied round the cork, and 

 fastened to the neck of the bottle, and a coating of 

 pitch wa* put over the wholr. When it was let down 

 to about the depth of 50 fathoms, the captain felt, by 

 the additional weight, that it had instantaneously filled ; 

 and, upon drawing it up, the cork was found in the 

 inside of the Dottle, which was of course filled with 

 water. 



Another bottle was prepared in a similar manner ; 

 but, in order to secure the cork, a sail needle was pass- 

 ed through the cork, so as to rest on the mouth of the 

 bottle, and the whole was covered over with pitch. 

 When the bottle had descended 50 fathoms, the captain 

 again felt that it had filled with water; but, upon 

 bringing it up, the cork and needle were found in the 

 same position, and no part of the pitch appeared to be 

 broken, although the bottle was completely filled with 

 water. The water had in this case obviously insinuated 

 itself through the pores of the pitch and the cork, and 

 not, as Mr CampiwII imagines, through the pores of 

 glass. The experiment of forcing mercury through 

 wood by the common pressure of the atmosphere, take* 

 away the apparent improbability of this explanation. 

 See Campbell's Travel*, p. 507. Note. Lond. 1815. 



menu Tor 



5. Experiment for iUtutratng the equality of the prei- { 

 ture offuidt in every direction. 



force 



If a soft or frangible substance is exposed to any i 

 ce in one direction more than another, it will 



idTm nay 

 dkeetion. 



