482 



HYDRODYNAMICS. 



HydrosUti- 

 . -I Instru- 

 ments and 

 Expert- 

 menu. 



I'LATK 



CCXVII. 



which contains about a pound of water, and abode a glass 

 tube, fixed to the end C of the beam of a balance, ami the 

 other end to a moveable bottom, by which the water in 

 the box is supported, the bottom and wire being equal in 

 weight to an empty scale suspended at the other extre- 

 mity of the balance. If a pound weight is put into the 

 empty scale, it will cause the bottom to rise a little, and 

 the water will appear at the lower end of the tube a. 

 The water will therefore press with a force of one pound 

 upon the bottom. If another pound is put into the 

 scale, the water will ascend to b, twice as high as the 

 point a, above the bottom of the vessel. If a third, a 

 fourth, and a fifth pound be put successively into the 

 scale, the water will rise at each time to c, d, and e, the 

 distances a b, be, c d, de, being equal to one another. 

 This result will be obtained, however small be the bore 

 of the glass tube ; and since when the water is at b, c, 

 d, e, the pressures upon the bottom are successively 

 twice, thrice, four times and five times as great as when 

 the water was contained within the box, it follows that 

 the pressure upon the bottom of the vessel depends 

 wholly on the height of the water in the glass tube, 

 and not upon the quantity which it contains. If a 

 long narrow tube, therefore, be fixed in the top of a 

 cask, and if both the cask and the tube be filled with 

 water ; then though the tube be so small as not to hold 

 a pound of the fluid, the pressure of the water in the 

 tube will be in danger of bursting the cask in pieces, for 

 the pressure is the same as if the cask was continued up 

 in its full size to the height of the tube, and filled with 

 water: (SeeChap.I. Sect.l. Prop. IV. Cor.2.) It follows 

 therefore, from this principle, that any quantity of water, 

 however small, may be made to exert a force of any as- 

 " signable magnitude, by increasing the height of the co- 

 lumn, and diminishing the base on which it presses. 

 This, however, has its limits ; for when the tube becomes 

 capillary, the attraction of the glass will support a great 

 quantity of the included water, and will therefore dimi- 

 nish the pressure upon its base. The preceding machine 

 should be so constructed, that the moveable bottom may 

 have no friction against the inside of the box, and that 

 no water may get between it and the box. The method 

 Fig- 2. of effecting this will be manifest from Fig. 2. where 

 A BCD is a section of the box, and a b c d its lid, which 

 is made very tight. The moveable bottom E, with a 

 groove round its edges, is put into a bladdery g, which 

 is tied close round it in the groove by a strong waxed 

 thread. The upper part of the bladder is put over the 

 top of the box, at a and d, all around, and is kept firm 

 by the lid a b c d, so that if water be poured into the 

 box through the aperture / / in its lid, it will be con- 

 tained in the space^E g h, and the bottom may be 

 raised by pulling the wire t fixed to it at the point E of 

 the moveable bottom. See Ferguson's Lectures, vol. ii. 

 p. 100. Edit. Edin. 1806. 



2. Desertion of the Hydrostatic Press. 



Bescription This ingenious and powerful machine, which has 

 f the hy- been recently brought into notice by the late Mr Bra- 

 drostatic mah, is founded on the doctrine contained in the corol- 

 lary to the fundamental principle of the equilibrium of 

 fluids, (see p. 427,)namely, that if any number of pistons 

 are applied to apertures of different sizes in the sides of a 

 vessel full of water, the forces with which the pistons are 

 applied will be in equilibrio, if they are proportional to 

 the apertures to which they are applied. Thus if a piston 

 PLATE G (Fig. 1.) is applied to an aperture at G, having an 

 GCCXIII. area of two square inches, it will be in equilibrio with 

 another piston applied to the whole aperture AB of 

 2000 square inches, if the force with which the piston 



G is applied, is to the force with which AB is applied Hydroiuti- 

 as -1 to 2000, or as 1 to 1000. Hence it follows, that a m \ Insm '- 

 force of one pound applied at G will raise 1000 pounds "E,. 

 placed upon the piston AB. The same result will be ments. 

 obtained if the vessel has the form shewn in Fig. 11. -y-^^ 

 the one piston being applied at a b, and the other at PLATE 

 AB. CCCXIII. 



The hydrostatic press founded on this principle was lg- 

 first proposed as a new machine by Pascal in his Trai- 

 tez de. L'Eqiiilibre des Liqueurs, et de la Pesanteiir, de la 

 Masse de L'air, Chap. II. Edit. 2d. Paris, 1664. He de- 

 scribes it as a new sort of machine for multiplying for- 

 ces, ( Nouvelle sorle de machine pour multiplier les for- 

 ces,) and he considers it as a new mechanical power 

 equal in value to the lever or the screw. Although 

 Pascal speaks so highly of his new machine, it is not a 

 little singular, that no attempt appears to have been 

 made for more than a century and a half to apply it to 

 the useful purposes of life. Mr Bramah had the very 

 great merit, not only of re-inventing the machine, (for 

 we believe he was not aware of its having been propo- 

 sed by any other person,) but of pointing out its appli- 

 cation to a great variety of useful purposes, such as- 

 working cranes, pulling up the roots of trees, packing- 

 goods of all kinds, &c. In our article CRANE, (see 

 Vol. VII. pp. 315, 316, &c.) we have given a full de- 

 scription of the hydrostatic press, as applied to a crane; 

 and by studying that part of the article, our readers 

 will have no difficulty in understanding the construc- 

 tion of the instrument. 



The hydrostatic press is represented by the parts PLATE 

 GHEFFL of Fig. 1. Plate CCXV. In the Figure, FF CCXV. 

 represents the wooden frame which supports the iron F 'S 

 cylinder L. This cylinder communicates with a small 

 copper pipe ggh, terminating in a common forcing- 

 pump at h, which stands in an iron cistern H contain- 

 ing the water. The power is applied to the handle G 

 of the pump, and the piston, pressing on the surface of 

 the water in the pipe at h, communicates its force, 

 through the intervention of the water, to the piston of 

 the cylinder L, to the top of which the work to be per- 

 formed is applied. See also our article JACK in this 

 volume, page 599. 



3. Hydrostatic Bellows improved by Ferguson. 



The common hydrostatic bellows consisted of a tube Comn 

 of glass or any other substance, about three feet high, hydrostatic 

 communicating with a cylindrical vessel, whose sides bellow*, 

 were made of leather like a pair of bellows, while its 

 upper and lower surfaces were formed of circular or 

 oval boards about 15 inches in diameter. When water 

 is poured into the tube, it flows into the bellows, and 

 separates the boards a little. Heavy weights to the 

 amount of 300 pounds, are then placed upon the upper 

 board, and by pouring water into the tube till it reach- 

 es the top, the moveable board with all its load will be 

 raised and kept in equilibrio by the column of fluid, al- 

 though the fluid itself does not weigh more than a quar- 

 ter of a pound. In order to shew the experiment with 

 more effect, a man may place himself upon the upper 

 board instead of the weights, and raise himself merely 

 by pouring water into the small pipe. 



The following very ingenious machine has been pro- Improved 

 posed by Mr Ferguson as a substitute for the common by Fergu- 

 hydrostatical bellows: A BCD, Fig. 3. is an oblong square 

 box, into one of whose sides is fixed the upright glass 

 tube o I, which is bent into a right angle at the lower 

 end, at i, Fig. 4. To this bent extremity is tied the 5. 

 neck of a large bladder K, which lies in the bottom of 

 the box ABCD. Over this bladder is placed the moveable 



