HYDROSTATICS. 



or oppressed by the weight of water, as 

 to burst the vessel, the weakest part, 

 wherever situated, will become the out- 

 let ; but, so soon as liberated, the fluid 

 will invariably descend, unless acted upon 

 by a syphon, as shown in treating of hy- 

 draulics. The pressure upwards is, how- 

 ever, merely in conformity with circum- 

 stances attendant upon general pressure, 

 and proves the tendency of fluids to find , 

 their own level. Thus, if you take a glass 

 tube of moderate diameter, open at both 

 ends, and stop one closely with your fin- 

 ger, when you immerse the other end in 

 any fluid, it will enter but little within the 

 vacancy ; because the columns of air 

 within the tube repress it. But when 

 the finger is withdrawn, the water will as- 

 cend within the tube, to the level of the 

 body in which it is immersed. 



As fluids press in all directions, it is 

 evident their whole weight cannot be ap- 

 plied against one part or side; while on 

 the other hand it is equally true, that, in 

 some instances, the bottoms of vessels re- 

 ceive a pressure which does not appear 

 to be their due. Thus, in a pan whose 

 base is narrower than its brim, the bottom 

 sustains only the weight of a column equal 

 to its area, multiplied by its height ; yet if 

 the pan be of a bell-shape, having its base 

 broaderthan its brim, the bottom will sus- 

 tain a weight equal to its area,also multipli- 

 ed by its height. Consequently, in a vessel 

 of a conical form, the base would be op- 

 pressed as much as if the sides were cy- 

 lindrical. This is called the hydrostatic 

 paradox ; but will be easily reconciled by 

 the consideration, that if a tube of glass 

 be made with a curved bottom, so as to 

 turnjup in the form of the letter U, but 

 with one leg or part much wider than the 

 other, the water will rise equally, in both. 

 If to each a piston be fitted, their weights 

 being equal, and that one piston be first 

 put into the wider leg of the tube, it will 

 cause the fluid to rise in the other in pro- 

 portion to its weight; but on applying 

 the lesser piston to the corresponding 

 smaller tube, the two will be held in equi- 

 librio. We have indeed further proof of 

 the pressure of water upwards, by means 

 of two boards, whose sides are joined by 

 leather, as in a pair of bellows; these may 

 be of any form, or of any size. At the top 

 of one of the boards cut a hole, and insert 

 a tube of about four or five feet in length, 

 so as to be perfectly tight : place on the 

 board several weights, according to the 

 size of the machine, and pour water into 

 the tube. The upper board will bear up 

 against the weights, provided they be not 



disproportionately heavy; and will admit 

 the water between the top and bottom to 

 the extent admitted by the pliable sides. 

 Some water ought to be poured in before 

 the weights are set on. *A circle of about 

 twenty inches in diameter will thus lift 

 and support three weights, of 100/6. each. 

 Where either air or any other fluid is de- 

 barred from access between two planks 

 annexed in the fwater, the lower one be- 

 ing kept to the bottom forcibly, they will 

 not separate, unless a force equal to the 

 weight of the superincumbent fluid be 

 applied ; because the lateral and superior 

 parts of the fluid are prevented from ex- 

 erting their pressure, except in that di- 

 rection which keeps the two (planks to- 

 gether ; but if the smallest opening be 

 given, the pressure of the atmosphere 

 will urge the fluid between them, and 

 by confining it to act as a wedge, force 

 the upper one to the surface. The com- 

 parative weights of fluids are ascertained 

 by the F^TDROMETER, which see. 



The comparative weight of fluids is 

 given with the table of specific gravities, 

 (see GRAVITY, specific}; but it may be as 

 well to point out in this place, that a gal- 

 lon of proof spirit weighs 7/6. 12oz. avoir- 

 dupois. 



If a vessel contain two immiscible fluids 

 (such as water and mercury), and a solid 

 of some intermediate gravity be immers- 

 ed under the surface of the lighter fluid, 

 and float on the heavier, the part of the 

 solid immersed in the latter will be to the 

 whole solid, as the difference between the 

 specific gravities of the solid and of the 

 lighter fluid is to the difference between 

 the specific gravities of the two fluids. 

 For a body immersed in a fluid will, when 

 left to itself, sink, if its specific gravity be 

 greater than that of the fluid ; if less, it 

 will rise to the surface : if the gravities 

 be equal, the body will remain in what- 

 ever part of the fluid it may be placed. 

 But in the case ad verted to, the one fluid 

 being heavier and the other lighter than 

 the body immersed, it is necessary to 

 combine their gravitieshy the mode above 

 shown. 



Balloons are properly hydrostatic ma- 

 chines, and derive their property of as- 

 cending from the earth into the upper 

 part of our atmosphere entirely from the 

 difference between the specific gravity of 

 the air, or gas, with which they are filled, 

 and the exterior, or atmospheric, air in 

 which they float. The weight of the ma- 

 terials must be taken into consideration ; 

 for unless the specific gravity of the intt- 



