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XXXIII.— ON A HYDEOSTATIC BALANCE. By J. JOLY, B.E., 



Assistant to the Professor of Civil Engineering, Trinity 

 College, Dublin. (Plate VII.) 



[Read, June 9, 1886.] 



The Hydrostatic Balance described in this Paper will be found il- 

 lustrated on Plate VII., reference to which will enable its principle 

 to be the more readily understood. It will be seen from figure 1 

 that it consists essentially of a vessel provided with one narrow 

 tubulure opening, and suspended so that this tubulure is down- 

 ward. Within is a second vessel ; this vessel is closed, and is made 

 of such slight material that it floats buoyantly in water. 



A fine wire is attached to the lower end of this inner vessel, and 

 passes through the tubulure. The tubulure of the outer vessel is on 

 a nozzle which, when screwed off, and the vessel turned up, enables 

 the space surrounding the float to be readily filled with water. 

 When filled, and the nozzle replaced, the vessel is hung up, as in 

 the figure, with the tubulure downwards. The diameter of the 

 tubulure being only some 3 mms., there is perfect security 

 against outflow : indeed the apparatus may be shaken or rolled 

 about upon a table with impunity. When the balance is hung it 

 is obvious that the inner vessel or float, in virtue of its buoyancy, 

 will be urged to ascend within the liquid, and if, as in fig. 2, we 

 hang a pan on the wire, and load weights on the pan, we find that 

 we can add weights up to a certain point, when the pan descends 

 with the sinking of the float within the vessel. This weight— just 

 adequate to cause the pan to descend— we assume for the present 

 to be constant, and equal W, suppose. W is evidently equal to 

 the weight of a mass of water having a volume equal to the dis- 

 placement volume of the float, less the weight of the float, of the 

 wire, and of the pan attached to the wire. We can evidently 

 ascertain, now, the weight of any mass not heavier than W. It 

 is as if we were using a balance, one arm of which was loaded 

 with an unalterable weight W. Thus, we place the substance to 

 be weighed on the pan, and add weights till the pan descends. 



