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XXVII. On a Hydrostatic Balance. By J. Joly, M.A., B.E., 

 Assistant to the Professor of Civil Engineering, Trinity 

 College, Dublin*. 



[Plate II.] 



r |lHE hydrostatic balance described in this paper will be 

 J- found illustrated on Plate II., reference to which will 

 enable its principle to be the more readily understood. It 

 will be seen from fig. 1 that it consists essentially of a vessel 

 provided with one narrow tubular opening, and suspended so 

 that this tubulure is downward. Within is a second vessel ; 

 this vessel is closed, and is made of such light 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, which is fixed onto the outer 

 vessel by a nozzle so that, when this is screwed off, and 

 the vessel turned up, the space surrounding the float can be 

 readily filled with water. When filled, and the nozzle re- 

 placed, the vessel is hung up, as in the figure, with the 

 tubulure downwards. The diameter of the tubulure being 

 only some 3 millim., 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 

 tend 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 displacement 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. At this 

 point we know that the weight W is in the pan. If the added 

 weight amounts to w, suppose, then a=W — iv. 



Practically, however, W is a quantity variable with the 

 temperature of the float and of the water, their densities 

 altering to different extents. When, therefore, accurate 

 results are required, we cannot assume any constant for the 

 balance, but must determine afresh the force W with each 



* Communicated by the Author. 



