270 Mr. J. Joly on a Hydrostatic Balance. 



air into the containing vessel, or expelling some of its con* 

 tents. For occasional use, where the balance is filled and 

 emptied frequently, as in travelling, this will cause no trouble 

 as the effects of small temperature change will be inconsider- 

 able. Thus, taking the case of concentric spheres of brass, 

 the inner displacing 179 grammes (diam. = 7 centims.), the 

 outer having a diameter of 9 centims., and the intervening 

 space filled with water, the effect of 1° C. change is a displace- 

 ment of 14 cubic millimetres of water, about half a drop. 

 The entry of a little air obviously does no harm : it simply 

 rises to the top of the vessel, and in no way interferes with 

 the truth or capacity of the machine. A little expelled water 

 is easily dried off. 



If it be desired, however, to render the arrangement nearly 

 permanent, so that the operation of filling need but very 

 seldom be repeated, the effects of temperature in expelling 

 water or drawing in air must be met in some way. In the 

 balance of fig. 2 this is done by providing the expansion 

 reservoir shown surrounding the tubulure, and which com- 

 municates with the interior of the sphere by the narrow tube 

 nearly reaching to the bottom of the reservoir, as shown in 

 the figure. The large surface of water exposed in this 

 reservoir bears to stand at a level above or below, by a couple 

 of millimetres, the surface level of the water in the tubulure, 

 as in the well-known experiment on capillarity in com- 

 municating tubes of very unequal bore. Hence, with rise of 

 temperature the reservoir receives the expelled water ; with 

 fall of temperature it parts with some of its contents, and no 

 water is lost. The annular reservoir communicates with the 

 air by a very small perforation, and the loss by evaporation is 

 very small. 



To enable the balance to be readily filled, the ring by which 

 it is suspended is arranged to screw out of a little tubulure 

 communicating with the interior. The balance is filled in a 

 few seconds by screwing out this ring, and immersing the 

 sphere in a vessel of water ; when no more bubbles ascend 

 through the tubulure, the ring is screwed home, while the 

 tubulure is still beneath the surface of the water. On with- 

 drawal a little water runs out at the lower tubulure, till the 

 head in the reservoir has been syphoned down to a position 

 of equilibrium with the surface-tension at the tubulure ; the 

 head is now still further reduced by applying a little bibulous 

 paper to the tubulure, in order to provide for a subsequent 

 rise in temperature. 



If the float be made of a substance having a low coefficient 

 of expansion, such as glass, and the containing vessel be of 



