268 Mr. J. Joly on a Hydrostatic Balance. 



the extent of surface, and as this area increases at a slower 

 rate than the volume inclosed by it, it appears that the 

 tediousness attending operations is, again, not fairly assumed 

 to be an attendant disadvantage which increases proportionally 

 with increase of power of the balance. The effect is indeed, 

 probably, complicated by the presence of currents or eddies 

 in the liquid. 



As regards the effects of solid friction, contact between the 

 movable and immovable parts might, indeed, be altogether 

 avoided. Thus we might attach the wire externally to a flat 

 cantilever, or flat spiral-spring, so that it is retained in the 

 centre of the tubulure by the horizontal rigidity of the spring, 

 while the spring may possess such small vertical rigidity as 

 not to interfere with the sensibility of the balance. It will 

 be seen, however, from the figures, that this plan is not re- 

 sorted to. It appears, indeed, unnecessary to do more than 

 guard against contact down the wall of the tubulure ; and this is 

 provided for in the little projecting collar placed at the point 

 where the tubulure meets the wider nozzle. The diameter of 

 the passage here provided for the wire is about 1*5 millim. ; 

 the tubulure is about 3 millim. in diameter. The edge of 

 the collar is sharpened to a knife-edge all round, but just 

 burnished smooth. With this arrangement, if the precaution 

 be taken of using a smooth piece of wire, there appears but 

 little retardation due to friction : this, doubtless, is partly due 

 to the position of the collar within the liquid, the liquid acting 

 as a lubricant. The effect of substituting a collar of burnished 

 agate for the brass collar has been tried as in the balance, 

 fig. 2, but with hardly appreciable gain in freedom. This 

 little balance (fig. 2) is represented half its actual dimensions. 

 The float is a sphere of thin blown glass, weighing about 

 12 grammes, its diameter being about 6'3 centimetres. The 

 outer vessel is of brass, parting, in a screw-joint, into hemi- 

 spheres. For convenience of weighing by replacement, a 

 double pan of thin brass is attached to the wire. This pan, 

 together with the suspending gear, weighs about 11 grammes. 

 The suspending wire traversing the surface of the liquid is of 

 brass ; its diameter is 0*09 millim. Its breaking strength is 

 403 grammes. 



The balance is protected from draughts and sudden changes 

 of temperature by a glass case, from the roof of which it 

 depends, hanging freely *. The weights are introduced at a 



* For delicate work the further precaution of preserving the whole in 

 a chamber not exposed to sudden fluctuations of temperature is, I find, 

 necessary. Trouble from this source might be guarded against by sur- 

 rounding the outer vessel with a non-conducting covering. 



