Mr. J. Joly on a Hydrostatic Balance. 269 



half door in the lower part of the case. The case needs no 

 levelling-screws. 



At 6°C. the load carried in the pan, when equilibrium 

 obtains, is 104* 660 grammes. A change of load of 1 milli- 

 gramme now causes displacement, and effects the descent or 

 ascent of the pan. This balance then estimates the weight of 

 100 grammes to an accuracy of 100,000. 



I must here observe, however, that working the balance to 

 this degree of accuracy needs some care. Where estimation 

 to an accuracy of, say, three milligrammes only is needed no 

 special precautions are likely to be required. But with the 

 construction shown in the figure for confining the travel of 

 the pan and float there would seem to be an amount of ad- 

 hesion before the pan is set in motion, which the small force 

 of one milligramme will sometimes be unable to overcome. 

 It is seen in figs. 1 and 2 that the double-eyed link to which 

 the suspension wire is attached moves through — but without 

 contact while moving — an eye which arrests its motion 

 ascending and descending, affording it only about one centi- 

 metre run. The float thus never reaches either to the top or 

 to the bottom of the containing sphere, and adhesion at these 

 points is avoided. The construction of this link is simple, 

 but necessitates the exercise of a little care in the process of 

 equilibration in order that the effect of adhesion at the link 

 may be guarded against. Thus it may be necessary to shake 

 the balance by tapping the case, or to bring the link to the 

 centre of its run with the forceps, when dealing with the last 

 couple of milligrammes. It would, doubtless, be easy to 

 arrange, so that the adhesion necessitating these precautions 

 when weighing with the milligrammes would be eliminated. 

 In fig. 3 two kinds of bearing are suggested : one (a) 

 where there is contact at two points ; the other (b) at one 

 point only, when the link is at either limit of its run. 



All these arrangements secure the advantages of guarding 

 the wire against the effects of a very excessive weight added 

 erroneously, and of rendering a slow motion of displacement 

 more readily observable. If care is taken in adding and 

 removing weights, the wire remains uniformly in the one 

 state of strain. 



If delicate weighing is to be carried out on these balances, 

 it is necessary to use water that has been carefully filtered 

 as sediment will settle down into the tubulure, and cloo- the 

 wire, adhering to it as it emerges from the liquid. 



In the balance depicted in fig. 1 there is no provision for 

 the effects of variation of temperature: any notable change 

 of temperature will, with that construction, result in drawin 



(T 



