352 Scientific Proceedings, Royal Dublin Society. 



link is at either limit of its run. The last plan, too, would have* 

 the advantage of starting motion with the suspension wire in a 

 central position. 



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 clog the wire, adhering to it 

 as it emerges from the liquid. 



Experiments extending over several weeks showed that there is- 

 no reason to expect that under varying conditions of atmospheric 

 pressure bubbles of air would be given off by the water to adhere 

 to the float, provided the water is not supersaturated with air in 

 the first instance. In my experiments the action, in this direction,, 

 of the variations of pressure was represented on an exaggerated 

 scale under the air-pump to tensions of 50 mms. of mercury. This 

 tension failed to withdraw visible bubbles from water previously 

 freely exposed for a long period to the atmosphere. What dis- 

 solved air was withdrawn probably slowly emanated at the surface. 

 The water was contained in a beaker, the float being represented 

 by an immersed spherical glass vessel. 



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

 effects of variation of temperature : any notable change of tempera- 

 ture will, with that construction, result in drawing air into the 

 containing vessel, or expelling some of its contents. For occasional 

 use, where the balance is necessarily filled and emptied frequently, 

 as in travelling, this is of no importance, and will cause no trouble, 

 if the precaution is taken of filling it with water appreciably at air- 

 temperature. The effects of temperature change will, indeed, be 

 inconsiderable. Thus, taking the case of concentric spheres of 

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

 outer having a diameter of 9 cms., and the intervening space filled 

 with water, the effect of 1° C. change is a displacement 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. 1 



1 If the float be made of a substance having a low coefficient of expansion, such as 

 glass, and the containing vessel be of material having a high coefficient of expansion, 

 as brass, such dimensions may be given to the apparatus that the water space shall, 

 with change of temperature, increase at the rate of expansion of water. In other 



