ANOMALOUS BEHAVIOUR OF DELICATE BALANCES, ETC. 



409 



differences in the temperature of the two arms will and must very naturally occur. 

 Every time a weight is introduced into the pan a current of warm air will ascend 

 from the observer's hand ; and this, on coming into contact with the arm of the beam 

 above, will inevitably result in a relative lengthening of that arm, and a consequent 



uncertainty in the true value of the weight sought. Now, although these relatively 

 large disturbances gradually die away and finally disappear, they only do so after a 

 considerable lapse of time ; but the smaller ones, to which attention has already been 

 directed, still remain, and may therefore introduce a very slight but perceptible 

 degree of inaccuracy. 



When, however, the beam is protected as described, it is never exposed to the open 

 air during the operations of loading and weighing ; and of the heat radiated from the 

 observer's hand in the direction of the beam, not more than about 10 per cent, will 

 enter the highly reflecting base-plate. The heat-rays that actually enter the base- 

 plate will be distributed within the metal in all possible directions with great rapidity ; 

 hence the small radiations from the upper surface of the plate towards the beam will 

 be approximately uniform ; and therefore the possibility of appreciable unequal 

 heating of the two arms of the balance is precluded. 



The base-plate itself guards the beam almost completely against the effects of any 

 ascending convection currents. But the necessary holes for the passage of the pan 

 suspensions and pointer render possible the introduction of an error from this source. 



It was with the object of minimising the slight disturbances that might be thus 



VOL. CCX. A. 3 G . 



