264 Prof. Potter on some properties of the Air Thermometer, 



dilated by Poisson's rule. I was, I believe, the first to point 

 out, in my paper in the Philosophical Magazine for September 

 1853, that there is a fundamental objection to the method, in 

 whatever form it is used, namely, that it is a dynamical experi- 

 ment, and the dynamical result is neglected. In the paper I say, 

 " the effect of the momentum of the moving column of water in 

 the gauge in passing from one position of equilibrium to another 

 has been attributed to the effect of heat developed in the sudden 

 condensation." 



It is clear that the discussion of the dynamical problem is 

 needed before we can judge of the thermometric results ; and the 

 desirable verification of Poisson's formula which MM. Clement 

 and Desormes invented may turn out after all to be nothing 

 more than fudge verification, although universally adopted in our 

 higher treatises on hydrostatics and hydrodynamics as valid veri- 

 fication. 



In the forms of the experiment which have been used, the 

 mathematical discussion is very simple if we consider only the 

 liquid in the tube of the gauge or manometer to be subject to 

 pressures which communicate motion to it ; but we must remem- 

 ber that motion is also produced in the liquid of the cistern ; and 

 this we cannot discuss for want of the requisite data. 



Taking the simplest case first, 



Prop. To determine the motion of the liquid in the tube of an air 

 thermometer arising from a sudden rarefaction of the air in the 

 receiver when the tube is horizontal. 



Let C D be the cistern containing the liquid mercury, water, 

 or spirit ; F the bulb or receiver of the thermometer, containing 



a, 



h 



A 



P 







A i 







BC 



1 





Tl 



• 







air, as well as its tube EAB as far as A. The tube passing into 

 the cistern below the surface of the liquid a b as in the figure, 

 let the space at the first position of equilibrium which is filled 

 with the liquid be A B. 



Then if the bulb has a pipe furnished with a stopcock G, and 

 is connected by it with an air-pump or other exhausting appa- 

 ratus, and a portion of air is suddenly withdrawn and the stop- 

 cock G then closed, the liquid A B will be put into motion, since 

 the pressures at its extremities A and B will be then unequal, 

 and the pressure of the atmosphere together with that of the 

 liquid in the cistern will cause an additional portion of liquid to 

 enter the tube, keeping it filled. Let be the place where the 



