M. Achille Cazin on Internal Work in Gases. 89 



the paper and traces a right line when it is motionless. The 

 pencil is carried by an elastic plate, h, to which the armature, t, of 

 an electromagnet is adapted. The moment the circuit is closed 

 the trace of the pencil on the paper changes its position, and it is 

 easy to tell from this the time of the closing of the circuit. 



We must first mark on the paper the instant the opening of 

 the stopcock C commenced. To this end the back part of the 

 stopcock carries a metal rod, 7, which moves with it and which 

 communicates with one of the poles of the battery by the con- 

 ducting-wire Imnop. When the stopcock begins to open, the 

 rod / immediately plunges into the mercury q, which communi- 

 cates with the other pole of the pile by the wire qvks. The 

 circuit is closed at this moment, and the pencil is displaced. 



The moment at which the manometer Q communicates with 

 the reservoir B must next be marked on the paper. For this 

 purpose the head of the rod of the valve R rests on a little lever 

 tm i which supports an iron rod plunged in mercury, n. So 

 long as the valve E, is not opened the circuit is closed ; but the 

 moment the stopcock C opens this valve the iron rod m is raised 

 and no longer touches the mercury, the current ceases, and the 

 pencil g returns to its initial position. 



Lastly, the positions of the level in the manometer Q at some 

 stated periods must be noted. For this purpose the observer reads 

 the number on the scale corresponding to the level in the mano- 

 meter, and at the same moment plunges the extremity u of the 

 conducting-wire v u into the mercury o. He thus closes the circuit 

 upskvuiov an instant only; and the displacement of the pencil 

 leaves a trace on the paper. To measure the time which has 

 elapsed between two displacements of the pencil, the distance of 

 the corresponding traces on the paper must be measured and the 

 movement of the paper known. 



This movement not being quite uniform, it must be determined 

 by some special experiments. To accomplish this the weights were 

 caused to descend, and the circuit of the electromagnet closed, 

 every five seconds, for example, after the departure of the weights. 

 We thus knew to what periods the successive displacements of 

 the pencil marked on the strip of paper corresponded ; and this 

 kind of graduation served afterwards to measure the times in 

 each experiment with sufficient accuracy. 



In several series the metal reservoir B was replaced by a glass 

 vessel of twice the capacity (60*617 litres) ; the circulation of 

 the water was then impossible, and the two reservoirs were either 

 enveloped in a layer of thick sawdust or left uncovered. In 

 these cases irregularities occurred, resulting from the inequality 

 of temperature of the two reservoirs. But such irregularities 

 are of little importance in the series in question. 



