454 



Lord Rayleigh. On the 



[Feb. 18, 



FIG. 2. 



~TO GENERATOR. 



TO PUMP. 



BLOW 

 Off. 



tube, which, being out of the main current of gas, might be supposed 

 to harbour impurities. After this the pump would be cut off, the 

 connexion with the generator re-established, and, finally, the tap of 

 the globe cautiously opened. 



The operation of filling usually occupied from two to three hours. 

 When the gas began to blow off under an excess of pressure repre- 

 sented by about half an inch of mercury, the blow-off cistern was 

 lowered so as to leave the extremity of the tube free. For two 

 minutes the current of gas from the generator was allowed to flow 

 through, after which the generator was cat off, and the globe lefb in 

 simple communication with the atmosphere, until it was supposed 

 that equilibrium of pressure had been sufficiently established. Doubts 

 have at various times been felt as to the interval required for this 

 purpose, If too little time is allowed, there will remain an excess of 

 pressure in the globe, and the calculated weight of the filling will 

 come out too high. On the other hand, an undue prolongation of the 

 time might lead to a diffusion of air back into the globe. In a special 

 experiment no abnormal weight was detected after half an hour's 

 communication, so that the danger on this side appeared to be small. 

 When the passages through the taps were free from grease, one or 



