250 On connecting and disconnecting a Receiver. [Mar. 18, 



aa to fit a conical opening to the upgoing spirit-bore tube AM ; and at 

 M the apparatus which is to be exhausted would be blown on. At C 

 there is an ordinary cup and stopper, ground to a very perfect fit, 



and the joint at C is made perfectly air-tight in the usual way by 

 pouring mercury into the cup. At the lower extremity of the part 

 CD is a stopper closed at the bottom, but with a fine hole drilled atp; 

 and in the tube of the cup EE there is a fine groove cut, which reaches 

 half way up the ground part of the tube and stopper to p ; but above 

 p there is a sufficient length of grinding to make a perfect joint.* 

 When the hole p is turned round to meet the groove, there is com- 

 munication through and through the tap, that is to say, from the 



* This cup and stopper form Mr. Grimingham's ingenious tap. 



