IMPROVED AIR-PUMP. - 203 



the bottom of the hole. Then it Aides down along the rod Defcriptlou of 



E T, and the air from the receiver has now free accefs to the Mendelfiohn's 



improved air- 

 cvlinder. When the pifton returns it lifts the rod E T, and pump. 



thus (huts up the valve. Then the pifton Aides again along 



the rod up to the top of the cylinder, condenfing the air 



above it, which air, by the Ieaft condenfation, opens a valve e, 



Fig. 2, and efcapes freely into the atmofphere. This laft 



valve has neither fpring or additional weight to ftiut it, but 



ftiuts by its own weight (about a quarter of an ounce) as 



foon as the pifton is arrived to the top of the cylinder. 



The cylinders are made of glafs, and the piftons of tin, fo 

 well fitted as to be air-tight, without the inlerpofitiun of any 

 leathers. The friction of thefe two bodies is fmall beyond ex- 

 pectation, a fufficient proof that they will be durable. They 

 poflefs the further advantage of being capable of Aanding for 

 even fix months, after which time they will ferve without 

 being cleaned or repaired, becaufe they are not liable to be 

 corroded by the oil which they contain, an inconvenience too 

 general in brafs cylinders. After all, if the pr'efent pump 

 Aiould want cleaning, it is an eafy operation to take off the 

 top piece gh, by unfcrewing the nuts Hand I, when this 

 piece, with all the apparatus upon it, will come off. Then 

 each cylinder may very eafily be Aid off from the pifton, 

 wiped out and replaced, afier having greafed it? infide with a 

 little of the cleaneft fweet oil : The top is then to be put again 

 in its place, and the two nuts H and I being fcrewed upon 

 it, the inftrument is ready. Neither racks or pinion need to 

 be taken out of their places, the cylinders ftanding above 

 them. 



The cock is conftructed (o, that, being in the fitnation re- 

 prefented in Fig. 1, the communication is open between the 

 cylinders, the receiver, and the barometer-gauge, and, by a 

 quarter of a revolution, the cylinders are excluded, the re- 

 ceiver and gauge being flill left in communication. A little 

 ftopper in Fig. 3, ground into the cock, being open, air is 

 admitted to the receiver if required. 



The receiver-plate is of glafs ground flat, as was men- 

 tioned before : this will be found preferable to brafs, becaufe 

 cleaner, and never corroded by acids or water j it will betides 

 often prove very convenient in making experiments on elec- 

 tricity in the vacuum. 



The 



