214 FERGUSON'S LECTURES 



LECT. the water in the phial ; from whence it will make its 

 ^2J\_ way, with the rest of the air in the receiver, through the 

 air pipe G G and valves a and b, into the open air. 

 When it has done bubbling in the phial, the ball is suffi- 

 ciently exhausted ; and then upon turning the cock k, the 

 air will get into the receiver, and press so upon the sur- 

 face of the water in the phial, as to force the water up 

 into the ball in a jet, through the neck c d; and will fill 

 the ball almost full of water. The reason why the ball is 

 not quite filled, is, because all the air could not be taken 

 out of it ; and the small quantity that was left in, and 

 had expanded itself so has to till the whole ball, is now 

 condensed into the same state as the outward air, and 

 remains in a small bubble at the top of the ball ; and 

 so keeps the water from filling that part of the ball. 



6. Pour some quicksilver into the jar D ; 

 and set it on the pump-plate near the hole i; 

 then set on the tall open receiver A B, so as 

 to be over the jar and hole ; and cover the re- 

 ceiver with the brass plate C. Screw the open 

 glass tube / g (which has a brass top on it at 

 A) into the syringe H, and putting the tube 

 through a hole in the middle of the plate, so 

 as to immerse the lower end of the tube e in 

 the quicksilver at D, screw the end h of the 

 syringe into the plate. This done, draw up the 

 piston in the syringe by the ring /, which will 

 make a vacuum in the syringe, below the piston; 

 and as the upper end of the tube opens into the 

 syringe the air will be dilated in the tube be- 

 cause part of it, by its spring, gets up into the 

 syringe ; and the .spring of the undilated 

 air in the receiver acting upon the surface 

 of the quicksilver in the jar, will force part of it up 

 into the tube : for the quicksilver will follow the piston 

 in the syringe, in the same way and for the same reason 

 that water follows the piston of a common pump when 

 it is raised in the pump-barrel ; and this, according to 



