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carefully ground to receive the lower surface of the ring UV. 
This ring falling on the flat surface of the bottom closes the 
apertures aa when it descends, but permits the passage of air 
upwards. 
Upon the upper surface of this ring are fixed balls (UV 
either solid or hollow as we use mercury or oil), the uses of 
which will be explained hereafter. 
Through the upper hemisphere of GO' the tube TT' 
passes airtight, and at SC has a stop-cock interposed, above 
which it terminates in PP'. 
L V. The lower valve consisting of a steel sphere pressing 
upon the concave end of the tube TT', raised by floating on 
M, the mercury, and prevented from falling by an open cage 
which partially surrounds it. 
H. The hole for the bolt connecting the lower end of the 
piston with a lever or any other source of power. 
M. The dotted lines indicating mercury or oil in the 
tubular piston rod PR, the same covering the upper surface 
of the piston, and the quantity required for the flotation of the 
ring UV in the globe GG'. 
PP. Is a solid metallic piston most accurately fitted to 
the internal surface of the cylinder. 
PR. Is a hollow piston rod attached firmly to the centre 
of PP, and open at its upper end. Below it is hermetically 
closed, flattened, and perforated for a bolt, connecting it with 
the motive power. 
PP'. 'Phe plate upon which is placed the vessel which is 
to be emptied of air. SC, stop cocks. 
UV. 'Phe upper valve contained in the globe GG' con- 
sisting of a flat ring sui’rounding the tube TT, and fitting 
accurately upon the flat bottom of the globe and closing its 
small tubes aa'. ZZ', receivers. 
When mercury is used, the small globes attached to its 
upper surface are required to sink the lower surface of the 
ring below the level of the liquid ; but when oil is employed 
