342 LECTURE XXIX. 



and many other liquids, have the property of emitting a vapour which pos- 

 sesses a very sensible degree of elasticity; so that if either water, or any 

 moist substance, be present under the receiver, it will be impossible to pro- 

 cure a total absence of pressure, the short mercurial gage commonly stand- 

 ing at the height of at least half an inch, in the best pumps. Hence, the 

 vacuum may be made more perfect when the receiver is ground to the plate 

 of the pump, with the interposition of an unctuous substance, than when 

 it is placed on wet leather, as it has sometimes been usual to do. The 

 quantity of atmospherical or incondensable air actually existing in the re- 

 ceiver, whether mixed with vapour or alone, is measured by means of 

 Smeaton's pear gage, which is left open under the receiver during the ex- 

 haustion, and. having its orifice then plunged, by means of a wire passing 

 through a collar of leather, into a bason of mercury, receives, upon the 

 readmission of the air, as much of the mercury as is sufficient to fill it, leaving 

 only in a tube rising from the neck of the gage, the small quantity of air 

 which had before filled the whole cavity, so that from the space occupied 

 by this air, compared, by means of previous measurements, with the capacity 

 of the gage, the degree of exhaustion of the pump with respect to a'lr may be 

 estimated. It is said that in an air pump of Cuthbertson's construction, such a 

 rarefaction has been procured that the air sustained but one hundredth part 

 of an inch of mercury, that is, it was expanded to nearly 3000 times its ori- 

 ginal bulk. The pear gage often indicates a much more complete exhaus- 

 tion, but this measurement relates only to the quantity of dry air presenti 

 (Plate XXIV. Fig. 328.) 



♦ 



A condenser is the reverse of an air pump; and sometimes the same 



machine is made to serve for both purposes ; but the condenser requires more 

 strength than the air pump, and less delicacy. The gage for measuring the 

 degree of condensation is a small portion of air contained in a graduated 

 cylindrical tube, the space that it occupies being indicated by a drop of 

 mercury which confines it. (Plate XXIV. Fig. 329-) 



Diving bells were formerly supplied with air by means of barrels let down 

 continually from the surface of the water, and taken into the bell by the 

 divers ; but it is now more usual to force down a constant stream by means 

 of a pump resembling a condenser in its construction and operation : the 



