262 LECTURE XXIX. 



total absence of pressure, the short mercurial gage commonly standing 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 receiver, 

 whether mixed with vapour or alone, is measured by means of Smeaton's 

 pear gage,* which is left open under the receiver during the exhaustion, 

 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 air may be es- 

 timated. 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,t that is, it was expanded to nearly 3000 times 

 its original bulk. The pear gage often indicates a much more complete 

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

 present.* (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 measur- 

 ing the degree of condensation is a small portion of air contained in a gra- 

 duated 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 heated air is suffered to escape by a stopcock at the upper part of 

 the bell. When proper care is taken to lower the machine gradually, 

 the diver can support the pressure of an atmosphere of twice or thrice 

 the natural density. It would be advisable that every diver should be 

 provided with a float of cork, or with a hollow ball of metal, which 

 might be sufficient to raise him slowly to the surface, in case of any 

 accident happening to the bell ; for want of a precaution of this kind, 

 several lives have been lost from confusion in the signals. (Plate XXIV. 

 Fig. 330.) 



Bellows are commonly made of boards connected by leather, so as to 

 allow of alternately increasing and diminishing the magnitude of their 

 cavities, the air being supplied from without by a valve. The blast must 



* Ph. Tr. 1752, p. 420. 



t Cuthbertson, Description of an improved Air Pump, 1783, 38. 



* See Nairne's Account of some Experiments made with an Air Pump, Ph. Tr. 

 1777, p. 622. Roz. Journ. xi. 159 ; xxv. 261. 



See Halley's Art of Living under Water, Ph. Tr. 1716, p. 492; 1721, p. 177. 

 Healy on Diving Bells, Ph. Mag. xv. 9. 



