194 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



taining a small receiver, from the hole at the centre of this 

 plate there is a canal communicating with that which goes 

 from the large receiver to the barrels ; under this receiver is 

 a small tube filled with mercury freed from air, this is called 

 the short barometer-gauge. As the air is taken out of the 

 small receiver at the same time that it is taken from the larger 

 one, the descent of the mercury in the tube will point out 

 the degree of rarefaction in the receivers ; the mercury does 

 not begin to descend in this tube till near three-fourths of 

 the air have been extracted ; and the air is said to be as 

 many times rarer than the atmosphere, as the column of 

 mercury sustained in this tube is less than that of the mer- 

 cury, at that time, in the common barometer. 



Price, $40.00 and $45.00. 



Double Barrel Air Pump. (Fig. 180, as below.) This 

 cut is a representation of the usual style of the double barrel 

 air pump described, with the omission of the gauge, and 

 being made of a smaller size. Price, $22.50 and $25.00. 



Fig. 180. 



Pike's Improved Double Barrel Air Pump. (Fig. 181, 

 page 195.) This instrument, in its general construction, 

 corresponds to the preceding one described (Fig. 179), 

 but of a larger and more elegant finish, the frame being sub- 

 stantially and elegantly finished, of rosewood or mahogany 

 highly polished, conveniently mounted from the floor on 

 four feet, and having the plate raised on a stage supported 

 on four pillars, beneath which the gauge is attached, which 

 is the same as described in the large lever pumps ; the 



