358 ODOKOGEAPHIA. 



until a maximum height is reached equal to the barometric 

 pressure. 



The pump figured Xo. 1 is sometimes made in glass and some- 

 times in brass. The tube A is about oh inches long and f inch 

 diameter: into this is fitted a smaller tube, B, the upper part of which 

 may be straight or take any convenient angle. This tube is generally 

 narrowed to the orifice C, which is inserted about half an inch 

 into the outlet tube D, which is also fitted into A. A connection 

 having been made l.»y means of tubing between the main hydrant 

 and F, the water is forced through C, and in passing into the 

 outlet tube it rapidly creates a vacuum, and A is rapidly exhausted, 

 the air being drawn through the annular space at D. A tube for 

 carrying off' the affluent water may be attached to H if necessary. The 

 inlet tube for admission of air is attached laterally, and is provided 

 with a stop-cock, and when this is opened the air rushes into A, 

 a continuous current being drawn through D to the outlet. In 

 this way a stronger or weaker current of air may be produced 

 according to the pressure of water. During the operation of 

 exhaust, an unvarying flovj of loater from the hydrant must he main- 

 tained, otherwise there is the probability that water will be drawn 

 back into the exhausted vessel, owing to the reduction of pressure, 

 or rather on account of the transference of the pressure from the 

 pump to the vessel, and for the same reason the stop-cock at E 

 must he turned off hefore the flow of water is stopped at the 

 hydrant. If it is desirable to have a vacuum-guage attached : it 

 should be fitted between the stop-cock and the vessel to be 

 exhausted. 



It has been recommended to interpose a bottle between the 

 aspirator and the condenser, in case the water should flow back 

 from the aspirator owing to reduction of pressure from any cause. 

 This bottle should carry a well-fitting india-rubber bung with 

 three holes, through which pass glass tubes, one of which is con- 

 nected with the aspirator, one with the condenser, and the third 

 with a long bent glass tube ('30 or more inches long), the open end 

 of which dips into a vessel of mercury ; by means of this tube the 

 degree of exhaustion can be ascertained, for the mercury rises in 

 the tube as the pressure decreases. 



Other forms of exhaust pump very convenient for laboratary use 

 are constructed in glass. The operation can then be watched with- 

 out a vacuum <?au<ze attached — as when the bubbles of air are not seen 



