264 APPLICATIONS OF MAEIOTTE'S LAW. 



On the same principle air may be driven by the ex- 

 ternal atmospheric pressure into an 'enclosed space in 

 which the pressure of the air is diminished by its 

 expansion. Contrivances for producing currents of air 

 are often called aspirators. In fig. 183 A, a short piece 

 of a wide glass tube, /, is closed at both ends by corks, 

 which may be. fixed by sealing wax. One cork is bored 

 with two holes, the other with one, and glass-tubes, having 

 respectively the form shown in the figure, pass through 

 the holes ; the lower tube should be from 40 to 80 cm 

 long. Both tubes are connected at the upper end with 

 short pieces of india-rubber tubing, one of which may 

 be closed by a screw pinch-cock, fig. 184 ; in this manner 



FIG, 184 (an. proj. real size]. 



the quantity of water which flow^s through the tube 

 may be regulated at will. The upper end of this india- 

 rubber tube is connected with a siphon, or, by means 

 of a glass-tube, with the reservoir described previously. 

 The other india-rubber tube, through which the air 

 enters into /, is connected with the vessel through 

 which a constant current of air is desired; in the figure 

 it is connected with a washing bottle, and this may be 

 done in every case, when the mere fact is to be demon- 

 strated that a current of air flows through the 

 apparatus. The pinch-cock c being opened, water is 

 allowed to flow through/ until it passes out at d; if 



