APPLICATIONS OF MARIOTTE'S LAW. 265 



e is not connected with a reservoir but with a siphon, 

 then b must be closed with the fingers and the 

 mouth applied at d, until water has been sucked 

 through the apparatus and flows out at d. It is evi- 

 dent that as long as the same quantity of water flows 

 out at d as that which passes through c, no alteration 

 takes place in the volume of the air enclosed in J\ and 

 hence no alteration in the pressure of the air, and 

 nothing will happen when b is opened. But if now 

 the passage in the tube # be made narrower by turning 

 the screw of the pinch-cock, more water will flow out 

 it d than can enter the space / from above, the air in 

 f will therefore expand, and its pressure will diminish, 

 initil the pressure of the external air exceeds' it suffi- 

 ciently to cause a current of air to enter the open 

 md of the tube ^, to overcome the resistance of the 

 column of water g h, and to flow into the space /, 

 .vlience it is carried away again by the water through d, 



If a weak current of air is required, the water should pass 

 h rough e merely in drops ; but as in that case the water is likely 

 ;0 flow down on the sides of the space / without sweeping any 

 ir out of it, the tube should have a loop below /, and close to it, 

 jS shown in fig. 183 B. The loop collects the descending drops 

 util they fill the section of the tube, the water that has accurnu- 

 jited then flows out all at once and pushes a, certain quantity of air 

 sefore it. 



i The screw pinch-cock, fig. 184, is made of a piece of stout brass 

 'ire, 16 or 18 cm long, one end of which is hammered flat, and 

 ijftened in the spirit-lamp ; a hollow (not a hole) is then punched 

 ;i it for the reception of the end of the screw. The other end of 

 wire is formed into a ring, about 4 mm wide, upon which is 

 >Klcred a round flat piece of brass, 8 or 10 mm wide and 2 or 3 mm 

 tick. This piece has a hole drilled in the middle and is tapped 

 fit a screw which has the finest thread that can be cut with 

 lie screw-stock ; the wire is next bent into the proper form 

 id hammered flat and elastic ; to prevent its becoming soft by 



