258 



APPLICATIONS OF MARIOTTE'S LAW. 



high, and as this pressure decreases upwards, it amounts at b to 

 10 m 20 cm =9 m< 8 only, and as there is equilibrium, the pressure of 

 the enclosed air is also equal to that of a column of water 9 m% 8 high. 

 Let the pipette now^e held in a horizontal position without removing 

 the finger from c, and suppose the volume of the enclosed air to he 

 50 CC . In the horizontal position of the pipette, fig. 178 B, both ends 

 of the column of water must evidently be under equal pressures if 

 there is to be equilibrium ; but the pressure at the outer end is lO 8 " 1 , 

 while that at the inner is only 9 m *8 ; hence the column moves, as 

 shown in the figure, and the volume of the enclosed air contracts, 

 until the pressure inside is the same as outside. By Mariotte's law, 

 if the .pressure increases from 9*8 to 10, the volume is diminished 

 in the inverse proportion, hence the volume of the enclosed air is 

 only 49 CC , for 10 : 9'8 : : 50 : 49. The column of -water consequently 

 moves towards the bulb, and the diminution of the volume of the 

 enclosed air by l cc will be distinctly observed at ,, as shown in 

 fig. 178 B. If the pipette is held vertical, the air will again expand, 

 and the water will move to the end of the instrument. 



In this experiment the pipette must be supported by holding the 

 tube, and not the bulb, between the fingers ;. otherwise the air in 



FIG. 179 (I real 



Q real size}. 



the bulb will .expand by the warmth of -the <hand, and the result 

 of the experiment will be very different. 



A welU3mown toy, the magic funnel, of which fig. 179 gives a 

 section, acts on the same principle. Two funnels of metal are placed 

 one inside the other and soldered together at the upper rim ; the tube 

 of the larger funnel projects l mm or 2 mm beyond the smaller, and 



