270 THE AIR-PUMP. 



exceeds that at ef by the weight of the immersed body, 

 and this excess will keep the body floating when the 

 vessel is inverted. If the body cdef be now pushed 

 somewhat upwards and deeper into the liquid so that 

 some of the latter escapes, the surface ef will be still 

 higher above a b than before, the pressure upon ef he- 

 comes still less while that upon ab remains the same; 

 the difference of the two pressures i-s now greater than 

 the weight of the body, and the latter moves upwards 

 in the inverted vessel while the liquid is forced out. 



For this interesting experiment two test-tubes are required, one 

 wider than the other. The smaller one should be about l m wide 

 and fit rather closely in the larger., but should be capable of moving 

 in it without friction ; the space between them should be very little 

 more than 2 naru , that is., the external width of the smaller test-tube 

 should be about O cn *'5 less than the internal width of the larger. Fill 

 the larger test-tube with water ; hold it upright, place the smaller in- 

 side, and wait until it sinks no longer ; the escape of the water may 

 be facilitated by loosely placing a finger upon the rim of the larger 

 test-tube.. When the whole is in equilibrium, invert it rapidly, 

 by pressing the thumb and the forefinger of the right hand at 

 opposite sides upon the rim of the larger and the side of the 

 smaller test-tube ; if slowly inverted, the water k apt to run out. 

 Hold the upper end of the whole in the position fig, 186 C with tlit 

 left hand, remove the right, and the smaller test-tube will remain 

 at rest ; push it with the linger a few millimetres up into the largei 

 tube, and it will continue to ascend if the finger is withdrawn, at 

 first slowly, bat more rapidly as the difference in height betweer 

 the rim of the larger tube and the curved end of the smaller, wind 

 respectively correspond to aJb and ef, increases. 



i 



28. The Air-pump. Experiments with the Air- 

 jpw^.^-Many experiments on the pressure and the. 

 expansive force of air are made with the air-pump. 

 of which a simple form is represented in fig. 187. ^ 

 tube of metal c c, fig. 187, A and B, with stout wall> 

 the interior of which is bored with great accuracy B< 



