ILLUSTRATED BY EXPERIMENTS. 159 



Let the little figure in the inverted jar AB represent the " Cartesian 

 Devil," surmounted by a bag-like crown of great size in proportion to 

 his body, filled with some very light substance, such as air, and we 

 shall therefore suppose that air is the body which it contains. The imp 

 himself must be constructed of glass or enamel, so as to possess the 

 same specific gravity as water, and therefore to remain suspended in 

 the fluid. A 



At the bottom of the vessel or jar, is 

 placed a diaphragm or bladder, that can be 

 pressed upwards by applying the finger to 

 the extremity of a lever eo, moving round o 

 as its fulcrum or centre of motion. The pres- 

 sure applied at a is communicated through the 

 water to the bag of air at m, which is thus 

 compressed, and consequently, the specific 

 gravity of the figure is increased, by which 

 it sinks to the bottom of the jar. 



By removing the pressure on the dia- 

 phragm at a, the figure will again ascend, 

 so that it may be made to oscillate, or rise 

 upwards and sink downwards alternately, and to dance about in the 

 jar, without any visible cause for its movements. 



Other figures, such as fishes made of glass, are sometimes employed 

 in this experiment, but the principle is nevertheless the same, and 

 when a common jar is used, the pressure is applied to the upper 

 surface of it, as at A. 



EXPERIMENT 6. The pressure of fluids at very great depths, is 

 beautifully illustrated by an experiment which has often been made 

 at sea, where the water is sufficiently deep to admit of the principle 

 being accurately put to the trial. 



The experiment is this : an empty bottle well corked is made to 

 descend to a great depth, on which the pressure of the fluid becomes 

 so great as to drive in the cork, and the bottle when brought up is 

 always filled with water. Several methods have been employed to 

 prevent the cork from being driven inwards, but although this has 

 been effected, yet the bottle on being brought to the surface, is con- 

 stantly filled with the fluid in which it has been sunk. 



The following experiments of this sort, are detailed by Mr. Campbell, 

 the author of " Travels in the South of Africa," published at London 

 in the year 1815; the experiments were tried on his voyage home- 

 wards from the Cape of Good Hope. He drove very tightly into an 



