PNEUMATICS. 159 



end to c, where the piston-rod may be always left 

 in. Place your feet on the pin, and the whole gun 

 serves instead of the handles i i (Fig. 8.) to con- 

 dense the air in the barrel. 



The magazine air-gun differs from the common 

 one, only by having a serpentine-barrel, which 

 contains 10 or 12 balls; these are brought into the 

 shooting barrel successively, by means of a lever ; 

 and they may be discharged so fast as to be nearly 

 of the same use as so many different guns. 



THE DIVING BELL. 



To illustrate the principle of this machine, take 

 a glass-tumbler, and plunge it into water with the 

 mouth downwards ; you will find that very little 

 water will rise into the tumbler ; which will'be very 

 evident, if you lay a piece of cork upon the sur- 

 face of the water, and put the tumbler over it ; for 

 you will see, that though the cork should be 

 carried far below the surface of the water, yet that 

 its upper side is not wetted; the air which was 

 in the tumbler having prevented the entrance of 

 the water ; but as air is compressible, it could not 

 entirely exclude the water, which, by its pressure, 

 condensed the air a little. 



The first diving-bell of any note was that made 

 by Dr. Halley. It was in the form of a great bell, 

 and was coated with lead, so as to sink empty; 

 weights being distributed about the lower part, to 

 keep it in an inverted position. It was three feet 

 wide at top, five feet wide at bottom, and eight 

 feet high. In the top was fixed a strong clear 

 glass, to let in the light from above, and likewise a 

 cock, to let out the hot air that had been deprived 

 of its vital principle by repeated breathing. It 

 was suspended from the mast of a ship, in such a 



