SCIENTIFIC RECREATIONS. 



Fig. 52 represents the experiment of the diving-bell, which is so simple, 

 and is explained below. It belongs to the same category of experiments 

 as those relating to the pressure of air and compression of gas. Two or 

 three flies have been introduced into the glass, and they prove by their 

 buzzing about that they are quite at their ease in the rather confined space. 



The DiviNG-BELL in a crude form appears to have been used as early 

 as 1538. It was used by two Greeks in the presence of the Emperor 

 Charles V., and numerous spectators. In the year 1720 Doctor Halley 

 improved the diving-bell, which was a wooden box or chamber open at the 

 bottom. Air casks were used to keep the inmate supplied with air. The 

 modern diving-bell was used by Smeaton in 1788, and was made of cast 

 iron. It sinks by its own weight. The pressure of the air inside is suffi- 

 cient to keep the water out. Air being easily compressed, it is always 



Fig- 54- The Hand Fire-Engine. 



pumped in to keep the hollow iron " bell " full, and to supply the workmen. 

 There are inventions now in use by which the diver carries a supply of air 

 with him on his back, and by turning a tap can supply himself for a long 

 time at a distance from the place of descent, and thus is able to dispense 

 with the air-tube from the boat at the surface. This apparatus was exhi- 

 bited at the Crystal Palace some years ago. 



THE PUMP. 



We have seen in the case of the Water Barometer that the pressure 

 of the air will sustain a column of water about thirty feet high. So the 

 distance between the lower valve and the reservoir or cistern must not be 

 more than thirty-two feet, practically the distance is about twenty-five feet 

 in pumps. 



