64 THE DIVING hT.tt. 



confined, and condensed into a smaller bulk than its "hatiifal 

 space, presses by its elasticity upon the surface of the water, 

 and forces it violently up the tube in a continual stream. It 

 is ujpon this principle that the engine for extinguishing fires 

 is constructed. 



Questions. — 1. What does the science of hydraulics teach ? 2. 

 What machines are constructed on the principles of this science.-* 



3. What are the diflerent ways in which water may be sot in motion ?^ 



4. What is the syphon ? 5. Describe the manner of its conveying 

 fluids. 6. How are intermittent springs caused ? 7. Describe the 

 common pump and show how it raises water. 8. How high can water 

 be raised in a common pump? 9. Describe the forcing pump. 10. 

 What engine is constructed on the principle of the forcing pump ? 

 11. Describe the common pump by fig. 21. and show its action. 12. 

 Forcing pump by fig. 22. and show how it acts in forcing up water. 



LESSON 26. 

 The Diving Bell, and Steam Engine, 



Ver'tically, in a direction perpendicular to the horizon. 

 Appara'tus, utensils and appendagea belonging to a machine. 



If you take a glass tumbler, and plunge it in water with 

 the mouth downwards, you will perceive that very little water 

 will enter into it. The air which fills the glass prevents the 

 entrance of the water ; but as air is compressible, it cannot 

 entirely exclude the water, which, by its pressure, condenses 

 the air in a slight degree. Upon this simple principle ma- 

 chines have been invented, by v/hich people have been abid 

 to walk about at the bottom of the sea, with as much safety 

 as upon the surface of the earth. The original instrument 

 of this kind was much improved by Dr. Halley, more than a 

 century ago. The machine was made of copper in the shape 

 of a bell. The diameter of the bottom was five feet, that 

 of the top three feet, and it was eight feet high. To make 

 the vessel sink vertically in water, the bottom was loaded 

 with a quantity of leaden balls. Light was let into the bell 

 by means of strong spherical glasses fixed in the top. Barrels, 

 filled with fresh air, were made sufficiently heavy, and sent 

 down, from which a leathern pipe communicated with the 

 inside of the bell, and a tube with a stop at the upper part 

 let out the air which had become unfit for breathing. The 



