THE BAROMETER. S# 



duced, till it is raised above a small hole in the barrel, when 

 the air rushes in, and is again discharged through the valve, 

 An instrument of this kind is used to produce what is called 

 the artificial fountain. 



Questions. — 1. What is Pneumatics ? 2. What is the atmosphere ? 

 3. What is said of its height ? 4. What is wind ? 5. What is said 

 of the weight and pressure of the atmosphere ? 6. What experiment 

 illustrates the upward pressure of the atmosphere ? 7. How does the 

 elasticity of air differ from the elasticity of bodies in general ? 8. 

 What is the weight of the atmosphere upon a square inch ? 9. Upon 

 the surface of a man's body ? 10. How is the pressure of the air upon 

 the body counterbalanced.' 11. Describe the air-pump. 12. Show 

 the method by which the air is drawn from the receiver. 13. What 

 are some of the experiments that may be performed by an air-pump ? 

 14. Describe the condensing syringe, and its action. 15. Look at fig. 

 16. and describe the air-pump, and show its action. IG. Look at fig. 

 26. and describe the artificial fountain. 



LESSON 28. 



The Barometer. 



Hermet'ically, a term applied to the closing of the orifice of a 

 glass tube by fusion, so as to render it air-tight. 



Respira'tion, the act of alternately inspiring air into the lungs, and 

 expiring it from them. 



The Barometer is a very useful instrument for determin- 

 ing the variations of the weather. If a glass tube of about 

 thirty-two or thirty-three inches long, hermetically sealed at 

 one end, be filled with mercury, and then inverted in a basin 

 or cup of the same fluid, the mercury in the tube will stand 

 at an altitude above the surface of that in the basin between 

 twenty-eight and thirty-one inches. The tube and the basin 

 are fixed on a board, for the convenience of suspending it; 

 the board is graduated for the purpose of ascertaining the 

 height at which the mercury stands in the tube ; and a small 

 moveable metallic plate, called a vernier, an inch of which 

 is divided into a hundred equal parts, serves to show that 

 height with greater accuracy. The height at which the 

 mercury will stand depends upon the weight of the atmo- 

 sphere, which varies much according to the state of the wea- 

 ther. The air is heaviest in dry weather, for it is then that 

 the mercury is found to rise in the tube and consequently 



