THE REASON WHY. 163 



" Blessed is the people that know the joyful sound : they shall walk, O Lord, in 

 the light of thy countenance." PSALM LXXXIX. 



Because the heat passes through the glass, in which the mercury 

 is enclosed, and expanding or contracting the metal within the 

 bulb, causes the small column above it to rise or fall. 



714. When does the thermometer vary most in its indi- 

 cation of natural temperature ? 



It varies more in the winter than in the summer season. 



715. Why does it vary more in the winter than in the 

 summer ? 



Because the temperature of our climate differs more from the 

 temperature of the torrid zones in the winter than it does in the 

 summer, and the inequalities of temperature cause frequent 

 changes in the degree of prevailing heat. 



The same remarks (714, 715,) apply to the barometer. 



CHAPTER XXXIV. 



716. What is sound ? 



Sound is an impression produced upon the ear by vibrations of 

 the air. 



717. What causes the air to vibrate and produce sounds ? 

 The atoms of elastic bodies being caused to vibrate by the appli - 



cation of some kind of force, the vibrations of those atoms are 

 imparted to the air, and sound is produced. 



718. TTow do we know that sounds are produced by the 

 vibrations of the air, induced by the vibrations of the atoms of 

 bodies ? 



If we take a tuning fork, and hold it to the ear, we hear no sound. 

 If we move it rapidly through the air, or if we blow upon it, it pro- 

 duces no sound ; but if we strike it, a sound immediately occurs ; 

 the vibration of the fork may be seen, and felt by the hand that holds 

 it ; and as those vibrations cease, the sound dies away. 



719. How do we know that without air thr*e would be no 

 sound ? 



