EEAT. 205 



the stream docs not become broken into bubbles and drops, neither sound or 

 air-bubbles will be produced ; but as soon as the distance becomes increased 

 to a sufficient extent to break the end of the column into drops, both air-bub- 

 bles and sounds will be produced. 



whatissneez- ^^- SneeziDg IS a phenomenon resembling 

 "'°' cougli ; only the chest empties itself at one 



effort, and chiefly through the nose, instead of through 



the mouth, as in cougliing. 



•What is laugh- ^oO. Laughing consists of quickly- repeated 

 ^^^ expulsions of air from the chest, the glottis 



being at the lime in a condition to produce voice ; hut 



there is not hetAveen the expirations, as in coughing, a 



complete closure of the glottis. , 



•wTiat is crying? 451. Crying differs from laughing almost 

 solely in the circumstance of the intervals be- 



t'^een the gusts or expirations of air from the lungs being 



longer. Children laugh and cry in the same breath. 



Insects generally excite sonorous vibrations by the fluttering of their wings, 

 or other membraneous parts of their structure. 



PRACTICAL QUESTIONS IX ACOUSTICS. 



1. The flash of a cannon was seen, and ten seconds afterward the report was heard: 

 how far oflF was the cannon ? 



2. At what distance was a flash of lightning, when the flash was seen seven seconds 

 before the thunder was heard ? 



3. How long after a sudden shout will an echo be returned from a high wall 1,120 feet 

 distant? 



4. A stone being dropped into the mouth of a mine, was heard to strike the bottom 

 in two seconds ; how deep was the mine ? 



5. A certain musical string vibrates 100 times in a second : how many times must it 

 Tibrate in a second to produce the octave ? 



CHAPTER XII. 



HEAT. 



452. Heat is a physical accent, known onlV 



What is heat? , . /« t i- ■, 



by its eiiects upon matter. In ordmary lan- 

 guage we use the term heat to express the sensation of 

 warmth. 



