ACOUSTICS. 67 



LISSAJOUS'S CURVES. 



The optical method of studying vibrations is attract- 

 ive to old and young, Jo students of science, and to 

 musicians ; but the apparatus generally used is so 

 costly that not many can afford to purchase it. The 

 following directions will enable any one to have a pair 

 of the tuning-forks made at the nearest blacksmith's 

 shop, that will be found even more satisfactory for pro- 

 jections than the more costly ones. 



Choose a piece of steel that is an inch broad, one- 

 fourth of an inch thick, and about four feet and a half 

 long. Have it made into two large tuning-forks, one of 

 them to be about fifteen inches long, and the other 

 twelve inches. Let the tines be two inches apart, and 

 the flat sides should face each other on each fork. A 

 stem may be now welded upon the bend ; it should be 

 about five-eighths of an inch in diameter, three or four 

 inches long, and made of round steel. When one of 

 these forks is struck in the manner of common tuning- 

 forks, it will be seen to vibrate through quite a large 

 arc, and will continue to vibrate perceptibly to the eye, 

 for half a minute or more. If, while the fork is vibrating 

 the stem be held upon a table or floor, or some other 

 resonator, a deep sound will be heard, and the larger 

 one will make about fifty vibrations per second, while 

 the shorter one will probably make seventy or seventy- 

 five vibrations per second. A stand will be needed for 

 each of these, and may be made by mortising a post 

 three inches square, and three or four inches high, into 

 a board eighteen inches long and ten inches wide 

 (Fig. 43). This post should have an inch-and-a-half 



