C. GENERAL PHYSICS. 129 



then see the distant telescope by looking at the teeth of the 

 wheel, which will be close to his eye; and, when the wheel 

 is turned, every time a tooth passes it shuts off both the view 

 of the distant telescope and the passage of the light from the 

 first telescojje. A series of flashes are thus sent, which on 

 their return to the observer are either cut off by a tooth of 

 the wheel or admitted between the teeth, according to the 

 movement of the wheel while the light was going and coming. 



In Cornu's experiments the second telescope was at a dis- 

 tance of 10,310 meters, or about six miles and a half. By 

 giving the toothed wheel a velocity of 800 turns a second, 

 six teeth would pass the focus of the telescope while the light 

 was soins: and coming^, and the flash would be caught by the 

 seventh tooth. The mean result of more than five hundred 

 trials gave a velocity of 298,400 kilometers, or 185,396 miles, 

 per second a result only 400 kilometers greater than that 

 of Foucault with the revolving mirror. 



Mr. Cornu's paper was ref&rred to the French commission 

 on the transit of Venus, and m'c hope our American commis- 

 sion may be able to make a similar determination with im- 

 proved apparatus. 



! 



EECENT DISCOVERIES IN ACOUSTICS. ' 



In the March number of the American Journal of Science, 

 Dr. A. M. Mayer published his fourth paper of original inves- 

 tigations in acoustics. The first paper of this series describes 

 simple and effective experiments with tuning-forks, shoAving 

 when a sounding body is moved the motion shortens the so- 

 norous waves in those portions of the air toward which the 

 sounding body moves, and lengthens the waves in those por- 

 tions of the air from which it recedes. 



This result, first indicated by Doppler in 1841, Professor 

 Mayer makes evident as follows: If two forks are in unison, 

 and one of them is sounded, while the other, placed at a dis- 

 tance, has touching one of its prongs a suspended ball of var- 

 nished cork, then the vibrations of the sounding fork will be 

 communicated through the air to the other fork, and, having 

 been thus set in vibration, it will project from its prong the 

 suspended ball. Now if the prongs of the sounding fork are 

 weighted, its vibrations will be sloAver, and therefore the waves 

 which it produces in the surrounding air are longer than they 



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