C. GENERAL PHYSICS. 131 



and attach to the latter a orum tube leadins; to another mem- 

 brane and box, whose gas jet is placed exactly below the jet of 

 the organ-pipe, and hold the opening of this sphere near the 

 pipe, we will see in the revolving mirror two series of serrations, 

 with the teeth of one series exactly over the teeth of the Other 

 series. Xow if we gradually move the sphere away from the 

 pipe, we wdll see the serrations corresponding to the vibra- 

 ting sphere of air gradually slide along those produced by the 

 vibrating air in the organ-pipe ; and when we have removed 

 the sphere to such a distance that the serrations again appear 

 exactly over each other, we will have moved the sphere from 

 its first position by the length of a sonorous tcave correspond- 

 ing to the note given by the pipe. Furthermore, if when the 

 sphere is placed at any distance from the pipe, so that the 

 serrations of one flame are exactly over those of tlie other, 

 we move the sphere around the pipe in all directions, so that 

 in every position the serrations remain stationary', then we 

 will have described in space the icave si(7]face of the vibrating 

 air ; for from all parts of that surface described by the mouth 

 of the sphere we have taken into the sphere the same phases 

 of vibration. Professor Mayer thus found that the wave 

 surface of an open organ-pipe was an ellipsoid, with its foci 

 at the top and bottom of the pipe. 



In his third paper. Professor Mayer describes his method 

 of measuring, with precision, the wave lengths of sound trav- 

 ersing tubes filled with air or any gas, and makes an im- 

 portant practical application of this method in an invention 

 which he designates as an "Acoustic Pyrometer," which in- 

 strument consists of a coil of tubing formed of a material re- 

 sisting very high heats, placed in the furnace whose tempera- 

 ture w^e would measure. 



A sound is sent through this tube, and the length of the 

 sonorous wave corresponding to this sound is measured by 

 means of the vibrating flames. These waves will increase in 

 length with the rise of temperature of the air in the tube, and 

 these wave lengths can be determined with such great pre- 

 cision that a temperature of even 2000 Centigrade can be 

 measured accurately to 10. 



The subject of the last paper of Professor Mayer's acous- 

 tical researches is "on the experimental determination of the 

 relative intensities of sounds, and on the measurement of the 



