18 Dr. A. M. Mayer on an Acoustic Pyrometer. 



whole surface. If we utter the consonant only at the moment 

 of making this movement with the hand, we generally succeed 

 in observing the most interesting part, namely the commence- 

 ment of the picture. 



To pursue these experiments further, perhaps it would be ad- 

 visable to employ a mirror placed in an oblique axis on which it 

 would be turned, and would then show the flame-picture in a 

 continuous circle instead of broken bands. 



The voiceless sibilants F, S, and CH give the same unsatisfac- 

 tory result as the whispered vowels. I could see nothing defined 

 in the confused dim light-bands. 



[To be continued.] 



II. On an Acoustic Pyrometer. By Alfred M. Mayer, 

 Ph.D., Member of the National Academy of Sciences, Professor 

 of Physics in the Stevens Institute of Technology, Hoboken, 

 New Jersey, U. S. A* , 



[With a Plate.J 



HAVING recently devised an arrangement of apparatus 

 (PI. III. fig. 7) — which is an instrumental simplification of 

 the method first practised by Zoch (Pogg. Ann. vol. exxviii.) — for 

 measuring the number of acoustic wave-lengths contained in a 

 given tubef, the idea occurred to me that I could use the method 

 for the determination of the variation in the number of wave- 

 lengths contained in this tube caused by a change in the tempera- 

 ture of the air which it contains, and thus succeed in readily 

 determining any temperature to which the tube might be exposed. 

 The accuracy of this (as far as I know) entirely new method 

 of pyrometry, and the facility of its application, can be judged of 

 by the following discussion. 



/ah A cT 



The formula V = \/ ?—=- (1 + at) — gives the velocity of 



sound in air of a known temperature. This formula, as is well 

 known, is reduced numerically to V = 333 \/\ + '00367^ ; in 

 which V = the velocity of sound at the temperature t Centigrade, 

 333 = the velocity of sound, in metres, at 0° C, and "00367 is 

 the coefficient of expansion of air under a constant pressure. 

 We will suppose that we have outside of the furnace whose tem- 

 perature we would determine, an ut 4 organ-pipe, and that we 

 have placed opposite its mouth an ut 4 resonator, and that tubes 



* Communicated by the Author. 

 . \ See my previous paper in the Philosophical Magazine (November 18/2), 

 " On a Method of detecting the Phases of Vibration in the Air surrounding a 

 Sounding Body, and thereby measuring directly in the vibrating air the 

 length of its Waves and exploring the form of its Wave-surface." 



