Dr. A. M. Mayer on an Acoustic Pyrometer. 19 



from the nodal capsule of the pipe and from the resonator lead to 

 contiguous gas-jets placed before the revolving mirror. We will 

 also assume that the air in and around the organ-pipe is at 0° C, 

 and that the serrations of the flames of pipe and resonator are, 

 by means of the manometric flame-micrometer, brought to coin- 

 cidence when 13 metres of metal tube, connecting the resonator 

 and its manometric capsule, are placed in a furnace which also 

 has the temperature of 0° C. Therefore the length of a wave in 



333 



the furnace-tube is — — =0*65 metre, audit will contain twenty 



wave-lengths. Now gradually raise the temperature of the fur- 

 nace to 820° C. As the temperature rises, we shall see the ser- 

 rations of the resonator-flame gradually slide over those of the 

 organ-pipe flame; and when the temperature has reached 820° C, 

 we shall have observed that the serrations of the resonator-flame 

 have glided over ten times the distance separating the centres of 

 two contiguous serrations of the flame of the organ-pipe ; for at 

 820° C. the air in the furnace-tube will have expanded to four 

 times its volume at 0° C, and therefore 



f x _ 333^/1 + '00367x820 ^ . 

 V 512 V 



it will contain half the number of wave-lengths it did when at 

 0° C. ; and the length of one of these waves in the tube will be 

 1*3 metre. 



We will now determine the limit of accuracy of the method 

 by elevating the temperature of the furnace 100°, or to 920° C. 

 At this temperature the velocity of the pulses in the furnace- 

 tube will equal 696*63 metres ; and the length of the wave at 

 this velocity will be 1*36 metre. But 1*36 — 1*3 = 0*06 metre, 

 the difference in wave-length produced by the increase in tem- 

 perature from 820° to 920°, and sufficient to cause the serrations 

 to be displaced 0*46 of the distance separating the centres of two 

 contiguous serrations of the organ-pipe flame. But by means of 

 the manometric-flame micrometer* one tenth of this displacement 

 can be measured ; therefore we can measure an increase of 10° in 

 temperature above 820°. 



From an examination of the well-established formula for the 

 determination of the velocity of sound, it will be seen that the 

 accuracy of our determinations of furnace-temperature will depend 

 only on the precision of the coefficient "00367, which is the 



* See my previous paper in the Philosophical Magazine, " On a Method 

 of detecting the Phases of Vibration " &c. In this paper I give the credit 

 of the suggestion on which I founded my micrometer to M. Radan, but I 

 find that it is due to Zoch (Pogg. Ann. vol. cxxviii.). Radan mentions it 

 in his V Acoustique without giving credit to the inventor. 



C2 



