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



lated in order to obtain / in equation (5), it follows that equa- 

 tion (4) is the simpler and the more readily worked numerically. 

 If we call T the absolute temperature Centigrade, then 



T = *' + 272-48, 



and equation (4) becomes 



H^m=^f' (6) 



in which equation the origin of coordinates is at the vertex of 

 the parabola. 



This paper is intended only to give a general account of the new 

 method of pyrometry ; and therefore I have not touched on the 

 details of apparatus and experiment. These I will present in a 

 subsequent communication, in which accounts of actual applica- 

 tions of the method will be given. I may, however, here remark 

 that if the tube in the furnace is 13 metres long and 0*015 metre 

 in diameter, it can be coiled into a spiral e 5 metre in dia- 

 meter, or into two spirals each of 0'25 metre diameter ; or the 

 tube can be bent on itself several times and thus form a compact 

 fascicle of tubing. Also the tubing should ascend from the re- 

 sonator to the furnace-tube and descend from the latter to the 

 manometric capsule, so that the rarefied air in the hot tube can- 

 not enter the tubing outside of the furnace. If the serrations 

 of the manometric flames are too dim to be readily observed, they 

 can be rendered distinctly visible even in broad daylight by pass- 

 ing the gas through a glass tube containing tow saturated with 

 benzole. I have thus been able to exhibit these phenomena at 

 night to an audience in a room brightly illuminated with many 

 gas-jets. "We can also make the vibrating flames brilliant by 

 sifting into them the fine scrapings of lead-pencil. 



In ascertaining the number of displacements produced by any 

 elevation of temperature, the furnace-tube is slowly moved into 

 the furnace, so that the displacements of the resonator-serrations 

 can be counted as the tube gradually attains the highest tempe- 

 rature, when the serrations become stationary. 



October 12, 1872. 



The reader is requested to correct the following errata occurring in my 

 paper " On a Method of detecting the Phases of Vibration " &c, Phil. 

 Mag. Nov. 1872:— 



Page 323, line 10 from bottom, for ventral read nodal. 

 „ „ 4 ., for in front of read behind. 



