Lord Rayleigh's Acoustical Observations. 155 



hydrogen-flame. With a larger resonator of the same kind 

 — a globe with a short neck, intended for showing the com- 

 bustion of phosphorus in oxygen, the pitch is 64 vibrations 

 per second. I have lately made some further experiments, 

 with the view of finding whether there is any obstacle to the 

 maintenance by flames of vibrations of still lower frequency. 

 The resonator, whose natural pitch is 64, was fitted with a 

 paste-board tube 2 inches in diameter and 14 inches long. 

 In this condition its calculated frequency* is about 25 ; and 

 it was found that vibrations could be maintained by a hydrogen- 

 flame, or even by a flame of common gas. The supply-tube 

 should be of considerable length ; and the orifice must not be 

 much contracted. Although the intensity of vibration was 

 such as to make it a matter of difficulty to keep the flame 

 alight, scarcely any thing could be heard. I saw no reason to 

 doubt that still slower vibrations might be maintained by 

 flames. 



In illustration of the mechanics of this subject, an apparatus 

 was contrived, in which by the aid of electricity a periodic 

 communication of heat to a limited mass of air could be 

 effected. By means of a perforated cork one leg of a U-tube 

 containing mercury was fitted air-tight to the neck of an 

 inverted bottle of about 200 cub. centims. capacity. The dia- 

 meter of the column of mercury was about 1 centim., and 

 the length of the column about 25 centims. The combina- 

 tion constituted a resonator, differing from an ordinary air 

 resonator by the substitution of mercury for air in the 

 channel joining the interior of the vessel with the external 

 atmosphere. Inside the bottle was a spiral of fine platinum 

 wire, at one end in communication through the cork with one 

 pole of a battery of two or three small Grove cells. The 

 other end of the platinum spiral was connected with a copper 

 wire, which terminated in the U-tube near the equilibrium- 

 position of the mercury-surface. The second pole of the 

 battery was in permanent connexion with the outer extremity 

 of the mercury column. As the mercury vibrates, the circuit 

 is periodically completed and broken. The current passes, 

 and the platinum wire glows, when the mercury rises in the 

 leg connected with the bottle. Thus the communication of 

 heat occurs when the air in the interior is condensed by the 

 vibration, which is the necessary condition for maintenance, 

 as is explained in the lecture referred to. 



JRijJce's Notes on a large scale. 

 The production of sound in tubes by heated gauze was 



* < 



Theory of Sound,' vol. ii. § 307 (8). 



