Sound through Narrow Slits. 155 



minimum to a minimum o£ excitation, measures the half 

 wave-length. When the sensitiveness o£ the fiame is suitably- 

 adjusted, the observation of the minimum, characterized by 

 recovery of the flame, admits of great precision. The bird- 

 call was mounted in a glazed earthenware drain-pipe. This 

 was closed at the hinder end with a wooden disk perforated 

 by two tubes, one serving as an inlet and the other as an 

 outlet for the wind, and in front by a second disk upon which 

 the adjustable slit was mounted. As has already been hinted, 

 the isolation of the sound requires much precaution. Although 

 the jaws of the slit fitted well, the sound could not be pre- 

 vented from escaping without the aid of grease. The wind 

 was from a bag of heavy rubber cloth suitably weighted and 

 controlled by observation of a manometer, and the outlet 

 tube was continued to a point outside the window by a rubber 

 prolongation. It was necessary to cement up every joint 

 until the whole was air-tight, otherwise sound could be heard 

 through a rubber tube connected with the ear and presented 

 at the outer end to the place under test. I should say that 

 my own ears are not now effective for observations at this 

 pitch, but Mr. Enoch's are very sensitive to it. 



The slit was of tin-plate with jaws carefully filed to a 

 knife-edge. The length was half an inch. The width could 

 be reduced to about 2 thousandths of an inch without too 

 great an uncertainty of measurement. With these arrnnge- 

 ments it had been expected that the desired observations 

 could be made without difficulty. An intensity observed 

 whether by ear or flame at a certain short distance in front 

 and with a certain width of slit might be recovered at a 

 greater distance with a Avider slit. On the basis of the usual 

 law of attenuation of sound with distance a measure would 

 be attained of the effect of widening the slit. 



But it soon appeared that nothing of value could be obtained 

 on these lines. In listening with the ear through a rubber 

 tube whose open end (usually provided with a conical ter- 

 mination of glass) was moved to and from the slit, the 

 intensity on receding was found not to fall in a continuous 

 manner, but to be subject to alternate risings and fallings, 

 almost as if it were due to a system of stationary waves. 

 The effect was not equally apparent with the flame, but it is 

 difficult to make good observations with a flame unless it can 

 be maintained in a fixed position. 



The earlier experiments were made in the laboratory where 

 the floor, ceiling, and walls might act as reflectors, but I was 

 surprised at the vigour of the alternations in view of the 

 proximity of the place of observation to the source. A 



