Chemistry and Physics, 55 



ing mirror and the conclusion was reached that it was not over 

 a ten millionth of a second. 



In addition to the determination of the velocities of revolution 

 the author mentions two other applications which have been 

 made of the use of the neon lamps. If a rapidly revolving 

 mechanism such as an air propeller is illuminated by a lamp 

 which is lit automatically by a break at each revolution, the 

 mechanism will appear at rest, so that strains or movements of 

 parts at speeds well over 1000 R. P. M. may be accurately exam- 

 ined under actual working conditions. In another case an inter- 

 nal combustion engine was illumined 99 times every hundred 

 revolutions in which case the engine appeared to be turning 

 smoothly at only 1/lOth of its actual speed and consequently the 

 movements of the valves, springs and other parts could be studied 

 with ease. — Proc. Camh. Phil. Soc. 19, 300, 1919. f. e. b. 



7. Harmonics in the Siren. — A source of sound of considerable 

 intensity and affording a pure tone whose frequency may be 

 varied at will is an important desideratum in acoustics and not 

 easil}^ obtainable. The investigations of E. A. Milne and R. H. 

 Fowler on the tone production of the siren, originally undertaken 

 while working on the problem of locating air-craft by sound, are 

 of interest in this connection. In its simplest form, commonly 

 designated the Seebeck siren, the instrument consists of a series 

 of circular holes in a plate which are made to pass in front of a 

 circular orifice from which a stream of air is issuing. The sound 

 vibrations result from the variable flow of air. For purposes 

 of calculation it may be assumed that the oscillations arise from 

 a flux which at any instant is proportional to the area of the 

 orifice exposed. The authors have computed this area in terms 

 of the spacing of the holes as a function of the position of the 

 disk. Their graph shows a notable departure from the form of 

 a sine curve and it is apparent at once that some of the lower 

 harmonics of appreciable intensity must be present in the dis- 

 turbance. By expanding the expression for the latter in terms 

 of a Fourier series they are able to determine the energy asso- 

 ciated with each harmonic. Numerical computation showed that 

 the proportion of the energy concentrated in the fundamental 

 depended significantly on the ratio of the diameter of the holes to 

 the distance between their centers and became a maximum when 

 this distance was about twice the diameter of the hole. 



The question of the relation between the intensity of the funda- 

 mental and the first harmonic was submitted to experimental 

 investigation by the aid of a Tucker hot wire microphone and 

 when allowance is made for the fact that the aural impression of 

 loudness is not exactly proportional to the square of the ampli- 

 tude a fair verification of their theory was obtained. At least 

 it was sufficient to justify its use in their attempt to design a new 

 siren which would give a purer tone. The authors proceeded to 

 do this by modifying both the shape of the holes in the disk and 



