SECTIONAL TRANSACTIONS.— A. 305 



The compilers of the Interna tioii a 1 Seismological Summary are to be congratulated 

 on the way in which they have published the ' additional readings ' at the end of the 

 classified observations, and it is to be hoped that, so far as the limitations of space, 

 and ultimately finance, permit, they will continue to print all the information at 

 their disposal. 



Department of Mathematics (A*). 

 Prof, A. C. Dixon. — Integral Equations. 



Tuesday, September 9. 



Prof. R. S. MuLLiKAN. — Molecular Spectra and Chemical Binding. 



Discussion on Flow in Gases. (Mr. E. Ower ; Mr. F. C. Johannsen ; 

 Mr. E. C. BiLHAM ; Mr. M. A. Giblett.) 



Mr. E. OwEEi — The Standard of Measurement of Gas Flow. 



Hitherto no instrument for measuring gas flow has been devised which does not 

 require experimental calibration. Therefore, if an instrument is to be adopted as a 

 standard the desirable features to be sought are that its calibration sliall not be liable 

 to change and shall vary as little as possible with the speed of the gas. Only one 

 instrument suitable for general use is known which possesses these characteristics — 

 the pitot-static tube. 



The properties of this instrument are described, attention being drawn to the fact 

 that the pressure at the orifices of the static tube is the resultant of two opposite 

 effects, a suction due to the flow round the nose and a pressure due to the presence 

 of the stem of the instrument. It is only when the holes are in such a position that 

 these two effects balance that true static pressure is recorded. Dimensions of an 

 instrument fulfilling this condition are given. It has a hemispherical nose and is 

 thus more robust than the N.P.L. standard type. 



The question of the use of the pitot-static tube for the measurement of low speeds 

 is next discussed, and a short account is given of a research by which it is hoped to 

 establish the calibration of the instrument down to an air speed of about 2 feet per 

 second. For this purpose a special manometer has been designed, which has a 

 sensitivity somewhat better than one hundred thousandth of an inch of water. 



Mr. F. C. Johannsen.— TAe Measurement of Air Speed. 



The precise calibration of a standard pressure-tube anemometer, with which 

 anemometers for ordinary service may be compared, involves the rotation of the 

 standard instrument on a whirling arm at known speeds over the ground. The true 

 speed of the instrument through the air is less than that over the ground as a result 

 of the swirl, or velocity in the orbit of rotation, imparted to the air by the motion 

 of the instrument under calibration. The discrepancy, of the order of 10 per cent. 

 of the true speed, introduces a serious difficulty into the calibration of a standard 

 anemometer, since no instrument exists whereby the swirl may be measured with 

 certainty, and since the swirl velocity is so small, at low speeds of the standard 

 instrument, as to be below the range of an anemometer for which an approximate 

 calibration is available. 



Apart from the difficulties of calibration, anemometers of the pressure tube type 

 are unsuitable for the measurement of low air speeds in consequence of the extremely 

 small pressures involved. In common, moreover, with vane anemometers their 

 dimensions are too large to permit of their being used for explorations in regions of 

 rapid velocit}' gradient. For such purposes, hot wire anemometers have many 

 advantageous features, notably a high degree of sensitivity at low air speeds, and a 

 rapid response to speed fluctuations. They are thus peculiarly well adapted for 

 investigation of the eddj'ing air flow associated with numerous aerodynamic problems. 

 Over a low speed-range much of the uncertainty arising from their liability to change 

 of calibration may be obviated if the heat-loss : wind-speed relation is expressed as a 

 function of the heat loss in still air. 



Mr. E. G. BiLHAM. — Becent Improvements in Meteorological Anemometry. 



A brief description is given of the modifications introduced into the design of the 

 standard Dines Tube Anemograph as a result of investigations carried out at the 



1930 TT 



