306 SECTIONAL TRANSACTIONS.— A. 



National Physical Laboratory between 1926 and 1928. As compared with earlier 

 patterns the modified instrument represents a substantial gain in accuracy in 

 recording both gusts and mean wind velocities. 



Details are given of the present distribution of anemographs from which data are 

 regularly published, with special reference to instruments of the new pattern. The 

 introduction of a non-Uquid type of recorder has made it possible to obtain records 

 of relative wind velocity at sea in recent j'ears. 



Particulars are also given of the three-cup Robinson anemometer designed by 

 J. Patterson of the Canadian Meteorological Service, and a comparison is made 

 between it and four-cup systems formerly used by the Meteorological Office, London, 

 on small anemometers. 



Mr. M. A. GiBLETT. — Reference is made to full scale experiments in the measure- 

 ment of wind speed and direction at the Meteorological Office, Royal Airship Works. 

 Cardington, Beds, on a very open time scale so revealing short period fluctuations 

 and typical records are shown. 



Dr. W. S. Tucker. — The Screening of Southend from Gun-Fire. 



The variability of ranges of audible sound in the open air was made the subject 

 of special study by Tyndall, who conducted exhaustive experiments for Trinity 

 House. The relationship between range of audibility and meteorological conditions 

 was worked out by Stokes, Osborne Reynolds, Rayleigh and Milne in terms of 

 variation of the velocity of sound with height above the earth's surface, the velocity 

 being affected by wind and temperature. Recent advances in meteorological 

 observation have made it possible to forecast to some extent the refraction of sound 

 in any given direction, using aeroplane observation for upper temperatures and pilot 

 balloons for wind. 



For explosions produced at a given point on the earth's surface it is possible from 

 the meteorological data so obtained to discover if any other point on the surface is 

 within sound shadow and to determine with some degree of approximation the 

 minimum depth of that shadow. 



The region surrounding Southend-on-Sea, which is five miles from the firing point 

 of heavy guns, is severely exposed to sound and low frequency disturbance under 

 those conditions under which there is an increase of velocity of sound with height, 

 towards the point of observation. 



This velocity gradient is not in general constant as height increases, but for the 

 purpose of forecasting, the nearest linear relationship of velocity with height has 

 been adopted. The depth of sound shadow over the Southend area, with the gun 

 as a source of sound, has then been calculated out for various directions of wind and 

 various wind and temperature gradients, and where this depth becomes zero the 

 point of observation is subject to the full violence of the disturbance. 



As an aid to the rapid forecasting of the extent of the region in shadow, a number 

 of celluloid discs have been produced which are capable of rotation about the gun as 

 centre. The depth of sound shadow for a range of five miles has been indicated by 

 lunar shaped areas extending half round the circle, and of maximum thickness 

 increasing with increase of wind gradients. 



Another set of discs deals with shadows produced by temperature refraction and 

 consists of circles thicked into black rings for negative temperature gradients and 

 red rings for positive gradients. These discs can be superimposed on the wind 

 refraction discs so that the blackened area due to wind can be extended or diminished 

 by the black or red areas due to temperature. 



This model is in actual use and is employed before the firing programme is carried 

 out, the meteorological data above referred to being obtained just prior to the 

 occasion. 



The efficacy of this method of selecting suitable times of firing to give immunity to 

 the Southend area from gun disturbance, has been proved by microphone and other 

 observation on the days in question. 



The paper is illustrated by records of microphone disturbance obtained under 

 various types of weather condition. 



Report of Committee on Seismological Investigations. 



