FIFTH ARMY : SPOTTING THE ENEMY GUNS - A knowledge of the wind speed and direction at various heights is necessary for accurate sound ranging. This is ascertained by the attached RAF Met. Unit, using a theodolite and a balloon filled with hydrogen. At intervals, the readings are called out by LAC S. Tyrer (right) of Stockton-on-Tees, to LAC May of London, , who, with the aid of a slide rule, computes the winds at various heights British Army

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