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_~_-- ,-IEXAMPLES OF TRANSMISSION SYSTEMS 39-~ - j:--- -~'.'-+ --+-,-, ,/'y--!1/ V I-. . .//• ~.' ,•~Cou•• •• -/ f- -c.­•ES.2.,.~ 047 - ./ -- ., ./~. ,S.. 0,2,iII• ., ., oS• •Snow density 9 cm- 3iFigure 3.7 - Calibration of snow-gauge with horizontal beam.Relationship between the time of measuring 3 840 pulses on theGeiger-MUller counter and the density of the snowThe radioactive source, Caesium 137, has an activity of 30 millicuries. Thehalf-life of this element is 34 years, instead of 5.5 years for Cobalt 60; the radia-'tion which is 0 little less penetroting than the radiation from Cobalt, results ingreater sensitivity.Compared with other automatic devices for measuring snow cover which have beentried for a number of years in various countries of the world~ (viz. fixed verticalradioactive snow-gauge, pressure pillow snow-gauge) the tele-snaw-gauge with movinghorizontal beam has the following advantages.(i)It gives not only the water equivalent, but also the thickness of the snowcover and the densities of the various layers;