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Show The principal stresses are < fc- VA % YA- L .* tosf * h ( 7) and are at an angle, o( , to the coordinates, given by T ™ *.<*- (*,-*) ' ( 8) For each depth, y, between 0 and h, 0*% and C7£ determine a circle, Cm, in the Mohr- plane ( Fig. 3)- A curve, Cr, can also be constructed to represent the strength of the snow at a depth, y. The usual criterium of stability is that this latter curve does not touch the Mohr's circle. This is called the " criterium of primary stability" as opposed to other kinds of stability which we will consider later on. If an overload is applied to the snow cover, or the slope angle increases, the circle, Cm, is enlarged and displaced. It is also possible that the strength curve may shift due to a change in the properties of the material ( e. g. a temperature rise or a metamorphism of the snow). In either case, the first layer for which the Mohr's circle touches the strength curve, is defined to be a cr it ical layer. 3- VARIATION OF PRIMARY STABILITY For abbreviation, let us call 3 Jo ( 9) The strength curve may be approximated by a straight line inclined to the 0~- axis at an angle, Mj . This line cuts the f - axis at ^ ( Fig. 3). If a line tangent to the Mohr's circle, inclined to the C- axis at an angle, cO , |