TOWER WITH CONSTANT PRESSURE, OR FUNNEL SURFACE  Cylindrical equation: (tower) or , i.e. (funnel). Cartesian parametrization: (tower). Cartesian parametrization the coordinate lines of which are the asymptotic lines: (view opposite). The tower with constant pressure is the surface of revolution obtained by rotating a logarithmic curve around its asymptote.
Its name comes from the fact that, if this surface is filled with a homogeneous material, then the pressure applied on any horizontal section by the upper part is constant.

 Derivation of the equation: The pressure at the altitude z is equal to ; assuming P constant and differentiating, we get the differential equation: which immediately gives the result, with . Remark: if we take the variation of g due to the altitude into account, , we get the surface , with , represented opposite. Much as the first one has a straight line as asymptote, this one has an asymptote cylinder. Note that this tower, extended to infinity, would have an infinite mass, but finite weight...   