  Thurston introduced shear deformations (cataclysms) on geodesic laminations -
deformations including left and right displacements along geodesics. For
hyperbolic surfaces with cusps, we consider shear deformations on disjoint
unions of ideal geodesics. The length of a balanced weighted sum of ideal
geodesics is defined and the Weil-Petersson (WP) duality of shears and the
defined length is established. The Poisson bracket of a pair of balanced weight
systems on a set of disjoint ideal geodesics is given in terms of an elementary
2-form. The symplectic geometry of balanced weight systems on ideal geodesics
is developed. Equality of the Fock shear coordinate algebra and the WP Poisson
algebra is established. The formula for the WP Riemannian pairing of shears is
also presented.
