2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/201070We provide a simple derivation of the Schwarzschild solution in General Relativity, generalizing an early approach by Weyl, to include Birkhoff's theorem: constancy of the mass; its deeper, Hamiltonian, basis is also given. Our procedure is illustrated by a parallel derivation of the Coulomb field and constancy of electric charge, in electrodynamics.8 pages. Clarifying remarks and historic references added, accepted for publication in Amer. J. PhysGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext