2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/33902I present a formulation of the second law of thermodynamics in the presence of black holes which makes use of the efficiency of an ideal machine extracting heat cyclically from a black hole. The Carnot coefficient is found and it is shown to be a simple function of the mass.3 pages; part of a talk given at EFI, at the University of Chicago; misprint correctedGeneral Relativity and Quantum Cosmologytext