Source of Volcanic Energy

A FEW words of explanation are necessary concerning my letter which appeared in NATURE, vol. xii. p. 396. Mr. Mallet's prime source of energy for producing tangential pressures is the force of cohesion developed in a cooling globe, gravitation giving only partial assistance; and when I spoke of ``gravitation of the whole mass to itself,'' I wished to convey that, setting aside altogether the force of cohesion and its accompanying motions, there still remains the force of gravitation, which, acting in a globe of such size as the earth, and composed of heterogeneous materials, must of itself produce enormous local pressures.