I AM in general agreement with Mr. Jeans's remarks on the difficulty of obtaining a source of stellar energy more powerful than the Helmholtz contraction. It may be added that there is a conceivable source, which was, I believe, once suggested by Mr. Jeans himself, viz. a gradual annihilation of matter by positive and negative electrons occasionally neutralising one another. This would provide an almost inexhaustible store of energy, but there is the grave objection that it affords no reason why the, dense dwarf stars should liberate so much less energy than rarefied stars of the same mass. One would have expected compression to be favourable to the process of cancelling of electrons. The search for an additional store of energy is not at all encouraging; but, on the other hand, there are important arguments against the short time-scale—notably Prof. Strutt's evidence of the age of terrestrial rocks, and the time needed for the tidal evolution of the earth-moon system. I have not felt myself able to combat the arguments on one side any more than on the other; accordingly, in the paper criticised by Mr. Jeans, the question was left entirely open. In the one place where it was necessary to consider the source of stellar energy, I attempted to show that my formula fairly represented both the radio-activity and the contraction hypotheses—having regard to the necessarily approximate character of the investigation.