of its atmosphere, would radiate into space twice as much energy as at the present time. He also showed that an increase of 25 per cent. in the absorption by the solar envelope would lower the surface temperature of the earth by 100°F. Langley's attention was directed chiefly to the earth, and not to the reflex action on the sun itself, which such an atmosphere must exert. A decrease in the outside radiation of energy, caused by any change in the enclosing solar envelope, means an increase in the energy contained in the sun. It is reasonable to assume that changes in the absorptive power of the atmosphere must arise, and Mr. Hahn presents the query: What becomes of the energy which is prevented from escaping into space by the solar envelope? He endeavors to show that there may occur in the atmosphere changes sufficient to lead to alterations in the thermal conditions of the sun's mass and attempts to decide how far such changes may lead to the known variations in the phenomena at the surface of the sun. Astronomers are generally agreed in accepting the theory of Helmholtz, which accounts for the generation of the sun's heat by the contraction of its mass. This theory, while it explains the generation of heat in any star, does not in itself give information as to whether the amount of heat thus formed is just sufficient to balance that which is lost by radiation. As a fact, there are doubtless suns, as indicated by the spectroscope, which are increasing, and others which are decreasing, in temperature, and in the life of each sun there is, probably, a period of increasing, and later one of decreasing, temperature. Our sun is perhaps an example of those stars in which the heat lost by radiation is greater than that gained by contraction. With this assumption the layer of maximum incandescence and radiation will be shifted nearer and nearer toward the sun's center. The result will be that, due to the increased absorption of the denser envelope, the solar radiation will be decreased, which will tend to raise the temperature of the inner layers of the sun itself. By this overheating the vertical temperature-gradient will become so steep that mechanical equilibrium will be impossible. Although retarded by the powerful convection currents which prevail, disturbances will sooner or later ensue as a result of these strained conditions of the internal overheating, and solar outbursts will occur. Mr. Hahn then proceeds to an analytical demonstration.
The problem consists in determining the changes in the amount of the outside radiation, caused by increased or decreased absorption. The method is an application of the Bouguer-Lambert formula for the extinction of light and heat in an absorbing medium. Formulas have been derived for the energy of radiation from the upper limits of the atmosphere, for the changes in the radiating power, and for the frequency of eruptions and spots. The results thus obtained appear to be in close agreement with observation. The object of the article is to give an abstract of the main principles upon which is built a new solar theory. In a paper which is to appear in the Annals of the Edinburgh Royal Observatory the author will enter more in detail into the various applications of the theory to periodic phenomena at the sun's surface. This theory differs from the views generally accepted, in that it involves the assumption that an increase in the dynamical forces at the surface of the sun indicates a decrease in the heat and light radiation, but here also the author believes that his theory accords well with the facts of observation.
SCIENTIFIC ITEMS.
The National Academy of Sciences held its annual stated session at Washington, beginning on April 15.—The spring meeting of the Council of the
