; 88 THE UNIVERSE A VAST SYSTEM OF PARTS according to A. G. McNish of the Carnegie Institution. This storm was accompanied by obstruction of transatlantic radio communication, numerous interruptions in wired communications services, and interference with the operation of electric power systems. There is some evidence that the weather is influenced by sunspots. W. B. Schotakovilch, after studying the records from 1869 to 1920, concluded that sunspots bring more rain on the average over the whole Fig. 20. The sun, showing unusual activity in both flocculi and prominences. (Photograph from the Yerkes Observatory, reprinted by permission of the University of Chicago Press). earth, because the earth receives more radiations during a sunspot period, and this increased radiation produces more heat, which evaporates more water, which precipitates as rain. The sun rotates about its axis in the same direction as does the earth but not being a solid, it does not rotate uniformly as a whole. Thus spots on the sun's equator rotate in about 24.6 days as compared with a rotational period of 34 days or more near the poles.
The Source of the Sun's Energy Is Still Unknown. THE SUN 89 The amount of energy that finally reaches the earth is very large. Measured by the pyrheliometer, it is found that the earth receives 230,000,000,000,000 horsepower, continuously, or about 160,000 horsepower for every human being. The amount of energy radiated by the sun would be sufhcient to melt a 40-foot shell of ice around the sun within one minute. The generators at Boulder Dam will be able to generate 663,000 horsepower, which means that it will be able to harness about 1/340,000,000 of the sun's energy which reaches the earth. When we consider that the earth and its atmosphere intercept only about one two-billionth of the total energy emitted by the sun, and when we consider how much energy this two-billionth represents, we wonder whether the sun is not gradually growing cooler as it loses all of this energy. Indelible records preserved in the rock layers of the earth indicate that there has been no noticeable permanent change in the amount of energy that the earth has received during the past billion years. This observation indicates either that there is so much energy in the sun that the relative amount lost in a billion years is negligible or that there are changes taking place within the sun which liberate energy. Hermann HelmhoUz (1821-1894), a German physicist, suggested that energy was evolved by a gradual condensation process within the sun, but later calculations showed that such a process would account for less than one hundred-thousandth of the energy it has already lost. Astronomers agree that the sun would show a decrease in temperature after a period much shorter than a billion years if the energy lost by the sun was not renewed continuously. Sir Isaac Newton suggested that matter itself might be changed into radiations, but it remained for recent studies of radioactivity and nuclear chemistry to provide some concrete evidence which places such an idea within the range of possibility. It is now supposed that this energy is liberated either by constructive or disintegrative processes taking place within the sun which change matter into energy. Radioactivity gives off a great deal of heat and produces helium as a by-product. The fact that there is considerable helium in the sun was responsible for the re :ioactive-decomposition theory of the source of the sun's energy. More recently, however, a reaction in which matter is built up rather than decomposed has been found to fit all of the known facts. A recent hypothesis, advanced by Bethe, that seems reasonable is