The history of science records one solitary attempt to explain the sun’s energy as coming in from outside. We have seen how the energy of motion of a bullet is transformed into heat when the speed of the bullet is checked. An astronomical example of the same effect is provided by the familiar phenomenon of shooting-stars. These are bullet-like bodies which fall into the earth’s atmosphere from outer space. So long as such a body is travelling through empty space, its fall towards the earth continually increases its speed, but, as it enters the earth’s atmosphere, its speed is checked by air-resistance, and the energy of its motion is gradually transformed into heat. The shooting-star becomes first hot and then incandescent, emitting the bright light by which we recognise it. Finally, the heat completely vaporises it, and it disappears from sight, leaving only a momentary trail of luminous gas behind. The original energy of motion of the shooting-star has been transformed into light and heat—the light by which we see it, and the heat by which it is ultimately vaporised. In 1849, Robert Mayer suggested that the energy which the sun emitted as radiation might accrue to it from a continuous fall of shooting-stars or similar bodies into the solar atmosphere. The suggestion is untenable, because a simple calculation shews that a mass of such bodies equal to the weight of the whole earth would hardly maintain the sun’s radiation for a century, and that the infall needed to maintain the sun’s radiation for 30 million years would double its weight. As it is quite impossible to admit that the sun’s weight can be increasing at any such rate, Mayer’s hypothesis has to be abandoned. In 1853 Helmholtz put forward a very similar theory, the famous “contraction-hypothesis,” according to which the sun’s own shrinkage sets free the energy which ultimately appears as radiation. If the sun’s radius shrinks by a mile, its outer atmosphere falls through a height of a mile and sets free as much energy in so doing as would be yielded up by an equal weight of shooting-stars falling through a mile and having their motion checked. On Helmholtz’s theory, the different parts of the sun’s own body performed the _rôles_ which Mayer had allotted to shooting-stars falling in from outside; they performed these same parts again and again, until ultimately the sun had shrunk so far that it could shrink no further. Yet Helmholtz’s theory, like that of Mayer, failed to survive the test of numerical computation. In 1862 Lord Kelvin calculated that the shrinkage of the sun to its present size could hardly have provided energy for more than about 50 million years of radiation in the past, whereas the geological evidence already noticed (p. 151) shews that the sun must have been shining for a period enormously longer than this.