During the lull of which we have spoken, a change in public opinion had occurred. From a series of retrospective calculations an astronomer had proved that the earth had already on several occasions encountered comets, and that each time the only result had been a harmless shower of shooting stars. But one of his colleagues had replied that the present comet could not in any sense be compared to a swarm of meteors, that it was gaseous, with a nucleus composed of solid bodies and he had in this connection recalled the observations made upon a comet famous in history, that of 1811. This comet of 1811 justified, in a certain respect, a real apprehension. Its dimensions were recalled to mind: its length of 180,000,000 kilometers, that is to say, a distance greater than that of the earth from the sun; and the width of its tail at its extreme point, 24,000,000 kilometers. The diameter of its nucleus measured 1,800,000 kilometers, forty thousand times that of the earth, and its nebulous and remarkably regular elliptical head was a spot brilliant as a star, having itself a diameter of no less than 200,000 kilometers. The spot appeared to be of great density. It was observed for sixteen months and twenty-two days. But the most remarkable feature of this comet was the immense development to which it attained without approaching very close to the sun; for it did not reach a point nearer than 150,000,000 kilometers, and thus remained more than 170,000,000 kilometers from the earth. As the size of comets increases as they near the sun, if this one had experienced to a greater degree the solar action, its appearance would certainly have been still more wonderful, and, doubtless, terrifying to the observer. And as its mass was far from insignificant, if it had fallen directly into the sun, its velocity, accelerated to the rate of five or six hundred thousand meters per second at the moment of collision, might, by the transformation of mechanical energy into thermal energy, have suddenly increased the solar radiations to such a degree as to have utterly destroyed in a few days every trace of vegetable and animal life upon the earth. A physicist, indeed, had made this curious remark, that a comet of the same size as that of 1811, or greater, might thus bring about the end of the world without actual contact, by a sort of expulsion of solar light and heat, analogous to that observed in the case of temporary stars. The impact would, indeed, give rise to a quantity of heat six times as great as that which would be produced by the combustion of a mass of coal equal to the mass of the comet.