Another source of heat may be sought in the downfall of meteoric bodies upon the solar surface; and it has been calculated that the inrush of all the planets of our system would suffice to maintain the present energy for 45,604 years. But to suppose the existence near the sun of anything like the amount of meteoric matter necessary to account, on this theory, for the annual emission of heat involves consequences which are quite at variance with observed facts, though it is possible, or even practically certain, that a small proportion of the solar energy is derived from this source. We are therefore driven back upon the source afforded by the slow contraction of the sun. If this contraction happens, as it must, an enormous amount of heat must be developed by the process, so much so that Helmholtz has shown that an annual contraction of 250 feet would account for the total present emission. This contraction is so slow that about 9,500 years would need to elapse before it became measurable with anything like certainty. In the meantime, then, we may assume as a working hypothesis that the light and heat of the central body of our system are maintained, speaking generally, by his steady contraction. Of course this process cannot have gone on, and cannot go on, indefinitely; but as the best authorities have hitherto regarded the date when the sun shall have shrunk so far as to be no longer able to support life on the earth as distant from us by some ten million years, and as the latest investigations on the subject, those of Dr. See, point in the direction of a very large extension of this limit, we may have reasonable comfort in the conviction that the sun will last our time. CHAPTER V MERCURY The planet nearest to the sun is not one which has proved itself particularly attractive to observers in the past; and the reasons for its comparative unattractiveness are sufficiently obvious. Owing to the narrow limits of his orbit, he never departs further from the sun either East or West than between 27° and 28°, and the longest period for which he can be seen before sunrise or after sunset is two hours. It follows that, when seen, he is never very far from the horizon, and is therefore enveloped in the denser layers of our atmosphere, and presents the appearance sadly familiar to astronomers under the name of 'boiling,' the outlines of the planet being tremulous and confused. Of course, observers who have powerful instruments provided with graduated circles can find and follow him during the day, and it is in daylight that nearly all the best observations have been secured. But with humbler appliances observation is much restricted; and, in fact, probably many observers have never seen the planet at all.