The height of the Monument of London is something more than two hundred feet. Other elevations, the produce of human labour, are considerably higher. It is in the nature of the mind, that we conclude from the observation that we have verified, to the accuracy of another, bearing a striking analogy to the former, that we have not verified. But analogy has its limits. Is it of irresistible certainty, or is it in fact to be considered as approaching to certainty, because we have verified an observation extending to several hundred feet, that an observation extending to ninety-five millions of miles, or to the incredible distances of which Herschel so familiarly talks, is to be treated as a fact, or laid down as a principle in science? Is it reasonable to consider two propositions as analogous, when the thing affirmed in the one is in dimension many million times as great as the thing affirmed in the other? The experience we have had as to the truth of the smaller, does it authorise us to consider the larger as unquestionable? That which I see with a bay of the sea or a wide river between, though it may appear very like something with which I am familiar at home, do I immediately affirm it to be of the same species and nature, or do I not regard it with a certain degree of scepticism, especially if, along with the resemblance in some points, it differs essentially, as for example in magnitude, in other points? We have a sensation, and we enquire into its cause. This is always a question of some uncertainty. Is its cause something of absolute and substantive existence without me, or is it not? Is its cause something of the very same nature, as the thing that gave me a similar sensation in a matter of comparatively a pigmy and diminutive extension?
All these questions an untrained and inquisitive mind will ask itself in the propositions of astronomy. We must believe or not, as we think proper or reasonable. We have no way of verifying the propositions by the trial of our senses. There they lie, to be received by us in the construction that first suggests itself to us, or not. They are something like an agreeable imagination or fiction: and a sober observer, in cold blood, will be disposed deliberately to weigh both sides of the question, and to judge whether the probability lies in favour of the actual affirmation of the millions of millions of miles, and the other incredible propositions of the travelling of light, and the rest, which even the most cautious and sceptical of the retainers of modern astronomy, find themselves compelled to receive.
But I shall be told, that the results of our observations of the distances of the heavenly bodies are unvaried. We have measured the distances and other phenomena of the sun, the moon, Mercury, Venus, Mars, Jupiter, Saturn, and their satellites, and they all fall into a grand system, so as to convey to every unprejudiced mind the conviction that this system is the truth itself. If we look at them day after day, and year after year, we see them for ever the same, and performing the same divine harmony.
Successive astronomers in different ages and countries have observed the celestial orbs, and swept the heavens, and for ever bring us back the same story of the number, the dimensions, the distances, and the arrangement of the heavenly bodies which form the subject of astronomical science.
This we have seen indeed not to be exactly the case. But, if it were, it would go a very little way towards proving the point it was brought to prove. It would shew that, the sensations and results being similar, the causes of those results must be similar to each other, but it would not shew that the causes were similar to the sensations produced. Thus, in the sensations which belong to taste, smell, sound, colour, and to those of heat and cold, there is all the uniformity which would arise, when the real external causes bore the most exact similitude to the perceptions they generate; and yet it is now universally confessed that tastes, scents, sounds, colours, and heat and cold do not exist out of ourselves. All that we are entitled therefore to conclude as to the magnitudes and distances of the heavenly bodies, is, that the causes of our sensations and perceptions, whatever they are, are not less uniform than the sensations and perceptions themselves.
It is further alleged, that we calculate eclipses, and register the various phenomena of the heavenly bodies. Thales predicted an eclipse of the sun, which took place nearly six hundred years before the Christian era. The Babylonians, the Persians, the Hindoos, and the Chinese early turned their attention to astronomy. Many of their observations were accurately recorded; and their tables extend to a period of three thousand years before the birth of Christ. Does not all this strongly argue the solidity of the science to which they belong? Who, after this, will have the presumption to question, that the men who profess astronomy proceed on real grounds, and have a profound knowledge of these things, which at first sight might appear to be set at a distance so far removed from our ken?
The answer to this is easy. I believe in all the astronomy that was believed by Thales. I do not question the statements relative to the heavenly bodies that were delivered by the wise men of the East. But the supposed discoveries that were made in the eighteenth, and even in the latter part of the seventeenth century, purporting to ascertain the precise distance of the sun, the planets, and even of the fixed stars, are matters entirely distinct from this.
Among the earliest astronomers of Greece were Thales, Anaximander, Anaximenes and Anaxagoras. Thales, we are told, held that the earth is a sphere or globe, Anaximenes that it is like a round, flat table; Anaximander that the sun is like a chariot-wheel, and is twenty-eight times larger than the earth.
