From the above it will be seen at a glance that we have entered upon a still greater scale of distance than in dealing with the diameters of the various bodies of the system. In that case the distances were limited to thousands of miles; in this, however, we have to deal with millions. A million being ten hundred thousand, it will be noticed that even the diameter of the huge sun is well under a million miles. How indeed are we to get a grasp of such distances, when those to which we are ordinarily accustomed--the few miles' walk, the little stretch of sea or land which we gaze upon around us--are so utterly minute in comparison? The fact is, that though men may think that they can picture in their minds such immense distances, they actually can not. In matters like these we unconsciously employ a kind of convention, and we estimate a thing as being two or three or more times the size of another. More than this we are unable to do. For instance, our ordinary experience of a mile enables us to judge, in a way, of a stretch of several miles, such as one can take in with a glance; but in our estimation of a thousand miles, or even of one hundred, we are driven back upon a mental trick, so to speak. In our attempts to realise such immense distances as those in the solar system we are obliged to have recourse to analogies; to comparisons with other and simpler facts, though this is at the best a mere self-cheating device. The analogy which seems most suited to our purpose here, and one which has often been employed by writers, is borrowed from the rate at which an express train travels. Let us imagine, for instance, that we possess an express train which is capable of running anywhere, never stops, never requires fuel, and always goes along at sixty miles an hour. Suppose we commence by employing it to gauge the size of our own planet, the earth. Let us send it on a trip around the equator, the span of which is about 24,000 miles. At its sixty-miles-an-hour rate of going, this journey will take nearly 17 days. Next let us send it from the earth to the moon. This distance, 240,000 miles, being ten times as great as the last, will of course take ten times as long to cover, namely, 170 days; that is to say, nearly half a year. Again, let us send it still further afield, to the sun, for example. Here, however, it enters upon a journey which is not to be measured in thousands of miles, as the others were, but in millions. The distance from the earth to the sun, as we have seen in the foregoing table, is about 93 millions of miles. Our express train would take about 178 _years_ to traverse this. Having arrived at the sun, let us suppose that our train makes a tour right round it. This will take more than five years.