tisfactoryto have some notion of what a second is on the circle of the heavens. And, as in the one case we took an inch as our unit, we shall now take the apparent breadth of the moon, the most familiar of the heavenly bodies. Suppose a string stretched from one border of the moon to the opposite one, how many stars could be strung upon it in order that they would be a second apart from each other, the diameter of the moon being about half a degree? No fewer than two thousand would be required, each star beiug regarded as a mere point of light. Of course this string of individual stars would appear to the eye as a perfectly continuous line of light. Yet the astronomer Strüve, with the great equatorial of Dorpat, could not only individualise each star, but though one hundred more were strung on between any two of the stars, he could still measure the intervals between these interpolated stars; or, in other words, he could, with his micrometer, measure, with certainty, a space in the heavens so minute as the one-hundredth part of a second.
We have considered the measurement of space; but that of time is still more difficult. On entering the transit-room, you will observe, on looking up, that there is a narrow slit in the roof running from north to south; and your view of the heavens is confined to the narrow strip of blue which is seen through this slit. Now, the astronomer, at his transit circle, ignores all the rest of the heavens. He has fixed his
