in the solar system—­from Mercury, from Jupiter, or Neptune—­there ought to be no mistake about the letter finding its way to Mr. John Smith.  But from his correspondent in the Great Bear this address would be still incomplete; they cannot see our earth from there, and even the sun himself only looks like a small star—­like one, in fact, of thousands of stars elsewhere.  However, each star can be distinguished, and our sun may, for instance, be recognized from the Great Bear by some designation.  We shall add the line “Near the Sun,” and then I think that from this constellation, or from any of the other stars around us, the address of Mr. John Smith may be regarded as complete.  But Mr. Smith’s correspondence may be still wider.  He may have an agent living in the cluster of Perseus or on some other objects still fainter and more distant; then “Near the Sun” is utterly inadequate as a concluding line to the address, for the sun, if it can be seen at all from thence, will be only of the significance of an excessively minute star, no more to be designated by a special name than are each of the several leaves on the trees of a forest.  What this distant correspondent will be acquainted with is not the earth or the sun but only the cluster of stars among which the sun is but a unit.  Again we use our own name to denote the cluster, and we call it the “Milky Way.”  When we add this line, we have made the address of Mr. John Smith as complete as circumstances will permit.  I think a letter posted to him anywhere ought to reach its destination.  To perfect it, however, we will finish up with one line more—­“The Universe.”

The Distances of the Stars.

I must now tell you something about the distances of the stars.  I shall not make the attempt to explain fully how astronomers make such measurements, but I will give you some notion of how it is done.  You may remember I showed you how we found the distance of a globe that was hung from the ceiling.  The principle of the method for finding the distance of a star is somewhat similar, except that we make the two observations not from the two ends of a table, not even from opposite sides of the earth, but from two opposite points on the earth’s orbit, which are therefore at a distance of one hundred and eighty-six million miles.  Imagine that on Midsummer Day, when standing on the earth here, I measure with a piece of card the angle between the star and the sun.  Six months later, on Midwinter Day, when the earth is at the opposite point of its orbit, I again measure the angle between the same star and the sun, and we can now determine the star’s distance by making a triangle.  I draw a line a foot long, and we will take this foot to represent one hundred and eighty-six million miles, the distance between the two stations; then placing the cards at the corners, I rule the two sides and complete the triangle, and the star must be at the remaining corner; then I measure the sides of the triangle, and how many feet they contain,