It will now be possible for us to attempt to account for the formation of the lunar craters. The most probable views on the subject are certainly those adopted by Mr. Nasmyth, as represented in the cuts, though it must be admitted that they are by no means free from difficulty. We can explain the way in which the rampart around the lunar crater is formed, and the great mountain which so often adorns the center of the plain. The first of these cuts contains an imaginary sketch of a volcanic vent on the moon in the days when the craters were active. The eruption is here in the full flush of its energy, when the internal forces are hurling forth a fountain of ashes or stones, which fall at a considerable distance from the vent; and these accumulations constitute the rampart surrounding the crater. The second cut depicts the crater in a later stage of its history. The prodigious explosive power has now been exhausted, and perhaps has been intermitted for some time. A feeble jet issues from the vent, and deposits the materials close around the orifice, and thus gradually raises a mountain in the center. Besides the craters and their surrounding barriers, there are ranges of mountains on the moon, and flat plains which were once named “seas,” before it was found that water did not exist there. Astronomers also see bright ridges, or lines, or cracks of light, hard to explain. One of the chief craters is called “Ptolemy,” and in size it is roughly calculated to be no less than one hundred and fourteen miles across. Another, “Copernicus,” is about fifty-six miles; and another, “Tycho,” about fifty-four miles. The central cone-mountain of Tycho is five thousand feet high. The crater of “Schickard” is supposed to be as much as one hundred and thirty-three miles in diameter. Astronomers have agreed to name these craters after the great discoverers who enlarged our knowledge of the solar and sidereal systems. It is fitting that these great names are suggested every time the moon is seen through a telescope. To Ptolemy we are indebted for what is known as “The Ptolemaic System of the Universe,” which makes the earth the center around which the sun, moon, planets, and stars all revolve, and explains the apparently erratic movements of the planets by supposing their orbits to be epicycles; that is, curves returning upon themselves and forming loops. Tycho Brahe was willing to allow, with Copernicus, that the planets all revolved around the sun; but he taught that both sun and planets turned around the earth. The system known as “The Copernican” is now known to be the only true one, and it is universally accepted. It is so called after Nicolaus Copernicus, who was born in Thorn, Prussia, February 19, 1473. He was educated for the Church, and studied medicine, but devoted himself especially to astronomy.