Of course, as we have already pointed out, fumaroles and solfataras may occur in the neighborhood of a volcano at different distances from its crater. CHAPTER XXII THE VOLCANOES OF THE MOON There can be no doubt that the moon was once the seat of very great volcanic activity. It was formerly believed that the very many volcanic craters which can be seen on its surface when it is examined by a comparatively small telescope, were all extinct. While this is nearly true, yet recent investigations have shown that in all probability a feeble volcanic activity still exists in a few of these craters. The distinctness with which the surface of the moon is seen does not depend so much on the size of the telescope employed, as it does on the steadiness of the atmosphere when the telescope is being used. When one wishes to examine a very distant body like a star, it is necessary to use a powerful telescope, but in the case of a comparatively near body, like one of the planets or the moon, a big telescope is not necessary. It is, however, necessary to make the observations at some time of the year, or in some part of the world, when the air is apt to be free from winds. A person on the earth's surface looking at the heavenly bodies through a telescope is practically in the position in which he would be were he at the bottom of the water in a large lake looking up through the water at some body in the heavens. He would have no difficulty in seeing such a body distinctly as long as the upper surface of the water remained quiet, and unruffled by waves. As soon, however, as waves were set up, the images seen in the telescope are so distorted as to become practically worthless. It is for this reason that it is customary to build great astronomical observatories in parts of the world where there are apt to be many days in the year when the air is almost entirely free from wind. Since the atmosphere is apt to be disturbed by winds in both the temperate and the polar latitudes, these parts of the world are not very satisfactory as sites for astronomical observatories. The conditions are more favorable near the equator, since, although at certain seasons of the year there are very severe storms in these regions, yet there are quite long periods when the air is almost entirely free from winds. It is for this reason that Harvard University has erected an astronomical observatory at Arequipa, Peru, at an elevation of 8,000 feet above the level of the Pacific Ocean. Here, with a comparatively small object glass, of about twelve inches aperture, magnificent photographs have been obtained not only of the moon but also of the planet Mars.