It is an interesting fact that we are, perhaps, better acquainted with that side of the moon which is turned towards us than we are with the surface of the earth on which we live. Of course, I do not mean in the small details of the moon's surface, but with such portions as can be seen through a good telescope when the air is quiet. While there are no parts of the moon's surface that have not been carefully examined in detail probably thousands of times by acute astronomers, there are still comparatively large areas of the earth that have never been once trodden by civilized man. When I speak of all parts of the moon's surface, I only mean those parts that are turned towards us. You may possibly be ignorant of the fact that the moon always turns exactly the same face towards the earth. Not only has no man ever seen the opposite side of the moon, but he never can hope to see it while he remains on the earth. This is because the moon turns or rotates on its axis in exactly the same time that it revolves in its orbit. When I say that the time of rotation is the same as the time of revolution of the moon, I do not mean that it is almost the same, but that it is exactly the same. If it differed even but a small fraction of a second, a time would come when we would be able to see the other side of the moon. Now, since astronomers have made careful pictures of the moon, many, many years ago, we can see by comparing them with photographs taken at the present time there has been no change whatever in that face of the moon which is turned towards us, and this, of course, proves beyond question, that the time of the moon's rotation during this great period has remained exactly the same as the time of its revolution. It may possibly seem to you that it cannot be a matter of great importance in a book like this on the Wonders of Volcanoes and Earthquakes, whether or not the moon always turns its face towards the earth; on the contrary, it is a matter of the greatest importance since by it we can prove positively that the moon was at one time at least in a partly fluid condition. It was the presence of this partly fluid interior that resulted in the time of the moon's rotation agreeing exactly with the time of its revolution. The tides of the earth set up in the moon's molten interior, tides, that instead of reaching twice every day the height of a few feet only, were set up in the molten mass in the moon's interior, probably reaching miles in height, rapidly decreased the time of the moon's rotation until the moon rotated once only during every complete revolution.