“Another machine is the Automatic hammer, which is a literal hammer though it may weigh many tons. The end of its handle is confined by a stationary wrist, while the hammer rises and falls under the effect of repulsion and attraction automatically alternated by shunting apparatus. Then we have the vertical parabolic railway; which consists of two steep inclined tracks, meeting each other at the foot. A car runs alternately down one and up the other on much the same principle as the pendulum machine. There are numerous other machines, but they all operate on the same principle, just as you have many forms of water wheels, all operated by the weight of water. So you see our power costs us nothing at all after the machine is built, except for the oil for its lubrication. As these machines have been known and used by us for many thousands of years, you may readily perceive what changes we have been able to make in all those conditions of our planet, that relate to our comfort and general purposes. You may add to this, that any exertion we make relating to the movement of heavy bodies, is ten times as effectual as the same exertion made on earth. Water and air with us are only one-sixth as heavy as on earth, and the average soil and rocks one-tenth as heavy; so that our laborers handle wheelbarrows holding a cubic yard of material as easily as yours do their little barrows containing two or three cubic feet.” Here I interposed again. “You speak of your atmosphere being only one-sixth as heavy as ours. That agrees with what our astronomers have told us, and they have pointed out that even if there is such an atmosphere, on the moon, animal life like ours is not possible there, because the air is too thin.” “Your astronomers do not consider that animal life and activity depend, not on the amount of air the animal is surrounded by, but by the amount of it he can use. The fishes in your waters have less air to the cubic foot of space than we have, yet are active, but if you take them out of the water and surround them with ten times as much air as they had, they nevertheless die, because they have not lungs suitable for breathing it. But furthermore it is not the amount of air that is of such consequence to animal life, but the amount of oxygen. Your air consists of about 21 parts of oxygen to 79 of nitrogen, and mixed with it is a considerable amount of carbonic acid and other impurities. In our air the proportions of nitrogen and oxygen are about reversed, and there is a far less amount of carbonic acid gas. There is also a much greater quantity of ozone, which as you know, is a concentrated and more active form of oxygen. And so on the whole, when I take a breath of air here on your earth, I get but a slightly greater quantity of oxygen than at home.” “Then you are not greatly inconvenienced in being transferred from lunar conditions to those of earth?”