re or less make no
difference in their apparent size, or in their positions in regard to
each other in the constellations as we know them in our maps.

As we were now fairly on our way, and moving rapidly in the direction we
wished to travel, I thought it quite time to put into operation a scheme
which John and I had previously decided upon, so I told M'Allister that
he must be prepared to take a little change of air.

"Why, Professor," he exclaimed, "that sounds almost like a proposal for
going to the seaside!"

"We certainly are not going there," I replied, "for we are rapidly
moving away from all seaside resorts, and you are not likely to visit
any of those places for a very long time to come."

"Well, mon, where are we going to get our change of air then?" he
inquired; "you know there's no air at all outside of this vessel."

"Quite true," I answered; "so we must get our change of air inside the
vessel."

"Yes," interposed John, "and, Kenneth M'Allister, you will have to make
up your mind to have rather short commons of it; the same as we shall!"

"Whatever do you mean?" he inquired, now appearing really scared--for a
dreadful thought had crossed his mind. "Mon, you surely do not mean that
our machinery is giving out!"

"Oh no! not at all, M'Allister," I replied; "but perhaps I had better
give you a full explanation of the matter:--

"You know we are bound for the planet Mars, where the air is very much
thinner than that which we have been accustomed to breathe, and very
probably it is composed of somewhat different constituents. In these
circumstances you will understand that, if we landed upon Mars without
having taken proper precautions, such thin air might make us very ill,
even if it did not kill us.

"That little compartment next the store-room was arranged and fitted up
for the special purpose of supplying a thin air in which we could
prepare ourselves for the atmosphere of the red planet. So we are really
going into training. The machines in that room will generate an
attenuated atmosphere somewhat similar to our own, and this will be
automatically mixed in a cylinder with a little oxygen and nitrous oxide
gas, so as to make it as near as possible like what we expect to find
upon Mars. When we commence it will be only slightly different from our
own air; then gradually we shall reduce its density and change its
quality until it is as thin as we shall require. Each of us must spend
about eight hou