arted."

"Then one only needs to get off somewhere and remain still in order to
make a trip around the World!" I exclaimed.

"You are quite right, and travelling upon the Earth's surface is the
most awkward method, because it is impossible to take advantage of the
Earth's own rapid motion. Around the World in eighty days was once
considered a remarkable feat, but if we were to travel steadily
westward we should make the circuit in very much less than twenty-four
hours. The motion of the Earth upon its axis is such an immense
advantage that if we were only going from Chicago to London, the trip
could be more easily and quickly made by going to the westward some
twenty-one thousand miles, rather than going directly eastward less than
four thousand miles. For going eastward we should have to travel a
thousand miles an hour in order to keep up with the Earth. It is
questionable whether we could make that speed tacking up and slanting
down."

"Then we shall have to follow the course of Empire, always westward!" I
laughed.

While we were talking thus, the whizzing and whistling of the wind,
which had been at first very loud and hissing, had gradually died down.
I looked at the barometer, and reported that there was scarcely
three-eighths of an inch of mercury in the tube.

"We are practically above the atmosphere, then," said the doctor,
turning in all the batteries. He tried the rudder in the ether, and
found it turned her when fully extended and turned rather hard over.

"I tried to sleep this morning at Whiting to prepare for to-night's
work," said the doctor presently; "but I find I am getting
uncontrollably drowsy. Come up, and I will show you the course we most
keep, and then I will lie down to get a little rest."

I mounted to his compartment and gazed through the telescope at Mars,
looking like a little, red baby-moon, floating in one side of the blue
circle.

"Keep him always in view, but in the edge of the field like that," said
the doctor. "We must always steer a little to the right of him--that is,
a little behind him."

"But he travels around the sun in the same direction the Earth does," I
objected. "I should think we ought to aim a little ahead of him, or to
the left, to allow for his motion forward in his orbit."

"That looks reasonable at first sight, doesn't it?" said the doctor.
"But a little learning is a dangerous thing. I will explain to you why
we must steer a little behind him after I have