clockwork to pass over the point of the pen, and so a curved
line is made which accurately registers the speed of the ascent in feet
per minute. No longer is the ascent at the rate of a thousand feet a
minute, and the Propeller complains to the Engine, "I'm losing my Revs.
and the Thrust. Buck up with the Power, for the Lift is decreasing,
though the Weight remains much the same."

Quoth the Engine: "I strangle for Air. A certain proportion, and that of
right density, I must have to one part of Petrol, in order to give me
full power and compression, and here at an altitude of ten thousand feet
the Air is only two-thirds as dense as at sea-level. Oh, where is he who
will invent a contrivance to keep me supplied with air of right density
and quality? It should not be impossible within certain limits."

"We fully agree," said the dying Power and Thrust. "Only maintain Us and
you shall be surprised at the result. For our enemy Drift _decreases in
respect of distance with the increase of altitude and rarity of air_,
and there is no limit to the speed through space if only our strength
remains. And with oxygen for pilot and passengers and a steeper pitch[11]
for the Propeller we may then circle the Earth in a day!"

Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
There may be greater wonders yet. Consider that as the speed increases,
so does the momentum or stored-up force in the mass of the aeroplane
become terrific. And, bearing that in mind, remember that with altitude
_gravity decreases_. There may yet be literally other worlds to
conquer.[12]

Now at fifteen thousand feet the conditions are chilly and rare, and the
Pilot, with thoughts of breakfast far below, exclaims, "High enough!
I had better get on with the Test." And then, as he depresses the
Elevator, the Aeroplane with relief assumes its normal horizontal
position. Then, almost closing the Throttle, the Thrust dies away. Now,
the nose of the Aeroplane should sink of its own volition, and the craft
glide downward at flying speed, which is in this case a hundred miles
an hour. That is what should happen if the Designer has carefully
calculated the weight of every part and arranged for the centre of
gravity to be just the right distance in front of the centre of lift.
Thus is the Aeroplane "nose-heavy" as a glider, and just so to a degree
ensuring a speed of glide equal to its flying speed. And the Air Speed
Indicator is steady at one hund