aking up of camp. In the photographic
dark-room at home we pass moments of as thrilling interest as any in
the field, when the image begins to appear on the plate and it is yet an
open question whether we have a picture of a flying machine or merely a
patch of open sky. These slow glides in rising current probably hold out
greater hope of extensive practice than any other method within man's
reach, but they have the disadvantage of requiring rather strong winds
or very large supporting surfaces. However, when gliding operators
have attained greater skill, they can with comparative safety maintain
themselves in the air for hours at a time in this way, and thus by
constant practice so increase their knowledge and skill that they can
rise into the higher air and search out the currents which enable the
soaring birds to transport themselves to any desired point by first
rising in a circle and then sailing off at a descending angle. This
illustration shows the machine, alone, flying in a wind of 35 miles per
hour on the face of a steep hill, 100 feet high. It will be seen
that the machine not only pulls upward, but also pulls forward in the
direction from which the wind blows, thus overcoming both gravity and
the speed of the wind. We tried the same experiment with a man on it,
but found danger that the forward pull would become so strong, that the
men holding the ropes would be dragged from their insecure foothold on
the slope of the hill. So this form of experimenting was discontinued
after four or five minutes' trial.

'In looking over our experiments of the past two years, with models and
full-size machines, the following points stand out with clearness:--

'1. That the lifting power of a large machine, held stationary in a wind
at a small distance from the earth, is much less than the Lilienthal
table and our own laboratory experiments would lead us to expect. When
the machine is moved through the air, as in gliding, the discrepancy
seems much less marked.

'2. That the ratio of drift to lift in well-shaped surfaces is less at
angles of incidence of 5 degrees to 12 degrees than at an angle of 3
degrees.

'3. That in arched surfaces the centre of pressure at 90 degrees is near
the centre of the surface, but moves slowly forward as the angle becomes
less, till a critical angle varying with the shape and depth of the
curve is reached, after which it moves rapidly toward the rear till the
angle of no lift is found.

'