252 
ABRIAL FLIGHT.? 
EGINNING with balloons, as having the priority 
in point of time, it may be remarked that the 
whole subject is included in the last 130 years dating 
from the experiment ofjthe Montgolfiers, who made 
their first ascent in 1785, but were at work for some 
years before this, and that other designs quickly 
followed containing in principle most of the appli- 
ances which are in use to-day. The ballonet, for 
instance, was proposed and tried by Charles and 
Robert. We find also designs for dirigible balloons 
of much the same shapes as dre now familiar to us. 
All attempts at propelling these vessels naturally 
failed for want of adequate power, and in some cases 
the proposed form of propulsion was impracticable, 
but in others a screw of nearly the same proportions 
as that now in use was actually tried. It was soon 
found, however, that the speed which could be 
developed by man-power or by any engine that the 
balloon could lift only amounted to a few miles an 
hour, less, that is, than the speed of a very light 
breeze. Thus, so far as directing the course of a 
vessel was concerned, the mechanism was almost 
useless, and few further attempts at mechanical pro- 
pulsion were made until the advent of the internal- 
combustion engine. 
Independently of outward form, balloons may be 
divided into two classes, according as the lifting gas 
carried is (a) constant in mass, or (b) constant in 
volume, and these again may be subdivided according 
to the relation of the pressure or density of the 
enclosed gas to that of the surrounding air. 
All the conditions, however, may be conveniently 
represented by supposing that the gas is contained in 
a massless vertical cylinder closed at the top by a 
fixed cover and. below by a moyable piston. The 
piston may be supposed to be free or clamped, and 
to be acted on by the gaseous pressures only or by 
any other additional force. 
I do not propose here to go into the questions of 
the relative merits of the rigid and non-rigid forms, 
questions which turn on structural details rather than 
on general principles, but something may be said 
on the nature of the envelope used for retaining the 
hydrogen which is now usually employed for lifting 
purposes. 
The best information on the subject is due to work 
recently carried out at the National Physical Labora- 
tory at the request of the Advisory Committee for 
Aéronautics, and will be found in detail in their 
published reports. 
It appears that among the fabrics in use there are 
enormous differences in their retentive power (that is, 
in the rate of the diffusion of hydrogen through them 
irrespective of actual leaks), differences of nearly two | 
hundredfold appearing between the worst and best 
specimens. 
Indiarubber coatings are the least satisfactory, 
allowing an escape in some cases of more than 0-7 
cubic foot for every square foot of material in twenty- 
four hours when new, and deteriorating as time goes 
on. The most retentive hitherto tested are various 
oiled silks, goldbeaters’ skin, and some other artificial 
membranes. 
When the large surface which all dirigible shapes 
expose to the air is considered, it will be seen how 
important is the choice of material, and that with 
the best the necessary hydrogen renewal is not a 
small matter, even if no ascents are made, and may 
well be more than 1000 cubic feet a day for a moder- 
ately large vessel. 
Much more than this, however, must be lost when 
1 Abridged from the ‘‘ James Forrest” Lecture, delivered before the 
Institution of Civil Engineers on April 19 by H. R. A. Mallock, F.R.S. 
NO. 2219, VOL. 89] 
NATURE 
| each such cycle work 
| from place to place 
_ this case 
[May 9, 1912 
the dirigible is in use. A thousand cubic feet of 
hydrogen gives a lifting force of about 75 lb., and 
the engines of one of the larger dirigibles will part 
with many times this weight in fuel and other ways 
in less than twelve hours. To keep the vessel at a 
constant height the lift has to be diminished or the 
downward force increased at the same rate. While 
travelling this may be effected to some extent by 
steering, but when stationary the balance can only 
be obtained by allowing the equivalent amount of 
gas to escape. To rise again an equal amount of 
ballast must be discharged. The number of ascents, 
therefore, which can be made without a fresh supply 
of hydrogen is limited by the quantity of ballast which 
can be carried. 
We may now direct our attention to the more 
promising field presented by true flying machines— 
machines, that is, which are heavier than air and 
are supported by the reaction of a downward current 
of air called into existence by the engines in ordinary 
flying or by the 
diversion of natural 
upward components 
of the wind in soar- 
ing. It is theoretic- 
ally possible also to 
maintain flight (with- 
out expenditure of 
work on the part of 
the flying machine) 
in a horizontal wind 
the velocity of which 
increases with the 
altitude or varies 
at the same level. In 
the flying 
machine has to 
descend in the direc- 
tion of the wind and 
then turn and 
ascend against it. In 
is gained, and the 
work is obtained 
from the difference 
of wind velocities. 
One or two ex- 
amples may be given 
illustrating the de- 
pendence of the 
power required on 
HiNem tien: nai nia] 
velocity. 
First take the case of a parachute, which may be 
supposed to be massless and to carry a long ladder 
up which a man climbs (Fig. 1). If the man is to 
maintain a constant elevation above the ground he 
must be able to climb as fast as the parachute falls. 
Now it is known from experiment that a surface such 
as a parachute experiences a resistance while falling 
through the air equal to about 14/1000 of a pound 
for every square foot of area at a speed of 1 foot per 
second. If we give the parachute a diameter of 
Fic. 1. 
| 36 feet, its area will be about 1000 square feet, and if 
we suppose the man to weigh 150 lb., the terminal 
150 
velocity will be given by Ue Se or v=3:3 feet per 
second. This, of course, is much more than a man 
can do. 
If we take a man-power as one-tenth of a horse- 
power, 55 feet per minute, or, at the outside, 1 foot 
per second, may be taken as the rate at which he 
can raise his own weight for any considerable length 
