364 
NATURE 
[Marcu 17, 1923 

motor, properly governed. Lines of light are recorded 
at definite intervals, e.g. two seconds. 
(b) Air-speed recorder, simply a Pitot-Venturi nozzle 
attached to suitable pressure-recording capsules. 
(c) Single component accelerometer, consisting in 
the main of a damped steel spring, one end of which 
is free. 
92 Second /0 42 Second 

Fic. 3.—Showing the curves of pressure on the entire surface of the 
rudder at various intervals during a left turn. 
(d) Three-component accelerometer, a combination 
of three of the previous instruments. The sensitive- 
ness of the three is adjusted corresponding to the 
amount of the acceleration to be expected. 
(e) Angular velocity recorder, making use of a 
high speed electric motor as a gyroscope. The curves 
obtained by this give, when integrated, angular 
displacement, and when differenti- 
ated, angular acceleration. 
(f) Three-component angular 
velocity recorder, a combination 
of three of the previous instru- 
ments. 
(g) Control position recorder, con- 
sisting essentially of three spring- 
controlled spools threaded on an 
axial screw, each spool actuated 
by a wire leading to the horn of 
the control surface. \ 
(h) Force recorder, using a carbon \ 
pile resistance method. 
Of course all these instruments are 
not used at the same time, but only 
as many as are needed for the study 
of each particular question. 
From a practical point of view these instruments 
allow the performance of an aeroplane to be recorded 
accurately, in a manner quite free from the personal 
impressions of the test-pilot ; and, further, the records 
taken in any manceuvre tell a story which is perfectly 
plain. Numerous questions, often raised by pilots, 
Radiography 


Floor liné Manhole 5'X 3' : 
have already been answered, and pupil pilots have 
received great assistance. 
The last research to be mentioned is one which is 
only beginning, but the apparatus has been carefully 
tested, and a few preliminary readings have been made. 
This refers to a new method of investigating the 
“scale effect.” In the ordinary wind tunnel the forces 
full scale are observed, and from these measurements 
deductions are made as to the promised performance 
of the full-sized aeroplane, or part thereof. It has 
been known for many years that in order for these 
deductions to be justified it would be necessary to have 
in the tunnel and in the flight of the actual aeroplane 
the same Reynolds’s number, as it is called. This 
number is the fraction pVL/j, in which p is the density 
of the air, V is the relative velocity of the air-stream, 
L is a linear dimension of the aeroplane (or part), and 
pis the coefficient of viscosity. It is clear that the 
Reynolds’s number in the tunnel is about one-twentieth, 
or less, that of the aeroplane in flight. To obviate 
this, a complete wind tunnel has been installed inside 
an elongated steel tank—35 feet long, 15 feet in 
diameter—in which the air is kept compressed to 20 
or 30 atmospheres (Fig. 4). The walls of the tunnel 
proper are hollow, and in this space the balances are 
installed, so as to be out of the air-stream. The 
attitude of the model in the tunnel may be varied, and 
the balancing weights may be shifted, etc., by small 
26'0" 
Monometer 
™o 




Beets eng | XZ SD) 
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Tube Connections J)- Stuffing box 
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Electric wires 
Fic. 4.—Compressed air wind-tunnel. 



Floor line 
electric motors, controlled from outside the tank. 
Readings consist in viewing through suitable small 
windows a number of Veeder counters. The import- 
ance of this apparatus from the point of view of aero- 
dynamics is sufficiently obvious, and from that of the 
aeroplane designer even more so, 
and Physics. 
By Dr. G. W. C. Kave. 
"THE frail, untrustworthy X-ray tube of 1895 
and the more robust and dependable tube of 
the present day do not differ in principle. The X-ray 
tube still remains a device for generating high-velocity 
electrons and suddenly depriving them of that velocity 
1 Abstracted from the opening address to the Society of Radiographers, 
October 31, 1922. 
NO. 2785, VOL. 111 | 
by hurtling them against a target. In fact, the tube 
possesses all the characteristics of a battlefield, except 
that as yet we lack the ability to give our shells speeds 
of the order of 50,000 miles a second. Then, just as 
a flash of flame accompanies the sudden stopping of 
the shell, so do the X-rays set out in all directions 
| from the target, travelling in straight lines just as 
and moments on a model of about one-twentieth the 
