June 19,. 1913] 



NATURE 



4'i 



the other end the air pressure is increased in the tube, 

 and this increase of pressure is a remarkably accurate 

 means of measuring the velocity of the wind. This 

 method is used in Dines's anemometer, and for 

 measuring the velocity of the air in the wind channel 

 at the National Physical Laboratory. In 1903 Dr. 

 Stanton read a paper before the Institution of Civil 

 Engineers (Proc. Inst. C.E., vol. clvi., p. 78) proving 

 the accuracy of this method of measuring air velocity, 

 and improvements have recently been made which 

 give even more satisfactory results. The delicate 

 measurement of the air pressure necessary for the 

 most refined work is made by the tilting water gauge 

 designed by Prof. A. P. Chattock and Mr. J. D. Fry. 

 This is a laboratory instrument of the highest order 

 of precision, and is far too delicate and accurate to be 

 used on a flying machine. It is a difference of pres- 

 sure that has to be measured — the increase of pressure 

 in the tube, above the air pressure outside — and a 

 second tube transmits this pressure (the static pres- 

 sure) to the manometer. It is found by experiment 

 that changes in the size of the opening of the Pitot 

 tube, or the thickness of the tube, or the 

 bevelling of its edge, make little or no differ- 

 ence in the pressure. With the opening of 



150 

 140 

 130 



120 



1 10 

 100 



7 8 9 10 II 12 13 



the static tube it is different, and its design 

 is important. In the design now adopted at the 

 National Physical Laboratory the pressure obtained 

 is almost exactly what we should expect from theo- 

 retical considerations. This is an advantageous 

 simplification, and this form of Pitot tube should be 

 used for all the most refined measurements. But the 

 static tube can be so made that it will give a pressure 

 below the true static pressure, and the Royal Aircraft 

 Factory has made use of this and has increased 

 the manometer readings by 20 per cent, in order to 

 give a more open scale. 



The tubes transmitting the pressure can be carried 

 a considerable distance to allow the manometer to be 

 placed in a convenient position for reading ; this is 

 often of great importance. If it is found advisable 

 to have a large amount of damping in the manometer 

 it is best to have long tubes of large diameter. This 

 gives the correct form of damping. Short tubes of 

 small diameter will also give a large amount of damp- 

 ing, but in this case the damping force will vary as 

 the square of the velocity of the air in the tube, and 

 the reading will not necessarily be the true mean. 

 For the same reason it is inadvisable to cause damp- 

 ing by throttling the passage of the air by closing a 



NO. 2277, VOL. 91] 



valve, or by means of letting it pass through a small 

 hole in a plate. 



If a Pitot tube speed-meter gives the correct speed 

 when flying near the ground level, it will not bu 

 correct when flying at a great altitude. The error is 

 caused by the change in the density of the air. As 

 you mount the air becomes less dense because the 

 atmospheric pressure is reduced, and more dense 

 because the temperature falls, and an error of 7 per 

 cent, may be expected at an altitude of 5000 ft. 



The simplest form of manometer is a U-tube con- 

 taining a liquid. The difference of the level of the 

 liquid is then a measure of the difference of the air 

 pressure in ths two tubes. For use on an aeroplane 

 this has two drawbacks : the scale is not open enough 

 to read the speed easily and accurately, and tilting 

 of the aeroplane causes an error. Mr. Short, of the 

 Royal Aircraft Factory, has designed a manometer 

 which overcomes both these objections. It is in effect 

 a U-tube manometer; he uses two liquids of different 

 densities, which do not mix, and thus obtains a 

 more open scale. One tube is placed inside the 

 other, and this overcomes the chief error due to the 

 tilting of the aeroplane, leaving only a small secondary 

 error amounting to \\ per cent, for a displacement of 

 10° out of the vertical. 



If the aeroplane has an upward or downward 

 acceleration or is changing its direction there is an 

 error. 



If a Pitot tube is fixed to the tips of the wings 

 of an aeroplane and it is flying in a circle, the speed 

 of the outer wing tip is greater than the speed of the 

 inner wing tip. If these Pitot tubes are joined to- 

 gether by a tube there will be a greater pressure at 

 one end of the tube than at the other, and at first 

 sight we should expect that there would be a flow 

 of air through the tube from the outer to the inner 

 wing tip. But this is not the case, because the aero- 

 plane is moving in a circle and there will be centri- 

 fugal force acting on the air in the tube. This will 

 tend to make it flow outwards, and will exactly balance 

 the tendency of the air to flow inwards due to the_ excess 

 pressure in the Pitot tube on the outer wing tip, and 

 there will be no flow through the tube. If there is a 

 side-slip this statement is only approximately true. 

 For accurate speed measurements at the Royal Air- 

 craft Factory two Pitot tubes are used, one at each 

 wing tip; both are connected to the manometer and 

 the mean speed is given. 



An instrument called a yaw-meter was described. 

 It measures the direction in which the air is moving 

 relatively to an aeroplane, and its action depends on 

 the fact'that the pressure in a Pitot tube becomes less 

 if it does not directly face the wind. Two Pitot tubes 

 are used, and the "indication is independent of the 

 speed of flight. 



A method of indicating the speed of ascent or 

 descent was also described. 



The Principle of Geometrical Design. 



Clerk Maxwell writes : — 



"Each solid piece of an instrument is intended 

 to be either fixed or movable, and to have a certain 

 definite shape. It is acted on by its own weight, 

 and other forces, but it ought not to be subjected to 

 unnecessary stresses, for these not only diminish its 

 strength, but (what for scientific purposes may be 

 much more injurious) they alter its figure, and may, 

 by their unexpected changes during the course of 

 an experiment, produce disturbance or confusion in 

 the observations we have to make. 



"We have, therefore, to consider the methods of 

 relieving the pieces of an instrument from unnecessary 

 strain, of securing for the fixed parts a determinate 



