4io 



NATURE 



[June 19, 1913 



which are for the most part contained in The Journal 

 of Agricultural Science. 



The report is accompanied by a circular of the 

 society for extending the Rothamsted experiments, 

 which gives details of the financial position of 

 the trust. Subscriptions are invited for the rebuild- 

 ing of the old laboratory, which must shortly be 

 undertaken, and for the maintenance of the per- 

 manent plots, which entails very considerable annual 

 expenditure. 



DESIGN AND USE OF SCIENTIFIC INSTRU- 

 MENTS IN AERONAUTICS. 1 

 A FTER expressing his admiration for the character 

 -'*• of Wilbur Wright, his brilliant engineering 

 work, and the scientific method by which he obtained 

 his results, the lecturer considered the resemblance 

 and differences of the manufactured aeroplane and the 

 living bird. The resemblance may be simply the 

 result of copying the bird, or it may be that similar 

 designs have been arrived at independently by bird^ 

 and men. The wings of both are roughly the same 

 shape : of wide span, and narrow in the direction in 

 which the bird flies ; both have concave wings with 

 thick leading edges. In many aeroplanes hollow 

 spars are used like bones and like the quills of the 

 feathers of birds. We copy plants also in this respect, 

 for they too have learnt the economy of material 

 in the use of hollow spars. 



These resemblances are remarkable, but there are 

 great differences. The Wright brothers found no 

 biplane bird to copy and did not flap their wings. No 

 flying animal uses a continuously rotating propeller 

 to drive him forward on soaring wings, and it is 

 perhaps scarcely too much to say that if birds only 

 knew how, they would now copy the Wright brothers. 

 Muscular action and the circulation of the blood, how- 

 ever, put supreme difficulties in the way of the 

 development of the continuous rotation of a part of an 

 animal. 



Instruments Used in Aeroplanes. 



It is important to realise beforehand the difficulties 

 of using instruments on aeroplanes during flight and 

 the errors that may be introduced in the readings. 

 The aeroplane shakes, it does not remain level, and is 

 subject to acceleration in all directions. The instru- 

 ment should be so designed as not to be affected by 

 any of these disturbances. A vertical acceleration has 

 the same effect as a change in the amount of the 

 downward pull due to gravity ; the tilting of the 

 aeroplane changes the direction of the downward pull 

 with regard to the instrument. A lateral or longi- 

 tudinal acceleration has the effect of altering both the 

 direction and the amount of gravity. But vibration 

 is a greater difficulty still. The hand of an instru- 

 ment may move so much and so rapidly that it is 

 difficult to estimate the mean reading on the scale, 

 and sometimes it is quite impossible to do so. And 

 this may happen when the quantity which is indicated 

 by the position of the hand only varies slowly and by 

 small amounts. The moving part of an instrument 

 should be well balanced. This reduces the vibration 

 from the shaking of the aeroplane as well as the 

 error caused by its tilting or want of level. 



In a compass as ordinarily made, the condition of 

 balance cannot be fulfilled. The magnet rests on a 

 steel point and is horizontal, and its centre of gravity 

 is below the steel point. The force on the north pole 

 acts in a downward direction towards the north, and 

 the force on the south pole in an upward direction 



1 From the first Wilbur Wrieht memorial lecture delivered before the 

 Aeronautical Society of Great Britain on May 21, by Mr. Horace Darwin, 

 F.R.S. 



NO. 2277, VOL. 91] 



towards the south, and the magnet is made to rest in 

 a horizontal position by arranging that the centre of 

 gravity of the magnet is between its south end and 

 its centre. It is below and to one side of the point 

 about which rotation takes place. Hence a sideways 

 movement must start it swinging. The magnet and 

 card in aeroplane and ship compasses are usually sur- 

 rounded by a liquid, so that any vibration which 

 may be caused by its want of balance is rapidly- 

 reduced. 



Instruments on aeroplanes should be damped, using 

 the word to damp in the sense of " to dull " or " to 

 abate the motion of." This damping is specially im- 

 portant if it should happen that the rate of vibration 

 of the whole instrument should agree with the natural 

 rate of vibration of the moving part. When this 

 happens with an undamped instrument, the vibration 

 is excessive. Damping is also required in cases where 

 the fluctuations in the quantity to be measured are 

 rapid ; it may then be difficult to read the instrument, 

 and the excursions of the hand may indicate a much 

 greater amount of variation of the quantity than really 

 takes place. If the mean reading is required the 

 instrument must be damped, and the damping should 

 be of a particular kind. 



The essential features of satisfactory damping are 

 that no force should be applied to the moving part 

 whilst it is at rest, but that as soon as it moves a 

 force should act opposing the movement. Friction at 

 the joint does damp the instrument, but does not 

 fulfil these conditions, and is bad. The force should 

 be small when the movement is slow, and it should 

 increase when the movement becomes more rapid. The 

 most usual method is to immerse the moving part, or 

 a paddle fixed to it, in a liquid more or less viscous, 

 or the paddle can be replaced by a fan in the air. 

 Another method is to damp by the movement of a 

 copper plate between the poles of a magnet. If a 

 Pitot tube is used, the flow of air through the con- 

 necting tubes damps the instrument. 



Mr. A. Mallock has pointed out that in order to 

 obtain a true mean reading with an instrument the 

 damping force should be proportional to the velocitv 

 of movement of its index. When the damping force 

 varies as the square of the velocity there may be no 

 error or there may be a considerable error. Suppose 

 that the quantity to be measured remains at 80 for 

 2/10 second, and then suddenly increases to 140 and 

 remains at that amount for 1/10 second, and 

 then it goes back to 80 and remains at 

 that amount for 2/10 second, and that this 

 rapid oscillation goes on indefinitely.' Suppose 

 also that the instrument is damped by a force which 

 varies as the square of the velocity of the index, and 

 that it is so much damped that the hand appears to 

 remain at rest. The reading of the instrument will 

 be 92 and the true mean in reality is 100, so that we 

 have an error amounting to 8 per cent., by no means 

 a small error. The diagram (Fig. 1) gives the sup- 

 posed variations of the quantity as it would be re- 

 corded on a moving sheet of paper, and gives the 

 true mean and the instrument reading. 



In the magnetic method of damping, the force 

 varies as the velocity and the true mean is obtained. 

 With liquid and air damping the force varies as the 

 square of the velocity, unless the movement is ex- 

 tremely slow, when it varies nearlv as the velocity. 



Speed of Aeroplanes. 



The speed of the aeroplane through the air is often 

 measured by a Pitot tube and a manometer. 



The principle of the Pitot tube is very simple. If 

 the open end of a tube faces the wind, the air wants 

 to pass down the tube ; if the tube is closed at 



