EVENING DISCOURSES, 



387 



by some accident, it could only be righted again with the greatest difBculty, if 

 at all. 



The lecturer finally referred to the difficulties which the aviator, as distinct 

 from the sailor, had with his navigational instruments. The aviator does not 

 know whether or not he is moving uniformly. A bubble level indicator will only 

 tell him that the acceleration, if any, is at right angles to the surface of the 

 bubble. The compa.ss is disturbed by the vibrations of the engine, to damp the 

 effect of these the whole compass is encased in a closed vessel usually filled with 

 alcohol. In the ordinary compass, the card carrying the magnets is fitted witb a 

 cup resting on a pivot. The vibrations make the card rotate and set the liquid in 

 motion; to reduce these movements the vessel is made spherical, the pivot and 

 cup are interchanged, the cup being attached to the support, and the card itself i.s 

 replaced by a short length of a cylinder having the cup on its axis. Even thus 

 improved the R.A.F. spherical compass, of which Fig. 4 gives a diagrammatic 

 section, will, on an initially northern course, move to the east or west, only, say, 

 10 deg., when a turn of 15 deg. has been made, because, in describing the curve, 

 the aeroplane is banked inwards so that the axis of the magnet card does not 

 remain vertical, and the directive force on the magnet is no longer the full 

 horizontal component of the earth's field. Moreover, the compass cannot take 

 u|) its position instantaneously. When the compass turns more quickly than 



FiqA. 

 R.A'.F. SPHERICAL COM PASS. DASHBOARD MOUNTINa 

 DIAGMMMAT/C SEC r-~\ Bezel. 



Filling Screw ■ j{ Rubber Jointing Strip. 



Glass wUh Spherical 



LubberLine.- 



Diaphry 



Outer Case_j. 



Expansion 



(eseao) 



liquid 



\ Cylindrical. Coj-d. 



Float CoaUwwrfflixgnets 

 Carrying Sapplure Cup. 



Holder for Adjusting MagriCts. 



the aeroplane, the observer would at first imagine that he was steering towards 

 the west when he was really turning eastward. This trouble could be remedied 

 by making the time of swing of the needles long compared with the time taken 

 by the machine to complete its turn. On a northerly course the turn would 

 then appear in the right direction, though too small in amount; on a southerly 

 course the turn would also appear in the right direction, but too large in amount. 

 The aviator's trouble with the sextant is that he is rarely able to take a 

 horizon reading, and even if he can that reading would have to be corrected for 

 the dip of the real horizon below the level of his niachine. In the bubble sex- 

 tant, illustrated in the diagram Fig. 5, the image of the bubble is visible at the 

 same time as the distant object. The observer has to bring the object to appear 

 in the centre of the bubble and to bring the two on the axis of his observing 

 telescope. But, as in the case of the compass, the observer should be sure that, 

 at the moment of reading, he is flying luiifoiinly without acceleration. His 

 sp«6edometer gives him his rate of motion through the air, and, with a steady 

 wind, he would know that he was flying at constant speed. His altimeter 

 aneroid, or his climbmeter, would enable him to keep on a horizontal path, but 

 he still needs something to tell him whether he is flying straight or on some 

 curve. The turn indicator gives him this information. When he banks on a 

 curve, the extremity of the outer wing is higher up and moving faster than 

 the inner-wing extremity, and there is a difference of pressui-e at these two 

 points; a. small difference, of course, yet measurable by Sir Horace Darwin's 



o 



