140 



DISCOVERY 



Mercury and Venus. As regards this coincidence it seems 

 to me it could not be otherwise whether the time taken by 

 the moon be twenty-eight days or 28,000,000 years. This 

 change of speed would still leave the same face of the moon 

 turned towards us. 



If I have, say, a cricket ball suspended on the end of a 

 few yards of string and swing that round in a circle above 

 my head, is there anj^thing strange in the fact that the 

 same face of the ball is always turned inwards ? 



Surely the strangest assertion would be that the ball 

 was turning upon its axis. Rotation has been defined as 

 " lines in a rigid body changing their direction." If the 

 moon could henceforth be made to turn upon its axis in a 

 reverse direction to its revolution, and in the same time 

 lines in it would not change their direction. But we have 

 now introduced axial rotation surely, because lunar in- 

 habitants (if any) would see the earth rising and setting. 

 If we introduce a new rotation in the same direction as 

 revolution, we shall get this rising and setting of the earth. 

 But what a difference this new rotation from the old ! 

 Now, as in the case of sun spots proving the rotation of the 

 sun, we shall have moon markings proving the rotation 

 of the moon. According to the definition, the pedal of 

 a push bicycle when ridden does not rotate upon the pin. 

 Take the familiar railway turntable. It rotates upon its 

 axis. Place a cricket ball exactly in the centre, when the 

 table turns the ball will rotate upon its own axis ; place 

 an orange resting near the circumference. Is that 

 rotating upon its oivn axis when the table is turning ? 

 If so, what is a spinning-top doing in a similar position 

 when the table is turning ? The earth has, of course, axial 

 rotation in addition to revolution, and we get rising and 

 setting of the sun, but it has also (if the moon has) axial 

 rotation because of revolution in its orbit. To call each of 

 these " axial rotation " without qualification is rather 

 perplexing. In the case of the planet Uranus I read that 

 its axis almost coincides with the plane of its orbit. 

 In this case I suppose we get one axial rotation because 

 of revolution and axial rotation pure and simple. 



If everything carried round in any particular path, cir- 

 cular, elliptical, or otherwise, is turning upon its axis, then 

 everv pebble on every beach upon the surface of the earth 

 is turning upon its own axis. If the moon could be brought 

 near enough to the earth to rest upon it, the same face 

 would, of course, always be turned towards it, but like 

 the other pebbles, it would of course be turning upon its 

 axis, according to the definition, but in about twenty-four 

 hours, instead of about twenty-eight days ; but there is 

 now nothing extraordinary in time of axial rotation 

 coinciding with revolution. Distance is, I think, the only 

 change imagined here, apart from time of revolution. 



As a familiar illustration take the face of your watch. 

 The minute and hour hands rotate on an axis, and the 

 seconds hand on another. On the heel of the seconds hand 

 is a disk. Let this represent the moon, the axis of rotation 

 the earth, and the axis of rotation in the centre of tlie 

 watch the sun. Can the disk (or moon) attached rotate 

 on its axis ? 



Imagine the disk detached but still revolving. Even 

 then it is not rotating upon its axis. 



Of course I can see that the moon, in a journey of about 

 1,500,000 miles, turns itself round, but surely this cannot 

 be axial rotation. When the moon really rotated, was 

 not the earth rising and setting ? and when this rising 

 and setting ceased, did not axial rotation also cease ? 



As an unscientific reader I shall be glad if you will deal 

 with the points I have raised, especially the everyday 

 illustrations. 



I am. Sir, 



Yours, etc.. 



J. Marshall. 



2 He.ath Villa, 



BiRCHwooD Drive, 

 Leigh-on-Sea. 

 February 14, 1923. 



To the Editor of Discovery 

 Dear Sir, 



Even Homer sometimes nods, and the Editor 

 of so excellent a magazine as Discovery ma)' be excused 

 for doing the same. But surely there is a slip in the last 

 of the Editorial Notes in the March number (p. 59). There, 

 in connection with the discovery of the new element 

 Hafnium, it is said : " The use of kathode rays, as they 

 are called — the bombardment of a crystal by electrons 

 from an X-ray tube — produces an equally characteristic 

 spectrum on a sensitive photographic plate." This 

 sentence seems to contain several inaccuracies. An X-ray 

 tube emits X-rays, and not electrons. The kathode rays 

 or electrons bombard the antikathode within the tube, 

 and the antikathode then becomes the source of X-rays. 

 The crystal in the X-ray spectrometer is " bombarded " 

 (if that term is appropriate) by the characteristic X-rays 

 of the element under examination, and by reflecting these 

 from successive planes of atoms within itself acts as a 

 difiraction grating for these rays, and so produces their 

 spectrum. The characteristic X-rays of an element may 

 be produced either by having the element in question on 

 the antikathode (the direct method), or by exposing it to 

 ordinary (mixed) X-rays which are " harder " than the 

 characteristic X-rays of the element. In place of the 

 element, of course, a suitable compound may be used. 

 Moseley used the direct method of generating character- 

 istic X-rays (Kaye, X-rays, p. 200). What method may 

 have been employed by Coster and Hevesy I do not know. 



Yours, etc., 



James Patrick. 

 United Free Church Manse, 

 Ballater, 



Aberdeenshire. 

 February 28, 1923. 



[The Editor is grateful and regretful. — Ed.] 



