630 



NATURE 



[February 12, 1920 



ever O may move in relation to other points of 

 observation, so that the centre of the system of con- 

 tracting and expanding spheres travels with the ob- 

 server, and each observer has his own system of 

 spheres. The approaching and contracting spheres 

 contain within them the whole future; the 

 receding and expanding spheres contain the 

 past. The present is the passage of a sphere 

 through O, the observer, when that sphere is concen- 

 trated on a point. This conception of a fourth dimen- 

 sion is thus not that of a simple spatial dimension 

 like the other three, but, as required in the theory of 

 relativity, it is intimately associated with time and 

 motion, and the observer's experience of it is simply 

 the happening of events with the flux of time. It is 

 very like the Flatlander's conception of the third 

 dirnejision derived from the invading sphere. It will 

 be noticed that to different observers the impres- 

 sions of the present are not quite the same. VVe 

 observe an event in a star. It is present to us. To 

 an observer in the star it happened years ago. 



The theory of relativity involves a change in the 

 unit of time, according to the motion of the observer 

 relative to the object observed. This complication 

 did not enter into the consideration of the space behind 

 the convex mirror, so that the dynamical problems in 

 that space were relatively simple. According to the 

 theory of relativity, if the observer is moving with the 

 velocity of light, time remains unchanged. This must 

 have been the case with the Mad Hatter. With him 

 it was always six o'clock, and always tea-time. 



W. G. 



Thermionic Valves on Aircraft. 



In a paper just published in the Proceedings of the 

 Royal Society (A, vol. xcvi.) Drs. W. H. Eccles and 

 J. H. Vincent give an account of some experiments 

 on the small variations of wave-length intro- 

 duced when changes are made either in filament 

 temperature or plate voltage of a thermionic valve 

 supplying oscillating energy to a wireless circuit. It 

 may be of interest to readers of Nature to know how 

 this effect influenced the design of wireless aircraft 

 generators used in the war. 



In 1916, when experimenting with continuous-wave 

 telegraphy and telephony from aircraft, I noticed a 

 small outstanding variation of wave-length radiated 

 from an aeroplane, which variation seemed to depend 

 mainly on the speed of flight, and therefore, possibly, 

 on the voltages supplied by the windmill-driven 

 generator. 



Following up this clue, I found in the -Mr Force 

 Laboratory that the changes of wave-length introduced 

 by variations of filament temperature and plate voltage 

 were more considerable than I had thought, especially 

 on short wave-lengths. 



It was the knowledge of this fact which led to the 

 inclusion of special regulating devices in the aircraft 

 dynamo circuits, so that the wave-length variation, 

 at the best of times noticeable owing to aerial sway, 

 banking, etc., should be reduced, at anv rate, tb a 

 minimum. R. Whiddington. 



The University, I^eds, February 5. 



Popular Science. 



I SHOULD like to be allowed to underline a few 

 remarks that occur in a review entitled "Scientific 

 Biography " in Nature for January 29. The writer 

 urges that science has neglected the populace and 

 offered its wares for ponular edification in a highly 

 unedifying way. I belieye this is very true. I am 

 old enough to remember different times, and can 

 recall with truth and gratitude the feeling of en- 



NO. 2624., VOL. IC4I 



thusiasm, and even of exaltation, which I had in 

 early days on hearing or reading popular science 

 lectures. I think of Huxley, Tyndall, Clifford, 

 W. B. Carpenter, Lockyer, Roscoe, and some others. 

 Science lectures then were aimed at showing how 

 science did its work, and they brought into view 

 something of the personality of the real scientific 

 worker. 



Remembering how much I had gained, I en- 

 de^avoured in my turn to carry on the good work 

 within the much-restricted range of my own powers, 

 but in the same spirit. In time I realised two 

 things : one, the debilitating tendency of publicity and 

 easily won applause ; the other, the invasion of the 

 science platform by the mere entertainer and his 

 entrepreneur. The work became suspect to all self- 

 respecting people. The degenerated Press has com- 

 pleted the havoc. 



Is it not possible to improve matters? I believe it 

 is. No doubt some knowledge of science is more 

 prevalent than it was, but there is yet ample room 

 for the simple, popular lecture of the genuine kind 

 by men who are the real workers. It is a serious 

 tax, but I am inclined to think a justifiable one, 

 on the time of these men to give, say once a year in 

 some large city, a really popular account of their 

 latest discoveries and have it printed to sell at a 

 popular price. That, and a vocal public opinion in 

 the world of science against comic, pyrotechnic, 

 mystic, or other profane tickling of the groundlings, 

 might do much in a good cause. Victorian. 



Mirage Effects. 



The mirage effect noticed by Mr. Quilter and Miss 

 Botley is very common on Woolacombe Sands, 

 especially on hot, sunny days when the observer is 

 looking south. The apparently wet patch keeps at 

 a half to three-quarters of a mile's distance from the 

 eye, but does not persist up to the southern limit of 

 the bay, which is bounded by high ground. I cannot 

 remember whether it is visible when the observer is 

 facing north. Spencer Pickering. 



Mirage effects similar to those referred to in Nature 

 of January 29 (p. 565) have been noticed by me several 

 times in Birmingham on tarred macadam or wood- 

 block roads. The effect on a hot, sunny day is of a 

 layer of water from 2 in. to 4 in. deep on the sur- 

 face of the roadway, immersed in which are the 

 feet of pedestrians and the wheels of vehicles about a 

 hundred yards from the observer. The effect is best 

 seen when the line of sight nearly coincides with the 

 surface of the roadway, as, for instance, just before 

 one breasts the summit of a slight rise, when the 

 eye is practically level with the ground beyond the 

 top of the rise. Stooping would produce a similar 

 effect. L. N. Norris-Rogers. 



I first saw a mirage on a road in Colombo, and 

 wondered how I was going to cross the apparent sheet 

 of water in front of me. Since then I have seen it 

 repeatedly in England, and instinctively look for it 

 when the conditions are right. For the best effects 

 these conditions are three : (a) Tarred roads (the 

 reason is obvious) ; (6) bright sun ; and (c) a slight 

 gradient rising from the observer. 



In very hot weather (c) may not be so necessary. 

 .■\t other times the mirage appears where the gradient 

 reaches towards the level of the eyes. It is very 

 clear, and reflections are as sharp as in water, 

 especially of objects crossing near the further edge. 



Harrv Hillman. 



117 Colniore Row, Birmingham. 



