'32 



NATURE 



[February 12, 1920 



its node and inclination. This puts the zodiacal 

 lig:ht out of court, even if its mass were suffi- 

 cient, which seems unlikely. Prof. Asaph Hall 

 suggested that the law of attraction should be 

 modified, the index of r being taken, not as —2, 

 but as — 2(i+d), where d is a small fraction, 

 chosen empirically so as to fit the case of Mercury. 

 This is the law adopted in Newcomb's tables, 

 and therefore in the Nautical Almanac. It would 

 give a centennial shift of the moon's perigee of 

 135"- (The discussion of Dr. E. W. Brown seems 

 to establish that there is no such excess of motion 

 in the perigee, which discredits the Hall hypo- 

 thesis.) 



Einstein's theory perfectly explained the excess 

 of motion of Mercury's perihelion, without intro- 

 ducing any arbitrary constant, or having any 

 other perceptible effect on the planetary or 

 lunar motions. By the method of exhaustion it 

 seemed to hold the field. There remained a small 

 excess of motion in the case of the node of Venus, 

 but it was only 2\ times the probable error, and 

 so was not unreasonable. ■ 



Prof. A. Fowler spoke on the attempts that 

 had been made to detect the shift towards the red 

 in the sun's spectral lines, and on the difficulties 

 in the way, which arose from the effect of vary- 

 ing pressure, the rotation of the sun, and pos- 

 sible convection currents in its atmosphere. A 

 series of cyanogen lines was selected for the test, 

 as they were not subject to shift through pressure ; 

 care was necessary to choose isolated lines, as 

 adjacent lines might influence the measures. The 

 sun's rotation could be eliminated by observing 

 opposite points of the limb. The results of the 

 measures of Evershed, St. John, Schwarzschild, 

 and recent Bonn observers were shown on the 

 screen. The mean of all gave a shift towards 

 the red of 0-003 ^- at the sun's centre and of 

 0004 A. at the limb, Einstein's predicted value 

 being o-oo8 A. Prof. Fowler inclined to the view 

 that the observed shift was due not to the Einstein 

 effect, but to cooler descending convection cur- 

 rents at the sun's centre, and to the "limb effect " 

 at the limb. 



Mr. E. Cunningham gave the following example 

 to show that the spectral shift need not necessarily 

 occur on the equivalence hypothesis : Imagine two 

 atoms each emitting light-vibrations in a non- 

 gravitational field, the periods of vibration being 

 the same. Referring them to a set of accelerating 

 axes, we simulate a gravitational field. The syn- 

 chronism between the two sets of waves is not 

 destroyed ; and on the equivalence hypothesis the 

 relation of physical sequences in the simulated 

 field is the same as in the real field. There is 

 the qualification that the atoms must be free to 

 fall — i.e. not constrained by neighbouring atoms ; 

 Mr. Cunningham doubted whether this was the 

 case on the sun's surface. He went on to say that 

 relativity did not necessarily imply the abandon- 

 ment of the a;ther; a unique a;ther could be con- 

 structed on a mechanical basis; -if it transmitted 

 light, it must also transmit stress and energy. 

 NO. 2624, VOL. 104] 



Prof. A. F. Lindemann spoke of the observed 

 average recession of 4 km. /sec. in the B-stars; 

 even those in the Orion nebula, which presumably 

 were at rest relatively to it, showed this differ- 

 ential shift ; he concluded that it was not a Doppler 

 effect, and might be the Einstein one. He noted 

 as a difficulty in the quantum theory of light that 

 to an observer at rest the mass of a quantum 

 would be infinite ; moreover, he considered that 

 since the mass of an electron changed with its 

 speed, its period of vibration should also change. 

 Speaking of Prof. Eddington's statement that a 

 sphere of water of radius 500 million km. would 

 fill all space, he preferred to say that to an 

 observer on the sphere it would appear to do 

 so, since all rays from it would be bent back 

 to it by its attraction ; but he thought there was 

 nothing to prevent other space from existing out- 

 side it. 



Prof. A. N. Whitehead showed a mathematical 

 method by which Einstein's first two astronomical 

 predictions might be satisfied without introducing 

 time as the fourth dimension. The method left it 

 uncertain whether the spectral shift would take 

 place or not. Should the latter be finally proved 

 not to exist, we might fall back on this method, 

 which agreed with the facts at present observed. 



A. C. D. Crommei.in. 



THE FLIGHT FROM CAIRO TO THE 

 CAPE. 



(i) Aviation .^xd Exploration. 

 ' I "HE enterprise of the Times in organising a 

 -^ flight from London to Cape Town via Cairo, 

 Khartum, the Upper Nile, the interior of East: 

 Africa, Northern and Southern Rhodesia, and the 

 Transvaal will certainly, if successful, greatly 

 advance the theory and practice of travelling' 

 through the air from one distant part of the world 

 to another. 



The bearing of the whole question — air travel 

 versus railway, ocean steamer, or road-motor 

 transit — was well put a few days ago by Capt. 

 Frederick Shelford in his address to the African 

 Society. There is no real cause for rivalry or 

 hostile competition between all four forms of 

 rapid transport. Air travel by aeroplane or air- 

 ship will for a long time to come be far more 

 dangerous to life than road or rail transit, and a 

 little more dangerous than sea voyages ; but it 

 will be very much quicker than all other methods. 

 It will be impossibly expensive for the transport 

 of goods or of many passengers. Sea travel by 

 boat is the cheapest mode of conveyance ; rail- 

 ways, on the whole, and especially in wild, little- 

 developed countries, are as cheaply made as motor 

 roads, and are much less expensive to maintain. 

 For mails and for passengers in a great hurry, 

 aeroplanes should have no rival, especially when 

 meteorology is better understood, and when the 

 great air routes of the world are duly provided 

 with aerodromes at convenient distances. 



It may seem to be stating too obvious a fact 



