NATURE 



[February 6, igo8 



to that of propagation of the primary rays as experiment- 

 ally determined is within 5 per cent, of that calculated 

 on' the ether pulse theory (see ?hil. Mag., February, 1908). 

 If Prof. Bragg can suggest a distribution of ejected pairs 

 that will produce such close agreement between the calcu- 

 lated and experimentally determined intensities, it will ho 

 time to consider the theory further. 



My argument has not been concerned with 7 rays, but 

 with the type of radiation with which I am experimentally 

 more familiar — X-rays of ordinary penetrating power. 



University of Liverpool. Cihrles G. Barkl.\. 



The Wave-length of Rontgen Rayp. 



In his theory of thcrmodynamical radiation, Planck has 



found the simple law e = /i „» = /!„-, where c is an element 



of energy, h„ = 6.55-io-"' a constant, n the frequency, \ the 

 wave-length of an electromagnetic resonator, c the velocity 

 of light; according to this "elementary law" the energy 

 of an electromagnetic resonator changes during a period 

 by a multiple of e. 



Applying Planck's elementary law on the emission of 

 Rontgen rays by stopped kathode ray particles, I have 

 found the following {Physik. Zeilschr., viii., 882, 1907). Let 



die = S — be the kinetic energy of a kathode ray, € its 



electric charge, V the freely traversed potential difference, 



the total kinetic energy may be, by stopping, transformed 



into energv of radiation. The smallest wave-length of the 



, _,.. ,...,. '2-hif - -2i„c , 



emitted Rontgen radiation is then ^t = — - = — rj ; for a 



working potential difference of 60,000 volts on a Rontgen 

 bulb At becomes bio-" cm. Haga and Wind (^Ann. d. 

 Phys., X., 305, 1903) have found by their experiments on 

 diffraction for the wave-length of the used Rontgen rays 

 the value A. = 5-io-' cm. 



It is clear . that the reversed phenomenon — the trans- 

 formation of Rontgen rays into kinetic energy of electrons 

 — gives the emission of secondary kathode rays by Rontgen 

 rays, or more generally by light. I have deduced from 

 Planck's elementary law that the maximum of the velocity 

 of secondary kathode rays is independent of the nature 

 and temperature of the radiating body, but inversely pro- 

 portional to the square root of the absorbed wave-length. 

 This statement is in agreement with the observations of 

 Innes (Proc. Roy. Soc, Ixxix., 442, 1907) ; the observations 

 cannot be explained by the hypothesis of J. J. Thomson 

 and W. Wien that the emission of secondary kathode rays 

 is produced by some radio-active process. 



It may be added that Planck's elementary law is also 

 confirmed by my observations on the Doppler effect on 

 Kanalstrahlen ; the simple or two-fold minimum of the 

 intriisilv ill this effect is explained by that law (Physik. 

 Zi-itsthi.. \iii., 913, 1907). Applying the law to a hypo- 

 thesis of the origin of banded spectra, it is possible to 

 calculate an inferior limit for the spectral position of the 

 banded spectra of the saturated and " loosed " valencies 

 in chemical compounds (PJiysik. Zcitschr., ix., 85, 1908). 



J. Stark. 



The Orientation of the Avebury Circles. 



I.N Sir, Norman Lockyer's notes on the orientation of 

 stone avenues printed in Nature, January 16, pp. 249-257, 

 in dealing with Avebury, he founds his argument as to 

 the existence and direction of the Beckhampton avenue 

 upon Stukeley's statement as to the remains of it visible 

 when he wrote in 1724. He then passes to the Kennet 

 avenue, and says : — 



" As will be seen from the map, this avenue apparently 

 was connected with the southern circle as the Beckhampton 

 one was with the northern one. If this were so, certainly 

 the enormous bank, erected apparently for spectacular pur- 

 poses, which is such a striking feature of Avebury, was 

 not made until after the Kennet avenue had fallen out of 

 anv astronomical use." 



In accordance with this statement. Sir Norman Lockyer 

 marks on the map reproduced to illustrate his notes the 

 course of the south-eastern or Kennet avenue as a straight 



NO. 1997, VOL. 7;] 



line making directly for the centre of the southern circle 

 across the existing bank and ditch well to the left of the 

 present road leading to Kennet. In this he entirely ignores- 

 the fact that Stukeley (in the map given by Long, " The 

 Temple at Abury surveyed by Dr. Stukeley in 1724 ") 

 marks two prostrate stones of the avenue actually in the 

 existing gap in the earthworks by which the Kennet road 

 enters Avebury, and furthermore notes that thev were 

 " broke 1722." Aubrey, too, in his plan taken in i66j 

 (reproduced in Jackson's "Aubrey," p. 319), shows seven 

 stones of the avenue as lining the sides of the existing 

 road immediately on its leaving the gap in the mound. 

 Lastly, there is standing at this moment a few yards 

 on the right-hand side of the Kennet road a large stone 

 which is the only one now remaining of those seen by 

 Aubrey and Stukeley at the point where the avenue struck 

 the earthwork circle. This stone was apparently not 

 noticed by Sir Norman Lockyer. 



Surely if anything can be said to be certain at all about 

 .Avebury, it is that the Kennet avenue joined the outer 

 circle through the existing gap in the rampart by which 

 the Kennet road enters it to-day, and did not make straight 

 for the centre of the southern circle over the bank and 

 ditch as shown in Sir Norman Lockyer's plan. Theoretic- 

 ally, perhaps, it ought to have done so, but as a matter 

 of fact, if any weight is to be attached to the statements 

 and plans of Aubrey and of Stukeley, and to the position 

 of the one existing remnant of the avenue on the spot 

 to-day, it did not. In the interests of accuracy it seems 

 desirable to point this out. Ed. H. Goddard. 



Stability in Flight. 



Mr. Mallock (January 30, p. 293) seems to presume,, 

 as a great many others do, that an apparatus on the 

 aeroplane principle " demands constant attention on the 

 part of the aeronaut " to maintain its stability in the air. 

 We are apt to get ideas from watching the behaviour of 

 little bits of paper floating in the gusts of wind, and to 

 forget that the flying machine of the future may run into 

 tons of weight. Though a frail canoe may easily capsize, 

 the big ship seldom turns over even in the roughest of 

 seas. Even so primitive a contrivance as we may pre- 

 sume that of Mr. Farman to be is some 33 feet across 

 and weighs, complete, half a ton. Such a structure is 

 not easily upset by mere puffs of wind. But it is also 

 evident that a machine can be designed possessing nearly 

 perfect automatic stability. Langley's model, away from 

 all human control, flew steadily on over the billows of the 

 air for a minute and a half. A well-designed and well- 

 balanced machine is automatically stable . without any 

 pendulums or other appliances ; in fact, it forms a pen- 

 dulum of itself. B. Baden-Powell. 



32 Princes Gate, S.W., February i. 



Referring to the letter which appeared under the 

 above heading in Nature of January 30, I have given 

 some little attention to this subject for the past few years,, 

 and thoroughly endorse your correspondent's views. 



.Any balancing apparatus must be automatic in its action 

 if it is to respond to the changes in the relative motion 

 of the air without delay. It would seem to nie that any 

 such apparatus must, as is suggested in the letter referred 

 to, depend on the conservation of angular momentum in a 

 pendulum or fly-wheel. .Such a pendulum (or system of 

 pendula) or fly-wheel may operate directly or indirectly, 

 i.e. the torque of resistance opposing change of angular 

 momentum may be employed to right the aeroplane, or 

 may operate mechanism to control the position of guide 

 planes or jockey weights, or rotate the main planes in a 

 suitable manner. The first case is analogous to the 

 Brennan mono-rail system, the second to the Obry torpedo 

 balance. Herbert Cihtlev. 



32 Britannia Road, S., Southsea, February i. 



The Stresses in Masonry Dams. 



I DO not think that Prof. Pearson proves his point. 

 Is it not an axiom of practical mathematics that nearly 

 identical functions (within certain limits) may have widely 

 different second differentials? Between and jr, for 



