April 12, 19 17] 



NATURE 



125 



(cm.-' sec.-^) in excess of those generated in the 

 same box on the ground : — 



Height km. o 



q o 



23456 789 



15 1-2 4-3 9-3 17-2 287 442 61-3 80-4 



The decrease in the first kilometre is due to the 

 cutting off of the {penetrating radiation from the radio- 

 active contents of the ground by the lower layers of the 

 atmosphere. The great increase in the ionisation from 

 2 km. to 9 km. is clearly shown. 



The war has naturally put an end to further observa- 

 tions in balloons, but not to the search for the origin 

 of this amazing radiation. 



In a paper published in the Elster and Greitel Fest- 

 schrift, E. V. Schweidler discusses several possible 

 sources of the radiation, only to reject them all. He first 

 calculates the absorption coefficient of the new radia- 

 tion, assuming that it is penetrating vertically down- 

 wards through the atmosphere, and finds /x = 7-46xio-* 

 cm.-' and /*/D = 577 x lo-' cm. ^/gram (the correspond- 

 ing values for 7 radiation from radium being given by 

 Rutherford as 6ox 10-^ cm.-' and4-6 x 10-- cm.^/gram 

 respectively). Applying these values to the observa- 

 tions, he finds that on the confines of the atmosphere 

 535 ions (cm.-* sec.-^) would be generated in air at 

 standard density. Assuming, then, that the radiation 

 is similar to that sent out by radio-active substances, 

 he calculates that if all the new radiation came from 

 the sun the latter would have to possess a specific 

 activitv 170 times as great as that of pure uranium. 

 This he considers to be a quite impossible value. 



Schweidler then considers the possibility of the 

 radiation being due to a radio-active gas in the atmo- 

 sphere, and shows that if the gas obeys Dalton's law, 

 the rate of increase of ^ with height would be entirely 

 out of agreement with the observed values. 



The only hypothesis considered by Schweidler which 

 is not entirely out of agreement with the observations 

 is that cosmical space is filled with a radio-active 

 gas. The calculation shows that, strange as it may 

 seem, the radiation would be independent of the density 

 of the gas, which would only need to have a specific 

 activity 1/1200 of that of uranium to provide the 

 observed ionisation. Needless to state, Schweidler 

 does not favour this latter explanation. 



In the Meteorologische Zeitschrift for April, 1916, 

 Linke attempts to solve the same problem. He shows 

 that the obser\'ations fit in very well with the ionisation 

 which would be produced by a layer of radio-active 

 substance spread uniformly throughout the atmosphere 

 at a height of 20 km. In this case the rays would not 

 penetrate only vertically downwards, but in all direc- 

 tions. This alters the coefficient of absorption from 

 /x = 7-46xio-* cm.-\ as calculated by Kolhorster, to 

 4-6x10-*, as calculated by Linke. 



Linke concludes that there is a layer of cosmical 

 •dust in the stratosphere, which is strongly radio-active, 

 and supports it by the following considerations : — 



(a) The presence of dust in the stratosphere is clearly 

 shown by several optical effects — for example, twilight 

 phenomena and Bishop's rings. 



(b) Dust which is present in the stratosphere cannot 

 fall into the troposphere except with great difficulty, 

 owing to the tempetature inversion, which is a well- 

 known trap for dust. 



(c) There was a considerable increase of this dust 

 after the earth had passed through the comet's tail in 

 May, 19 10. 



(d) On this occasion Thomson, in America, observed 

 a sudden increase in the penetrating radiation measured 

 near the ground. 



Many more observations are necessary before Linke 's 

 hypothesis can be accepted, so it is no use considering 

 it in further detail. For physicists, however, the most 



NO. 2476, VOL. 99] 



interesting fact is that these observations leave little 

 doubt of the existence of a new extremely penetrating 

 r diation, which increases as one ascends in the atmo- 

 sphere. G. C. SiMPSo.s. 



Airplanes and Atmospheric Gustiness. 



I.\ a recent discussion of the action of an airplane 

 encountering gusts, it is stated that a velocity of 

 about six metres per second may be regarded as a 

 mild gust. Making use of an exponential equation 

 and starting from a condition of srill air, increasing 

 to a certain intensity, the value of the exponent is 

 taken as determining the sharpness of the gust. With 

 a value of i, the gust reaches nearly its maximum 

 value in one second, which would be a decidedly 

 sharp gust. 



It is evident from the discussion that data for 

 the natural conditions are meagre; in fact, it seems 

 plain that the engineers have entirely under- 

 estimated the velocities likely to be met with in the 

 free air at low altitudes. And gusts do not as a 

 rule begin from a still condition. Moreover, since 

 the flow of the air may be upward, downward, 

 inclined, or on the level, straight or rotar\- and super- 

 imposed on steady or intermittent general motion, it 

 will be difficult to express in a general formula the 

 condition of flow in a gust ; and possibly no two gusts 

 will be alike. 



The problem of the stability of an airplane in a 

 gusty atmosphere belongs without doubt to the 

 aeronautical engineer; but there is another problem, 

 that of systematically recording the general character 

 of the air flow with regard to gustiness, which belongs 

 to the aerographer ; and it will be readily conceded 

 that this latter problem is now one of some moment. 

 The question is then, How shall gustiness be re- 

 corded in the various observatories of the world? 



We are attempting at Blue Hill to record each day 

 the number of hours during which aviation is con- 

 sidered safe and unsafe. Our method is doubtless 

 crude, for we use the wind velocities indicated on 

 an anemo-kinemograph, countmg as safe those hours 

 during which the average velocity does not exceed 

 10 m./s., and there is no variation greater than 50 

 per cent, in five minutes. For example, the records, 

 of March 2 and 5 (not reproduced here) illustrate 

 days on which respectively there were 24 and o 

 hours suitable for aviation. Incidentally we have 

 been able with another instrument to obtain records 

 showing a variability of 50 per cent, in three seconds ; 

 also velocities as high as 60 metres per second; and 

 one true gust in which the total air fk)w was 370 

 metres in ten seconds, of which 300 metres occurred 

 in five seconds. 



Since we have no International Committee — and 

 let it be said, not in bitterness, but sadness, that it is 

 quite unlikely that representatives of certain nations 

 will be welcomed at any international conference for 

 years to come — there is no way now open to reach 

 an agreement unless the British Meteorological Office 

 will be willing to formulate a definition. L'nder its 

 progressive director it has become the leading and 

 representative Service, and one the methods of which 

 will be generallv accepted. 



This particular feature of the weather has not 

 heretofore received much notice, other than the record- 

 ing of days on which gales occurred ; but it is evident 

 now that a more detailed record of the condition 

 known as gustiness must be kept. 



Perhaps some of the readers of Nature can offer 

 suggestions? Alexander McAdie. 



Harvard L'niversit\% Blue Hill Observatory, 

 Readville, Mass. 



