526 



NATURE 



[October i, 1903 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Radium and the Geological Age of the Earth. 



At various times since the appearance of Mr. W. E. 

 Wilson's suggestion (Nature, July 9) that the presence of 

 radium in the sun might enter as an important factor in 

 contributing to solar radiation, I had intended directing 

 the attention of geologists to the direct application of this 

 suggestion to the views entertained as to the extent of 

 geological time. Absence from home led me to defer 

 doing so. 



Prof. Darwin has in a large measure anticipated my 

 remarks (Nature, September 24) by pointing out that the 

 age of the sun can no longer be determined from dynamical 

 considerations if supplies of energy from radio-active bodies 

 go towards maintaining solar temperature. It will have 

 to be shown, indeed, that such bodies do not enter even as 

 a small ingredient into solar stuff (see Mr. Wilson's letter), 

 or else that they do not retain their heat-generating proper- 

 ties at high temperatures. So far as experiments go — 

 especially on the radio-active emanations — the latter con- 

 tention seems improbable. 



The gross dynamical supply of solar heat must no longer 

 be regarded as affording a major limit both to solar age 

 and geological time. 



But there was one other good argument from the physical 

 side opposed to the geological estimate of the earth's age : 

 that derived from the observed gradient of temperature 

 from the earth's surface inwards. Prof. Perry has pointed 

 out (Nature, Jan. 3, 1895) that an increase of conductivity 

 towards the interior would lead to extension of Lord Kelvin's 

 minor limit of time since the Consistentior Status. Quite 

 equivalent to increased supplies from the interior would be 

 a source of supply of heat in every element of the material. 

 The establishment of the existing gradient of temperature 

 inwards may, in fact, have been deferred indefinitely during 

 the exhaustion of stores of radium and similar bodies at 

 greater or shallower depths. In fact, we find these bodies 

 here ; the only question is as to how much of them exists, 

 or at one time existed, in the earth's interior. 



The remaining physical objection (that based on tidal 

 retardation) being condemned for good reasons, it would 

 appear that the estimates derived from physical speculations 

 are now subject to modification in just the direction which 

 geological data required. The hundred million years which 

 the doctrine of uniformity requires may, in fact, yet be 

 gladly accepted by the physicist. J. Joly. 



Trinity College, Dublin, September 26. 



Some Overlooked Zoological Generic Names. 



In the course of my reading, I have found a few generic 

 names of animals which have been overlooked in the pre- 

 paration of the invaluable* " Index Zoologicus," recently 

 published by the Zoological Society of London. It may be 

 as well to direct attention to them, so that zoologists may 

 take note of them, and avoid duplicating them for other 

 animals. They are : — 



Callobombus, Dalla Torre, Cat. Hymenop., x. p. 503 

 [nom. emend.]. 



Cephalacanthus, Lapworth, tenth Ann. Rep. U.S. Geol. 

 Surv., p. 641 [nom. praeocc.]. 



Fiorentinia, Dalla Torre, Cat. Hymenop., x. p. 334. 



Helenia, Walcott, Proc. U.S. Nat. Mus., 1889, p. 39 

 [not Helena, Hartm., 1881]. 



Holmia, Matthew, 1890 (subg. of Olenellus). 



Isoxys, Walcott, tenth Ann. Rep. U.S. Geol. Surv., p. 

 625. 



Leptomitus, Walcott, Bull. U.S. Geol. Surv, 1886, p. 89. 



Linnarssonia, Walcott, Amer. Journ. Sci., 1885, p. 114. 



Olenoides, Meek, cf. Amer. Journ. Set., 1888, p. 165. 



Protopharetra, Bornemann, Geol. Zeitschr., 1883, p. 274. 



Protocaris, Walcott, cf. Bull. U.S. Geol. Surv., 1886, p. 

 148. 



NO. 1770, VOL. 68] 



Protospongia, Salter, cf. Bull. U.S. Geol. Surv., No. 30, 

 p. 90. [I suppose Protospongia, Kent, 1880, is different.] 



Authorities will differ as to whether Helenia should be 

 changed because of Helena. I think it should not ; the 

 difference of a letter is enough to constitute it a distinct 

 name. T. D. A. Cockerell. 



Colorado Springs, Colorado, U.S.A. 



Height of the Atmosphere Determined from the Time 

 of Disappearance of Blue Colour of the Sky after 

 Sunset. 



The extreme height of our atmosphere has been deter- 

 mined heretofore from the observation of meteors, which 

 begin to glow when the friction becomes sufficiently intense 

 to vaporise the materials of which they are composed. 

 This method is very satisfactory from most points of view, 

 and will perhaps continue to be used by astronomers. 

 Nevertheless, I think it worth while to direct attention to 

 another method, which is more simple, and which, I believe, 

 will be found equally accurate. It consists in observing 

 with the naked eye the gradual disappearance of the blue 

 colour of the sky as darkness comes on. It is surprising 

 how accurate a person of good sight can make this observ- 

 ation when the atmosphere is perfectly clear. The time of 

 sunset should be noted, and the time of the last sensible 

 blue of the sky. With the data in the Nautical Almanac 

 a simple computation by spherical trigonometry gives the 

 depression of the sun at the instant the blue fades out into 

 black, and we at once calculate the height of the illumin- 

 ated particles overhead. The following are the results of 

 some observations taken by the writer at Arrtiapolis, Md. : — 



1903. Height. Remarks. 



August 10 ... 125 miles ... A trace of blue remaining. 



,, 21 ... 130 ,, ... Blue just vanishing. 



,, 22 ... 133 ,, ... Sky just black. 



,, 23 ... 135 ,, ... Blue has disappeared. 



,, 24 ... 132 ,, ... Blue vanishing. 



Average height, 131 miles. 



The uncertainty of this value will probably be between 

 five and ten miles. 



The instant the blue disappears from the sky is a little 

 indefinite, owing to the gradual thinning out of particles 

 in the upper air sufficiently dense to reflect blue light which 

 can be seen by the eye against a black night sky, but I 

 have not found this indefiniteness so great as might be 

 expected. It does not seem to lead to greater uncertainty 

 in the height of the atmosphere than the method depending 

 on meteors. 



Prof. NewGomb, in his " Popular Astronomy," p. 397, 

 say-i that, from observations taken at Richmond and 

 Washington during the meteoric shower of November 13, 

 1867, " the general result was that they (the meteors) were 

 first seen at an average height of 75 miles, and disappeared 

 at a height of 55 miles. There was no positive evidence 

 that any meteor commenced at a height greater than 100 

 miles. It is remarkable that this corresponds very nearly 

 to the greatest height at which most of the brilliant meteors 

 are ever certainly seen. These phenomena seem to indicate 

 that our atmosphere, instead of terminating at a height of 

 4:; miles, as was formerly supposed, really extends to a 

 height of between 100 and no miles." 



According to Lord Rayleigh's theory the blue colour of 

 the sky is due to reflection of sun-light from minute particles 

 of oxygen and nitrogen in the upper layers of our atmo- 

 sphere. This theory receives its most striking confirm- 

 ation from the long duration of the blue colour after sun- 

 set, showing the great height of the particles which scatter 

 the blue light. There can, I think, be very little doubt 

 that our atmosphere extends to a height of about 130 miles. 



Washington, D.C., September i. T. J. J. See. 



The Lyrids of 1903. 

 Being absent I did not see the letter on the Lyrids of 

 1903 at the time of its appearance in Nature of July 23. 

 The Lyrid maximum occurred this year, it would seem, on 

 the night of April 22, or a day later than an important 

 display observed by Mr. Denning on April 21, 1901. The 

 night of April 22 happened to be overcast here. There 

 was a fair amount of meteoric activity seen by the present 



