496 



SCIENCE 



[N. S. Vol. XLIV. No. 1136 



clear there at that time, it adds one more to 

 the long list of wonders associated with that 

 observatory. 



In regard to the third matter, relating to the 

 transmission of terrestrial radiation, I am quite 

 unable to understand Mr. Very's logic. His 

 mind seems to let through the consideration of 

 rays that rise vertically from the earth's sur- 

 face, but to abolish all thought of those which 

 rise obliquely. Like every other surface, all 

 parts of the earth's surface emit rays in all 

 directions within a hemisphere, and tend to 

 cool by the loss of the energy of all these rays 

 which they emit. The loss is to some extent 

 compensated by rays which reach the earth 

 from every one of these directions, and 

 which at night come mainly from the emis- 

 sion of the atmosphere itself. Mr. Angstrom 

 and others have measured at night the ex- 

 cess of the radiation emitted by a horizontal 

 blackened surface, at terrestrial temperature, 

 over the radiation received by such a sur- 

 face from above. There is no great dis- 

 agreement in the observation. All observers 

 find the net loss of radiation at 20° 0. to be 

 from 0.12 to 0.20 calories per sq. em. per min- 

 ute, depending on the state of the atmosphere. 

 But Mr. Very maintains that the whole of this 

 loss represents energy that is transmitted en- 

 tirely through the atmosphere in direct beams 

 from the earth's surface to space. I see no 

 reason to admit this at all. What is meas- 

 ured is a difference between the energy of 

 two beams of rays, one leaving the surface, the 

 other reaching it. If the atmosphere (taking 

 its entire thickness) was totally opaque to these 

 rays, there would still be a difference in these 

 amounts of energy, because the atmospheric 

 sources are at a lower temperature than the 

 earth's surface. 



To determine the transmission of the earth's 

 surface-radiation through the atmosphere, as 

 I define it, one must sum up the total of all 

 radiant energy which, having been emitted by 

 a horizontal fragment of the earth's surface, 

 escapes outside the atmosphere into space, by 

 whatever path, without having suffered true 

 absorption and re-radiation. The sum total 

 just described divided by the original quantity 



emitted by the same element of surface is the 

 transmission. Perhaps Mr. Very has in mind 

 the coefficient of vertical transmission. This 

 is naturally larger than mine, but it does not 

 serve to indicate the rate of loss of heat of 

 the earth's surface by radiation. That depends 

 on the rate of loss by oblique rays as well as 

 that by normal ones. 



C. G. Abbot 

 Mount Wilson, Calif., 

 August 17, 1916 



A REMARKABLE AURORAL DISPLAY 



Between eight and nine o'clock on the eve- 

 ning of August 26 I stepped out on the porch 

 of our cottage on the shore of Lake Douglass 

 in northern Michigan and noticed what I at 

 first mistook for an unusually bright twilight 

 for that date and hour. 



Looking up through the tree-tops I saw a 

 curious flickering as of sheet lightning on a bit 

 of cloud. But there was a peculiar streaming 

 movement which at once suggested an auroral 

 phenomenon, although I was looking towards 

 the south! Passing around the house to an 

 open field, I was fairly staggered with such a 

 spectacle of light in motion as had never been 

 dreamed of by any of our family group of 

 eight which at once answered my cry of 

 amazement. 



Practically the whole vault of the heavens 

 was alive with light. Light in patches, bands 

 and arches; in streamers, sheets and delicate 

 pencillings. Clear from the northern horizon 

 to the zenith, and far beyond until the south- 

 ern sky was invaded to within about four de- 

 grees of the horizon, and was utilized for the 

 unfolding of the display. 



I had seen what I thought to be fine auroras 

 much farther to the north, but had never even 

 heard of one which required almost the entire 

 expanse of the heavens for its staging. 



The focus of the spectacle was the zenith 

 itself, and around this was a shifting and 

 irregular zone of light below which almost 

 the entire sky was set with masses of shifting, 

 shimmering radiance constantly changing 

 shape as if the sky were a vast kaleidoscope. 

 It seemed, indeed, as if we stood beneath the 



