448 



8CIEJSCE. 



[Vol. VIII., No. 198 



a triangular black flag, for temperature, to be 

 hoisted above the other flag for higher temper- 

 ature, below for lower temperature ; and a square 

 white flag, with square black centre, for a cold 

 wave, as at present. When suspended from a 

 horizontal pole or rope, a small white streamer 

 will be used to indicate the end from which the 

 flags are to be read. This system of signals 

 is superior to the Ohio system, —red and blue, 

 sun, star, moon, — now in general use, by reason 

 of its simplicity, visibility, and cheapness ; and 

 the absence of red among its colors removes the 

 objection that many railway managers have felt 

 to the display of the other signals on the sides of 

 cars. 



THE FALL MEETING OF THE NATIONAL 

 ACADEMY. 



The semi-annual meeting of the National acad- 

 emy of sciences was held Nov. 9-11, 1886, in Bos- 

 ton. By the kindness of the Massachusetts 

 institute of technology, the academy was accom- 

 modated in its spacious buildings on Boylston 

 Street. More than half the members of the acad- 

 emy were present, the nuhiber being larger than 

 usual, owing to the interest taken by many in the 

 two hundred and fiftieth celebration of the found- 

 ing of Harvard college, which event was cele- 

 brated on the preceding days. The only business 

 of general interest related to the publication of 

 the annual volumes of memoirs. The president 

 announced that the text of vol. iii. was nearly all 

 printed, and that authors are cautioned to see that 

 the manuscript and ilkistrations are always in 

 proper shape, and complete for the printer when 

 handed in to congress early in December of each 

 year, as otherwise they are likely to be rejected. 

 Of the scientific papers read, a full list of which 

 is given on another page, we note the follow- 

 ing:— 



S. P. Langley, in a paper on ' The solar-lunar 

 spectrum,' stated that for some years past we 

 have suspected, but never actually been able to 

 demonstrate, the existence of radiations from the 

 sun of wave-lengths greater than three microns, 

 and have been in doubt whether our atmosphere 

 had entirely absorbed these if they really existed, 

 or whether they were absorbed already in the 

 sun's atmosphere and never reached ours at all. 

 He has during the last year shown that the former 

 hypothesis is more probable, and that the trouble 

 lay partly in the fact that the terrestrial absorp- 

 tion here was almost total ; partly in the apparatus, 

 wherein diffused solar radation of shorter wave- 

 lengths entirely obscured the almost infinitely 



feeble portion of these longer waves, which our 

 atmosphere had in fact transmitted. By the use 

 of very perfect rock-salt trains, and by an elabo- 

 rate device for sifting out extraneous radiations, he 

 has now been able to show the existence of cer- 

 tain of the longer solar waves, even down to the 

 extreme length of seventeen microns, to which 

 waves lamp-black is as transparent as glass is to 

 the shorter or light waves. This selective absorp- 

 tion of lamp-black has been before surmised, but 

 its existence to this degree is a new fact. On ex- 

 amining the radiation of the moon. Langley finds, 

 in spite of the feeble heat, some of these long 

 waves more easily distinguished than in solar ra- 

 diation, owing to the fact that in the case of the 

 moon, whose radiation, he observes, is mainly dark 

 heat of these very great wave-lengths, he is not 

 troubled with the enormous disturbances due to 

 the diffusion of the intense shorter waves in the 

 case of the sun. He states then that there 

 is found, by the aid of the rock-salt trains, a 

 minute amount of solar heat between three and 

 five microns, below which the cold bands which 

 have been growing closer and closer, and letting 

 less and less heat between them, practically co- 

 alesce into one almost unlimited cold band, extend- 

 ing to eleven microns; and that probably the earth's 

 atmosphere absorbs practically all the solar radi- 

 ation between five and eleven microns, and, in- 

 deed, beyond ; except that there is one band of 

 most feeble transmission from this point to about 

 sixteen microns. This transmission is here so 

 feeble that the energy of the strongest radiations 

 in this latter part of the normal spectrum is 

 less than one one-thousandth of that in the 

 visible region, and the total radiation here even 

 less in proportion to that in regions already 

 known. 



These new researches, then, while enlarging 

 the extent to which the solar infra-red spectrum 

 has been examined, to the great probable length 

 of over seventeen microns, and while confirming 

 the previously announced fact that almost no solar 

 heat reaches us in this region, are chiefly inter- 

 esting in their bearing on the question of the 

 transmissibility of our atmosphere, and as show- 

 ing that its apparent action in allowing lunar 

 heat to pass where no solar heat was found is 

 consistent with the possible existence of the latter, 

 outside our atmosphere, of every wave-length. 

 Professor Langley's researches on lunar heat are 

 not completed, but he announced the conclusion 

 as probable that the temperature of the moon's 

 sunlit surface is neither as high as assumed by 

 Lord Rosse nor as low as he himself was once in- 

 clined to think, and probably may be little higher 

 than that of melting ice. 



