78 



A'A TURE 



[November i8, 1909 



show the Martian features in spring (southern hemisphere), 

 and later at the beginning of summer, and it is seen that 

 at the tater epoch the markings were more numerous and 

 better defined. 



Perrine's Comet, 19096. — From the recent observations 

 (August-October) of Perrine's periodical comet Dr. Kobold 

 has calculated a set of elements, which he gives in No. 

 4368 of the Asitonomische Nachrichten (p. 405, 

 November 10) ; this gives the perihelion passage as 1909 

 October 31865 (M.T. Berlin). An ephemeris is also pub- 

 lished showing that the comet is apparently travelling, 

 nearly due south, through Gemini, as shown by the 

 following positions: — November 18, 7h. 3-8m., +17° 46-8'; 

 November 30, 7h. ii-gm., +5° 45'5'. On November 19 

 the comet will be some minutes west of A Geminorum, and 

 of about magnitude 12-3. 



The Liverpool Astronomical Society. — The annual 

 report of the Liverpool Astronomical Society shows that 

 its activity and membership are being well maintained. 

 In the presidential address Mr. W. E. Plummer gives an 

 interesting paper on satellites, and other notes, by Mr. 

 Thorp and Father Cortie respectively, deal with diffraction 

 gratings and sun-spot spectra. There is also a paper on 

 Morehouse's comet, illustrated by a frontispiece repro- 

 ducing photographs taken by four of the society's members. 



The P.ar,\ll.\x of the Double Star 2 2398. — In these 

 columns on September ih we directed attention to Dr. 

 Bohlin's re-determination of the parallax of the double star 

 2, from which it followed that this star, with a parallax 

 of 0-484", is the nearest to us in the northern hemisphere. 



Since Dr. Bohlin's publication several observers have 

 directed attention to the discordance of the above value 

 from several previously determined, Prof. Schlesinger point- 

 ing out (Astroiiomische Nachrichten, No. 4365, p. 359) tha^ 

 it is probably 0-15" too high. 



Dr. Bohlin now states [ihid.) that an error crept into 

 his calculations, and that, on correcting this, the parallax 

 derived from his observations is +0-251" ; there are, at 

 least, eight or nine stars in the northern hemisphere for 

 which greater parallaxes than this have been found. 



TN/-: 



MEASUREMENT OE SOLAR 

 RADIA TION} 



T N the long memoir referred to below Dr. Bemporad 

 -*■ discusses a variety of problems relating to the measure- 

 ment of solar radiation, a subject to which he has made 

 previous contributions. The material employed consists 

 mainly of observations made with pyrhehometers of the 

 now common Angstrom pattern at the peak of Teneriffe 

 during five days in June and July, 1896. Prof. Angstrom 

 and an assistant observed at two different heights, the two 

 stations simultaneously occupied being one at a level of 

 3252 metres, the other at one or other of the three levels 

 3683, 2125, and 360 metres. 



On an average day there were about sixteen observa- 

 tions at each of the two stations occupied. The observa- 

 tions were scattered over the day, the zenith distances of 

 the sun usually ranging from a little more than 5° to 

 between So° and 90°. The pyrheliometer reading on re- 

 duction gives 9. the heat received in unit time by a unit 

 of surface placed orthogonally to the sun's rays. .As the 

 sun's zenith distance alters, there is at any station a 

 change in the length of the path of the sun's rays through 

 the atmosphere. The longer the path, the greater the loss 

 by absorption, and the first problem considered by Bemporad 

 is which of several formula^, due to Crova, Bartoli, 

 Pouillet, and himself, best represents the variation of q 

 with zenith distance. The formulse all contain a quantity 

 e defined as the " mass " of air traversed by the rays 

 (for this purpose a ray may be regarded as a tube of unit 

 section), unity representing tlie value of c for a vertical 

 ray. The author refers to a previous memoir, in which 

 he has tabulated c as a function of s, the zenith distance, 

 at sea-level. Treating the morning and afternoon observa- 



' " L'Assorhimento pelettivo della Radiazione ^olare neU' Atmosfera 

 terrestre e la sui variazione coll' altezza." By Dr. A. Bemporad. Pp. iii. 

 From Reile Accademia del T.incei, anno c:cv., 190S. (Roma : Tipografia 

 della R. Accademia dei Lincei. 1908.) 



NO, 2090, VOL. 82] 



tions of each day at each station separately, he calculates 

 the best values for the constants g,, and ra in a formula 

 of Crova 's type g = 5„(i -f e)- '". The average difference 

 between the individual observed and calculated values is 

 usually well under i per cent. The author seems, how- 

 ever, to prefer a three-constant formula of the type 

 log g = a — be", and makes numerous comparisons with 

 formuUe of this type. 



If we neglect the earth's curvature and the small varia- 

 tion in E at different levels due to refraction, and regard 

 the atmosphere as formed of " layers " bounded by parallel 

 planes, an increase of z increases the length of path in 

 ail layers in the same proportion. Thus the formulse 

 involving e do not really assume that the absorption in 

 travelling through a given " mass " of air is the same at 

 all levels. With the view of ascertaining the variation 

 in absorption with height, the author proceeds to calculate 

 the " mass " of air traversed by rays, having different 

 values of s, in passing from the level of one of Angstrom's 

 stations to another. This is got by a slight modification 

 of the difference between the air " masses " from sea-level 

 up to each of the two heights, as given in a table 

 previously published by the author. One assumption in 

 the process which rather invites criticism — though there is 

 no obvious means of avoiding it in the absence of direct 

 observations — is that the temperature gradient was the 

 same at all levels and deducible from the observations at 

 the two stations. Having found M, the " mass " of air 

 traversed between the levels of the two stations, the author 

 assumes that the simultaneous values q and q' of q at 

 the two stations are connected by a formula of the type 

 log ((;'/<)) = cMi where c is the mean absorption per unit 

 mass of air for the layer between the two heights. 

 Angstrom's four stations supply, of course, three layers, 

 though the data for the three refer really to different days. 

 Thus three values of c — answering to three different layers 

 or to three different mean heights in the atmosphere — are 

 obtained for a series of- different values of c. One note- 

 worthy feature is the tendency of c, for a given value of 

 c,to be greater- in the afternoon than in the forenoon. 

 The author finds c to diminish notably with the altitude 

 of the layer, and he concludes in favour of the empirical 

 law that the absorption at different heights varies as the 

 fourth power of the density of the air. This conclusion 

 was derived primarily from : Angstrom's observations alone, 

 but the author tests it by reference to results obtained by 

 Millochau at Mont Blanc, Langley at Mount Whitney, 

 and Rizzo at Rocciamelone. On p. 97 he considers the 

 relationship of his formula to the law of variation with 

 height usually proposed for aqueous vapour. 



In an earlier paper the author had tabulated the 

 value M of the " mass " of air traversed by a ray 

 between sea-level and heights varying up to 5000 metres 

 for different values of z. His calculations assumed 

 the temperature gradient j3 to be 62° C. per km. In 

 an appendix to the present memoir he gives the correc- 

 tions to be applied to the previous table when the 

 value of /3 is not 62°, but 6°, 5° . . . o'. By a curious 

 oversight the table for /3 = 2° is printed twice over on 

 p. 103, that for /3=t° being omitted. 



The author refers in various places to the fact that the 

 absorption by the atmosphere is selective, the presence of 

 aqueous vapour in the lower strata being specially important ; 

 but Angstrom's observations in 1896 dealt only with the 

 total radiation. He also recognises that the presence of a 

 mountain may disturb the atmospheric conditions in its 

 immediate neighbourhood. He apparently ascribes to this 

 — and with good reason — certain anomalous results derived 

 from Angstrom's two highest stations, according to which 

 the absorption in the intervening layer was at times 

 negative. One cannot but entertain some doubt whether 

 the subject is yet quite ripe for mathematical calculations 

 of so elaborate a tvpe as the author's. There would 

 certainly have been a much more suitable field for their 

 application if he had had at his disposal absolutely 

 synchronous data from seven or eight stations — preferably 

 in the free atmosphere — at heights differing by regular 

 intervals of, say, 500 metres, results being obtained, not 

 merely for the total radiation, but for the portions passed 

 by a series of filters after the method recently nronosed by 

 Angstrom. C. Chree. 



