April 9, 1903 



NA TURE 



545 



Variation of Solar Radiation Received on the Earth's 

 Surface. — In a paper published in No. u (1903) of the 

 Comptes tendus, M. Henri Dufour discusses a series of 

 observations, extending from October, 1896, to March, 1903, 

 which show that the amounts of the solar radiation recorded 

 ■during December, 1902, and January, February, and the 

 first half of March, 1903, were considerably below the aver- 

 age amounts received during these months, respectively, fcr 

 the last seven years. 



The observations on which the above statement is based 

 were made at two stations about 20 kilometres apart, and 

 during the whole of the period each set of observations has 

 been recorded by the same observer. The observers have 

 used exactly similar instruments, the actinometers of M. 

 Corva, one of which has been verified by the inventor him- 

 self and the other checked by it, and the observations 

 exactly corroborate each other. 



The figures obtained for December were so small as not 

 to warrant any conclusive statement as to the decreased 

 insolation, but the figures obtained during January, 

 February and part of March corroborate them, and show- 

 that for these three months the insolation, per sq. cm., was 

 o-ii, 015 and 019 (calories — gramme-degrees — minutes) 

 less than the mean for the same months during the past 

 six years. 



M. Dufour seeks to explain this decrease by supposing 

 that the atmosphere at the present time contains some matter 

 which is absorbing an abnormal proportion of the solar 

 radiation, and suggests that the volcanic dust thrown out 

 by Mont Felc-e may be the cause. 



Annals of the Royal University Observatory of 

 Vienna. — Vol. xiv. of these Annals, edited by Prof. 

 Edmund Weiss, director of the observatory, contains the 

 detailed results of the observations of minor planets and 

 comets made with the i6'2-cm. Fraunhofer refractor during 

 the period from August, iSqs;, to January, 1899, and with 

 a 67-cm. Grubb refractor and a 38-cm. equatorial coude 

 •during the years 1897 and 1898. 



The tables include the details of the observations of the 

 positions and magnitudes of twelve cornels ( 1 s;, , ^ iii. to 

 1898 x. inclusive), the positions of twenty-nine NGC nebula: 

 and one new one, and the nositions and magnitudes of manv 

 minor planets, including those of Eros observed during 189S. 



Vol. xvii. of the same Annals contains a "dictionary " 

 of B.D. stars, wherein references are given, opposite each 

 star's B.D. number, to all the other catalogues containing 

 ■details about the star in question. 



A Variable, or Temporary, Star in Lyra. — Herr See- 

 liger, in a communication to the Astronomische Nach- 

 richten (No. 3857), describes and gives a chart showing 

 the position of a faint star (10, 1903, I. via) which appears 

 on two plates obtained with the 42-inch telescope of the 

 Munich Observatory by Herr E. Silbernagel on September 

 2 and 3, 1902. The star in question occupies the position 

 ■a=i8h. 48m. 42s., 5 = 4-32° 39 ,p o (1S53), and is about 30s. 

 preceding and i2''o south of the Ring Nebula ; on the two 

 plates mentioned above it was equal in magnitude to two 

 twelfth magnitude stars between which it is situated, but 

 on plates taken on June 28 and December 10, 1902, on which 

 these two stars are plainly visible, it does not appear. 

 Neither is it shown on any one of thirteen plates, showing 

 thirteenth magnitude stars, obtained with a 6-inch tele- 

 scope on various dates between July, 1895, and July, 1902, 

 nor does it appear on two plates taken with a 16-inch objec- 

 tive on July 10, 1901, and July 10, 1002, although these 

 plates show stars of magnitudes 15 and 13 J respectively. 



Prof. Max Wolf obtained two photographs of this region, 

 one on January 14 and the other on February 6, 1903 ; the 

 first showed images of stars of the thirteenth magnitude, 

 and the second, which had 2h. 10m. exposure, showed much 

 fainter objects, but on neither plate does the star 10, 1903, 

 Lyrae appear. 



In an editorial note appended to Ilerr Seeliger's notice i^ 

 a communication from Prof. Hartwig, in which he states 

 that he observed the star 10, 1003, Lyra; on the morning of 

 March s (May 7, 1025I1., M.T. Bamberg) with a 10-inch 

 refractor, and found it to be of about the fourteenth magni- 

 tude, 02111. brighter than its nearest neighbour. 



NO. 1745, VOL. 67] 



THE FORMATION OF DEFINITE FIGURES BY 

 THE DEPOSITION OF DUST. 



Ti was hardly to be expected that a fine dust when separ- 

 ■*- ating out from the air could easily be made to deposit 

 in perfectly sharp, clear, and constant figures, but this is 

 easily done by simply raising the plate, on which the deposit 

 is to take place, a few degrees above that of the surrounding 

 air, and in five to six minutes, in place of a uniform deposit, 

 which would naturally lie expected, a perfectly definite figure 

 is formed ; the dust will be neaped up in certain places, and 

 in others the plate will be without a trace of deposit upon 

 it. That a plate, bombarded on every side by a thick dust, 

 should be able to compel by means of a very small amount 

 of heat added to it the failing particles to arrange them- 

 selves in such definite forms is undoubtedly remarkable. 



The active agents in bringing about these results are, no 

 doubt, the currents of air set up round and on the plates, 

 but that their flow should be so regular, so persistent, and 

 so powerful, is more than could have been anticipated. The 

 figures, although very easily formed, are in many cases 

 verv complicated, and, notwithstanding the deposit giving 

 a clear and constant record, still at present it remains an 

 unsolved problem how these complicated effects are brought 

 about. Diminished atmospheric pressure does not affect 

 the figures formed. 



The material of the plate on which the dust is to settle 

 is not a matter of consequence ; it may be of metal, glass, 

 ebonite, india-rubber, or cardboard, and the same figure 

 will be formed, but obviously on some materials the dust 

 will be more visible than on others. A glass plate is pro- 

 bably the best substance on which to receive the deposit, 

 and the best dust to use is that produced by burning mag- 

 nesium ribbon, for it is brilliantly white, and is readily 

 obtained in any quantity. A glass receiver, or a box of any 

 kind without a lid, will serve as a receptacle for the dust. 

 Light the magnesium and invert the receiver over it, and 

 if sufficient magnesium be used, a dense atmosphere of dust 

 is formed. The plate on which the figure is to form should 

 be raised about an inch above the table on a small support, 

 and then the receiver, filled with the dust, placed over it 

 and left there for six or seven minutes. The plate, previous 

 to placing it in the dust must be warmed ; if it be glass, pass 

 it over the flame of a lamp until the moisture, at first con- 

 densed on the under side, disappears ; other materials may 

 be treated much in the same kind of way, or heated in an 

 air bath. The essential point in order to obtain a good 

 figure is that the plate should be a few degrees, 10° or 

 15° ('., above that of the dust atmosphere. If it be of nearly 

 the same temperature, then the figure is but faint, and the 

 same happens if it be' some 100° to 120° above the tempera- 

 ture of the surrounding air, and if of still higher tempera- 

 ture, no deposit of dust takes place. 



Suppose now the experiment is made with a square glass 

 plate, treating it as above described ; on removing the plate 

 from the dust receiver, most of the dust having subsided, 

 the plate will be found not covered all over with a fine 

 deposit, but a clear and most delicately drawn cross, con- 

 sisting of four rays, each starting from a corner of the plate 

 and reaching to the centre, is seen. Under the above con- 

 ditions, the figure is absolutely constant : it may be dense 

 or faint, and it may be slightly distorted by conditions now 

 well known and described, but on a plate of this shape it 

 is always a cross that is formed. The figure starts from the 

 four corners, but vary the form of the plate and you vary 

 the form of the figure deposited on it. The corners being 

 the agents which principally, if not entirely, determine the 

 figure, and in this simplest case a square, it is not difficult 

 to imagine that even the slight heating of the plate is 

 sufficient to start currents of air, which, flowing round the 

 edges of the plate, carry the dust with them, and allow 

 it onlv to fall where a comparatively still atmosphere exists. 

 In other cases, the flow of the currents seems very difficult 

 to follow, still with such definite and easily produced pictures 

 it may be possible to follow the changes they undergo. 



On the square plate, the action of each corner is evident, 

 and this action of corners is still more clearly shown if 

 a plate in the form of an octagon be used (Fig. 11. With 

 a triangular plate, a figure of three limbs is produced, and 

 so on with other shapes, the corners always determine the 

 general figure, and if there be no corners, if the plate be 



