352 



NATURE 



[July 3, 19 19 



negative anomalies of gravity over the trough, and 

 positive anomalies on either side of it; these are, in 

 fact, the discrepancies which required explanation. 

 After showing the agreement of the theory with the 

 Himalayan and Gangetic observations, Sir S. G. 

 Burrard similarly discusses the data for other great 

 Indian troughs, and finds further confirmation oi the 

 existence of isostasy. 



The Geophysical Journal of the Meteorological 

 Office, or the British meteorological and magnetic 

 year-book, for' 1917, recently received, gives daily 

 values of the several elements observed in the British 

 Isles. Data are dealt with for solar radiation, 

 meteorology, atmospheric electricity, terrestrial mag- 

 netism, and seismology. Results for the upper air 

 are given for certain stations situated in different 

 parts of the United Kingdom, and nephoscope ob- 

 servations are made at Aberdeen, together with tables 

 showing the occurrences of aurora. The hours of 

 bright sunshine are given for several stations and the 

 percentage of the possible duration ; the normal values 

 for some stations are for thirty-five years. Meteoro- 

 logical results comprise pressure, _ temperature, wind 

 direction and velocity, and precipitation ; the values 

 are taken from self-recording'instruments. Estimation 

 is made of the cloud amount and the weather. Mag- 

 netic data are given for the observatories at Kew and 

 Eskdalemuir (Dumfriesshire). Earth temperatures 

 and the mean level of underground water are given for 

 each day at Kew Observatory. Referring to the 

 anemographs and to the wind factor derived from the 

 revolution of the cups of the anemometers, it is noted 

 that "recent investigations have shown that the 

 correct factor depends on the speed." 



A NOTE from the Nela Research Laboratory which 

 appears in the February issue of the Journal of the 

 Franklin Institute deals with the observations of Mr. 

 M. Luckiesh on the influence of temperature on the 

 transmission of a number of commercial coloured 

 glasses. In general, the transmission decreases as 

 the temperature of the glass is raised from 30° C. to 

 350° C, and in some cases there is a slight change 

 of colour of the light transmitted, which, from the 

 table of results given by the author, appears to be 

 towards the red end of the spectrum. For medium 

 red glass coloured by copper the transmission at 

 350° C. is 84 per cent, of that at 30° C, for deep red 

 copper glass 42 per cent., and for blue-green copper 

 glass 82 per cent. For pink gold glass, purple man- 

 ganese, and dull yellow glass it is 90 per cent, or 

 more, while for lemon-yellow glass it is 71 per cent. 

 The cobalt glasses transmit well, deep violet showing 

 no diminution at 350° C, while light blue transmits 

 at 350° C, 8 per cent, more than at 30° C. For a 

 yellowish-green chromium glass the transmission is 

 67 per cent. only. 



Mr. Harry J. Powell's paper on glass-making 

 before and during the war, recently read before the 

 Royal Society of Arts, is a valuable summary of the 

 achievements of the British glass trade in the very 

 trying conditions of war. Many new types of manu- 

 facture were undertaken by individual firms, and 

 especially in the field opened up by the war, which 

 deprived this country of the different classes of 

 scientific glassware obtained prior to 19 14 from Ger- 

 many. Thanks to assistance from Sir Herbert Jack- 

 son and the Institute of Chemistry (who supplied 

 recipes of certain German glasses), this particular 

 branch of the industry has obtained a good sitart in 

 the direction of rendering our country Independent of 

 German supplies In future. The author, however, 

 warned his listeners that Germany (and especially 

 Jena) have probably made progress as well during the 



NO. 2592, VOL. 103] 



war. It therefore behoves British science and the 

 glass industry to cooperate more clearly than in the 

 past ; and no doubt the new Institute of Glass Techno- 

 logy at Sheffield University will contribute in no small 

 measure to the attainment of this object. 



Lichtenberg's dust figures caused by an electric 

 spark were observed for the first time in 1777. Since 

 then they have formed the subject of a long series 

 of investigations. P. O. Pedersen has recently pub- 

 I lished in English the first part of a detailed examina- 

 I tion of the subject {Dei Kgl. Danske Videnskabernes 

 j Selskab, Mathem'atisk-fysiske Meddelelser, i., 11). 

 '■' In order to obtain pure and simple figures the 

 i Lichtenberg gap must be subjected to a very high 

 I Impulsive voltage of very short duration. The size, 

 ' shape, and character of the figures are Independent of 

 the nature of the plate and the mechanical and 

 physical condition of its surface. They are controlled 

 ! almost exclusively by the nature and pressure of the 

 surrounding gas. The difference between the positive 

 and negative figures is very striking. The pure nega- 

 tive figure appears as a white disc broken up into 

 separate parts by a number of fine dark radial lines. 

 It is attributed to lonisation by collision produced by 

 I electrons moving outwards from the electrode. The 

 j positive figures consist of sharply defined stems or 

 I trunks with short, well-defined branches or offshoots. 

 I It Is suggested tentatively that they are due to posi- 

 tive particles moving outwards from the electrode. 

 I One difficulty in the way of this view is the fact 

 j that the velocltv with which the positive figure spreads 

 j out from the electrode is two or three times greater 

 I than the corresponding velocity for the negative 

 j figure. The results already obtained seem to indicate 

 that the elucidation of the .formation of the figures 

 ' will prove of considerable theoretical Importance. 



j Although surveying by means of photography is a 

 j comparatively old art, and was actually employed more 

 i than twenty-five years ago for mapping some 25^000 

 i square miles in America under conditions that 

 i rendered surveying by the usual method quite impos- 

 sible, it is the recent war that has brought it into 

 prominence, and done more than any other circum- 

 stance to demonstrate its advantages. Moreover, the 

 recent methods are new so far as they allow the use 

 of a very high viewpoint, and also the vertical position 

 of the camera, which brings the sensitive plate parallel 

 to the ground, instead of, as is usual, perpendicular 

 to it. New conditions and new desiderata have led 

 to the designing of new forms of cameras, and these 

 we referred to a few weeks ago. But these new 

 conditions have given rise to new problems, many 

 of which were solved during- the war, but for obvious 

 reasons are only now getting published. In the 

 British Journal of Photography for May 30 there ap- 

 pears a small series of articles on " Calculations in 

 Aerial Photography," by M. L. P. Clerc, the results 

 of which were empoyed by the French Aerial Photo- 

 g-raphic Service. In these M. Clerc considers " the 

 lowering of the horizon line in photographs taken 

 from high view-points," and o-ives a diagram which 

 shows the extent of the lowering in mm. for 

 various heitrhts and various focal lengths of the ob- 

 jective. " The estimation of the height of objects by 

 the measurement of their cast shadows in aerial photo- 

 graphy " is also accompanied bv a chart, in which a 

 series of curves gives the height sought under the 

 various conditions that affect the shadow. " The limit 

 of admissible ansfling in vertical or horizontal photo- 

 graphy " is, as in the other cases, worked out mathe- 

 matically, and the results expressed In curves on charts 

 for convenience in practice. 



