;o6 



NA TURE 



[January 30, 1896 



semi-axis major of the primary orbit is 2274 astronomical units, 

 while that of the secondary orbit is i '5 astronomical units, and 

 the combined mass is i '6 times that of the sun. The whole 

 system of 70 Ophiuchi is thus contained in a space less than that 

 occupied by the solar system ; the orbit of the bright companion 

 being intermediate in size between those of Uranus and Neptune, 

 while the action of the dark satellite causes it to describe a 

 secondary orbit corresponding in size with that of Mars 

 {Astronomical Jotirnal, No. 363). 



Minor Tianet Photograi'HY.— The great value of the 

 photographic method of recording the positions of known minor 

 planets, and in searching for new ones, is admirably illustrated by 

 the results obtained by M. Charlois {Rulletin Astronotnitjue, 

 January). Between November 18, 1894, and August 29, 1895, 

 forty-one plates were exposed by him at the Nice Observatory 

 on suitable parts of the sky, and only nine of these failed to show 

 traces of the objects sought. In the remaining photographs 

 forty-four known planets and eleven new ones were recognised. 

 Four of the new ones were of the nth magnitude, three of the 

 I2th, one of the 13th, and three of the 14th, while eleven of the 

 old planets were of loth magnitude or brighter. The newly- 

 discovered planets are thus among the smallest of this class of 

 bodies. Up to the end of last year the patience of M. Charlois 

 had been rewarded by the discovery of eighty-three minor 

 planets, or a little more than one-fifth of the total number at 

 present known. 



Theory ok Comeis' Tails. — It has long been imagined 

 that the phenomena of comets' tails are in some way due to a 

 solar electrical repulsion, and additional light is thrown on the 

 subject by recent physical researches. Several investigators 

 have shown that when ultra-violet light falls on an uncharged 

 body the surface disintegrates, the particles which fly off" being 

 charged negatively, while the body itself becomes positively 

 charged. Applying this to the case of a comet. Prof. Fessenden 

 suggests that negatively charged particles are emitted from that 

 side of a comet which is turned towards the sun, while the 

 nucleus has a positive charge (Astrophysical Journal, vol. iii. 

 No. i). According to J. J. Thomson's experiments, the fact that 

 the C line of hydrogen is brighter than the F line indicates that 

 the sun's chromosphere is negatively electrified, and hence the 

 disintegrated particles of the comet will be subjected to four 

 forces ; namely, the force due to gravitation, a second force to 

 the repulsion of the negative charge on the sun, a third due to 

 the attraction of the positively charged nucleus, and a fourth 

 due to the repulsion of all the other similarly electrified particles. 

 The shape of the tail is the resultant effect of these four forces. 

 The observed effects do not demand an improbably great solar 

 potential, the value calculated being 15,000 volts. Accept- 

 ing the theory, the contraction of the head, the partition of 

 comets, multiple tails, and other appearances seem to find a 

 reasonable explanation. The increasing positive charge of the 

 nucleus as the sun is approached will result in an increased solar 

 electrical attraction, and the effect will be the same as that which 

 would be produced by a resisting medium ; that is, the period 

 will be shortened, as in the case of Encke's comet. 



The fact that the most frequently observed spectrum of a 

 comet's tail is like that of a candle flame, indicates, according 

 to J. J. Thomson's experiments, that the particles of carbon are 

 negatively electrified, and this is quoted in favour of the theory. 

 It may be pointed out, however, that there is no such direct 

 evidence to show that the nucleus is positively charged, as 

 required by the theory. 



The Rotation Period of Venus. — A valuable contribu- 

 tion to the study of the rotation period of Venus has been 

 made by Prof. Tacchini {Atti Reale Acad. Lmcei, vol. v. p. 3). 

 Observations made at the Collegio Romano during last summer, 

 tended in favour of Schiaparelli's view that the rotation period 

 of the planet is 2247 days, that is, equal to the sidereal revolu- 

 tion. He now announces that continued observations, made 

 under the best atmospheric conditions towards the end of 1895, 

 have led to the same conclusion. The observations terminated 

 on December 19, and on some occasions they extended over 

 very considerable intervals on the same mornings. On 

 November 28, for example, work was commenced at 5.45 and 

 continued until 11 o'clock, and during this time the same 

 features were constantly observed on the illuminated part of the 

 planet. A nebulous arc on the dark part of the planet, near the 

 southern cusp, observed in September last, was not seen in the 

 recent observations. 



THE INTERNATIONAL GEODETIC CONGRESS 

 A T BERLIN 



T AST month, in the new Reichstag palace, in Berlin, the 

 ■'— ' official international Congress of Geodesy met together. 

 The members represented seventeen States of Europe, Asia, and 

 America. 



At this conference the delegates of France were : MM. H. 

 Faye, Vice-President of the Bureau des Longitudes ; Tisserand, 

 Director of the Paris Observatory ; Bouquet de la Grye, the 

 retired Engineering Hydrographer in charge of the Marine ; 

 Bassot, Superintendent of the Geodetic Section of the Geo- 

 graphical Service of the Army ; and Ch. Lallemand, Director of 

 the General Levelling of France. 



The Congress was welcomed, in the name of the Prussian 

 Government by Dr. Bosse, Minister of Public Instruction. 

 After recalling the fact that the International Geodetic Associa- 

 tion was founded by the Prussian General Baeyer, Dr. Bosse 

 briefly sketched the history of progress made in the different 

 sections of geodesy during the last ten years, under the happy 

 influence of the Association. 



In reply to the Minister, M. Faye, President of the Association, 

 very appropriately remarked that although Germany has done 

 much for geodesy in the last fifty years, still France has the 

 honour of having, dliring the last century, set the example. 



Following this Prof. Foerster, Director of the Berlin Obser- 

 vatory (President of the Congress), described the recent discovery 

 of the variation of latitude. 



M. Fergola (Director of the Naples Observatory) has proposed 

 since 1883 that observations should be organised in a permanent 

 manner in observatories equally distributed round the earth, and 

 situated at more or less the same latitude, in order to observe 

 the small possible movements of the terrestrial axes. The first 

 signs of these movements, noticed by M. Ktistner at the Berlin 

 Observatory, were reported to the Conference held at Salzburg 

 in 1888 by the International Geodetic Association. The As- 

 sociation took up the question, and instituted two years after- 

 wards an astronomical station of observations at Honolulu 

 (Sandwich Isles), to control the results made in Europe. The 

 success of this undertaking has now led the permanent commis- 

 sion of the Association to propose the realisation of M. Fergola's 

 plan. This realisation will be greatly facilitated by the recent 

 construction of a special photographic telescope, of which the 

 first results were very interesting. 



We will confine ourselves to naming some of the principal 

 scientific communications made to the Congress. M. de Kalmar 

 (Austrian delegate, and reporter on accurate levelling) announced 

 the fact that, in the last three years, the total length of these 

 levellings in Europe has increased from 20,000 kilometres, and 

 exceeds to-day 120,000 kilometres. Colonel Bassot reported 

 that three geodetic bases have just been measured in Roumania 

 with the instruments of the Geographical Service, and with the 

 help of French officers. Another base must be measured next 

 in Turkey under the same conditions. M. Bouquet de la Grye 

 announced that the Bureau des Longitudes have just undertaken, 

 with the help of the officers of the French Marine, the execution 

 of a new magnetic map of the world. M. Lallemand reported, 

 amongst the principal networks of accurate levellings of Europe, 

 the existence of systematic errors — the cause as yet unknown — 

 the probable value of which, although much greater than those 

 of accidental errors, only considered up till now, is calculated 

 to be between o'l mm. and 0"2 mm. per kilometre for the French, 

 Spanish and Prussian networks. This being so, the researches 

 and efforts of those in charge of great levelling undertakings 

 should certainly aim at reducing the systematic errors. 



According to verifications made in Austria by Colonel Sterneck, 

 and confirmed by other observers, the intensity of gravity has a 

 slight diurnal oscillation. The communication of the captain of 

 the ship Voti Kalmar states that the officers of the Austrian 

 Marine have determined the intensity of gravity in thirty-nine 

 stations, situated in different seas of the world. Prof. Vogler, of 

 Berlin, exhibited an accurate levelling instrument {niveau de 

 pri^cision) constructed on the principle of the cathetometer, and 

 metallic levelling rods, formed of two rods of steel coupled with 

 a rod of zinc, and covered with a layer of aluminium. These 

 new arrangements would permit the inventor to reduce the 

 accidental errors of the levelling ; but it is doubtful if it would 

 be the same for systematic errors. 



The principal and most laborious task of the Congress con- 

 sisted in drawing up a new diplomatic convention, in place of 



NO. 1370. VOX.. ^^ 



