July 5, 1906] 



NA TURE 



235 



of the positions of Ihc stars for a particular period is a 

 work which must precede all exact investigations of their 

 propir motions. Hence it constitutes a fundamental 

 problem of astronomy. The multitude of stars seen on a 

 brii;ht night is bewildering to the casual observer. They 

 are described in poetical writings as innumerable, but when 

 an actual count is made, it is found that their number is 

 really moderate, and it is doubtful if more than two 

 thousand stars have ever been visible lo the naked eye 

 at the same time. The use of the telescope consider- 

 alily increases this number, according to the size of 

 the object-glass or reflecting mirror used. Thus Argelandcr 

 in his great star catalogue included nearly 324,200 stars 

 wliich he observed through his telescope of four inches 

 apiTlurc. The advent of photography and the manu- 

 f.u-ture of suitable lenses to be used in connection with 

 photography increased the astronomical output of a tine 

 nighi to such an extent that it became possible to 

 make a further and very substantial advance. The Inter- 

 national Star Catalogue which is at present being con- 

 structed owes its origin chiefly to the hard work of 

 :\dniiral Mouchez, who was at the time director of the 

 Paris Observatory, and who became converted to the 

 feasibility of the plan by the excellent results obtained 

 by the Brothers Henry, the pioneers in star photogriijjhy. 

 He was assisted by the energetic support of Sir David 

 Gill, to whom the first suggestion was due. The pro- 

 gramme of work was determined upon at an International 

 Conference which met at Paris in the year 1887. Eighteen 

 observatories were to take part in the work, the telescopes 

 to be used were to have an aperture of thirteen inches, and 

 such a focal length that a millimetre on the plate corre- 

 sponded to one minute of arc. Each observatory had a 

 certain region of the sky assigned to it, and undertook to 

 cover this region four times, twice with plates of short 

 exposure, twice with plates of long exposure, and to 

 measure all the stars appearing on the short-exposure photo- 

 graphs. The long exposures were intended for reproduc- 

 tion in the form of charts, and are only taken by some 

 of the observatories. As there are about 400 stars on each 

 plate, and it takes about 600 plates to cover the share of 

 one observatory once, this means that each observatory 

 has to measure nearly half a million star places, and that 

 the complete catalogue will give the positions of nearly 

 45 million stars. This includes all stars down to the 

 eleventh magnitude. 



The following is a list of observatories taking part in 

 the work : — For the northern hemisphere, Greenwich. 

 Oxford, Paris, Bordeaux, Toulouse, Potsdam, Helsingfors, 

 Rome, Catania, Algiers. For the southern hemisphere, 

 San Fernando, Tacubaya, Santiago de Chile, Cordoba, 

 Cape of Good Hope, Perth (W. Australia), .Sydney, 

 Melbourne. 



The work connected with the ultimate completion of the 

 catalogue, and especially the reproduction of the star maps, 

 requires a considerable expenditure. Each country has to 

 make its own arrangements, which in the British Empire 

 usually means that each body concerned has to pay its own 

 expenses. There was, however, in this case sonie official 

 help. The Astronomer Royal obtained a contribution of 

 5000/. from the Government for the reproduction of charts, 

 and in the case of the Cape of Good Hope the necessary 

 expenses have been met from Imperial funds. Prof. Turner, 

 of Oxford, has obtained a grant of 1000/. from the Govern- 

 ment grant of the Royal Society, and a further sum of 

 2000/. for publication from the Treasury and the University 

 of Oxford jointly ; but the .Australian colonics are much 

 hampered by the want of funds, and their work will be 

 delayed in consequence. The four French observatories, 

 on the other hand, are well supported. Each of them has 

 received a Government contribution of 25,700/., making a 

 total of well over 100,000?. More than half this goes 

 towards the reproductions of the long-exposure photographs 

 as a series of charts, which, however, have proved to be 

 so costly that they will probably never be completed. 

 Jndeed, if completed, their utility may to some extent be 

 impaired by the ditTiculty of storing them in an accessible 

 manner. Prof. Turner calculates that the series of maps 

 will form a pile of paper 30 feet high, weighing about 



NO. 19 14, VOL. 74] 



I now pass on to those undertakings which are intended 

 to fix standards of measurement, or to establish a general 

 agreement on matters in which uniformity is desirable. 

 The foremost place in this division must be given to the 

 Bureau International des Poids ct Mesures, which was 

 established in the year 1873 at Sivres, near Paris. This 

 bureau was the outcome of an International Commission 

 constituted in 1869, which had for its object the scientific 

 construction of a series of international metric standards. 

 By a convention entered into by the different countries at 

 a diplomatic conference held at Paris in March and April, 

 1875, means were created for carrying out the work of 

 verifying standards under a new International Metric Com- 

 mittee, and for the purpose of enabling the committee to 

 execute its duties effectually, as well as of securing the 

 future custody and preservation of new metric prototypes 

 and instruments, the Permanent Metric Bureau w'as 

 founded. The original cost of the bureau was 20,000/., 

 and the annual budget was fixed at 3000/. for the period 

 during which the prototypes were being prepared, after 

 which time it was expected that the expenditure could be 

 reduced to 2000/. In iqoi, however, it reached 4000/., 

 the maximum lo which, by the terms of the convention, 

 the annual budget could be raised. Great Britain did not 

 join the convention until 1884, when it declared its 

 adhesion. A first payment of 1787/. was made as entrance 

 fee, and the annual contribution now ranges between 200/. 

 and 300/. Major MacMahon, to whom I owe the above 

 details, is at present the British representative on the 

 International Committee. 



The work carried out at Sevres is not confined to the 

 reproduction of metric standards, but measurements of pre- 

 cision in various directions have been made with con- 

 spicuous success. Scientific thermometry owes much to 

 the International Bureau, and in some respects it may be 

 said that exact thermometry was created there. Prof. 

 Michelson's work, in which the length of the metre was 

 compared directly with the length of a wave of red light, 

 is another classical investigation carried on in the labor- 

 atories of the International Bureau. More recently Mr. 

 Guillaume examined the physical properties of alloys, 

 notably those of nickel steel, and proved the possibility 

 of manufacturing a material which shows no sensible 

 expansion with rise of temperature. The importance of 

 metallic rods the length of which does not depend on 

 temperature is obvious, provided they prove to be of 

 sufficient permanence. 



Time does not allow me to give an account of the con- 

 ference and conventions which have led to a general agree- 

 ment on the standards of electric measurements, but it is 

 a satisfaction to know that these standards are essentially 

 those proposed and first constructed by the British Associ- 

 ation. The old British Association ohm no doubt was 

 found to be wrong by more than i per cent., but it has 

 remained the prototype of the present international unit, 

 and in principle the old ohm, volt, and unit of current 

 stand as they were given to us by the original committee.' 



While in the case of scientific units complete agreement 

 is absolutely essential, uniformity is desirable in other cases. 

 There are matters of nomenclature in which confusion 

 has arisen purely from want of general agreement. Thus 

 the great recent improvement in the optical power of 

 telescopes has led to the discovery of many details on the 

 surface of the moon. Small craters or other distinctive 

 features named by one observer were not correctly identified 

 by another, so that at the present time the sanie name is 

 applied to quite different things by different observers. It 

 is quite clear that an international agreement in lunar 

 nomenclature is called for. 



There are other deficiencies of uniformity which perhaps 

 appear trivial, but which yet lead to the waste of a good 

 deal of time. Such, for instance, is the position of the index 

 in scientific books. The index is placed sometimes at the 

 beginning, sometimes at the end, and sometimes neither at 

 the beginning nor at the end. Some books have no index, 



1 The orieinal commiitee was appointed in iSfii. and consisted of 

 Profs. A. Williamson, C. Wheat«tnne, W. Thomson (Lord Kelvin), W. H. 

 Miller, Dr. A. Matthiessen, and Mr. F. Fenkins. In the following vear, 

 Messrs. C. Varlev. Balfour Stewart. C. W. iSir William) Siemens, Prof. 

 Clerk Maxwell, Dr. Joule, Dr. Esselbach, and Sir Charles Bright were 

 added to the 



