October 26, 1905] 



NA TURE 



641 



zero, should be presented to him from many points of view. 

 Force should be taken as the rate of change of momentum. 

 All these facts should be brought out and illustrated by 

 experiment, and it should be the object of the teacher to 

 turn out a pupil with a thorough grasp of mechanical 

 principles, and not one crammed with formulce which he 

 soon forgets. 



With the report of the Mathematical Association com- 

 mittee on the teaching of mechanics Prof. Perry is sub- 

 stantially in accord, although he differs from it in wishing 

 to retain the term " centrifugal force " and to abolish the 

 " poundal. *' 



Lord Kelvin communicated a paper on the kinetic and 

 statistical equilibrium of ether in ponderable matter at any 

 temperature. If two small spheres, one covered with 

 black, the other with white cloth, were placed in space 

 at the earth's distance from the sun, the temperature of 

 the black sphere would be greater than that of the 

 white. If the spheres were at a distance from the sun 

 1000 times as great, and qqg other suns were scattered 

 through space, all at about that distance from the spheres, 

 the difference of temperature would be one-thousandth of 

 the former difference. Dr. Chree has found, using thermo- 

 meters, that in bright sunlight the difference of tempera- 

 ture is 1° C. to 3° C. On a starlight night we might 

 therefore expect a difference of o-ooi° C. or 0003.° C. 



Dr. J. T. Bottomley described his experiments on the 

 cooling of a lamp-blacked or silvered copper sphere in an 

 evacuated spherical copper enclosure kept first at the 

 temperature of liquid air, then, when the sphere has cooled, 

 raised to the temperature of boiling water. Temperatures 

 were observed thermoelectrically. The present results agree 

 with those found previously by Dr. Bottomley, and do not 

 support Stefan's law. 



The writer reviewed the recent experimental work on 

 the thermal conductivities of substances, and pointed out 

 that the balance of evidence is in favour of many sub- 

 stances decreasing in thermal conductivity as their 

 temperature is raised. 



Mr. A. Word gave a resume of the work done during 

 the past year in the Cavendish Laboratory and elsewhere 

 which justifies the conclusion that all substances are more 

 or less radio-active. 



Prof. Beattie described his observations on atmospheric 

 electricity in South Africa, and his attempt to connect the 

 observed conductivity of the air with other meteorological 

 phenomena, an attempt which he considered had proved 

 unsuccessful. 



Communications on the meteorology of South Africa 

 by Dr. Mill and by Mr. R. F. Kendall were read, and 

 Prof. Beattie gave an account of the present state of the 

 magnetic survey of the country, and exhibited charts 

 embodying the results for the declination. Necessarily the 

 work has had to be confined to positions near the rail- 

 wavs, and it will bo necessary to provide some means of 

 extending the field of operations, especially along the 

 western coast of South .\frica. The association made a 

 grant of looi. towards the expense of this extension. 



Great interest was shown in Sir David Gill's account 

 of the geodetic survey in South ."Xfrica and the .African 

 arc of meridian. After the completion of the survey of 

 Cape Colony and Natal in 1892, It became necessary 

 to determine with greater accuracy the position of the 

 twentieth parallel of longitude north of the colony at 

 points where it formed the boundary of British and German 

 territory. The work was placed in Sir David Gill's hands 

 bv the two Governments, and completed in 1903. kt the 

 same time, under the auspices of the Rhodesian Govern- 

 ment, surveys of northern and southern Rhodesia were 

 being carried out, partly In connection with the Anglo- 

 Portuguese boundary. Since the war, surveys of the Trans- 

 vaal and Orange River Colony have made steady progress, 

 and the results so far obtained were embodied in the chart 

 of South .Africa exhibited by Sir David Gill. Throughout 

 the work the bases taken were measured with the help 

 of wires which were compared with a standard base 

 400 feet long before and after use. The discordance in the 

 measurements of the Gwlbl base of about 70,000 feet 

 amounted in the aggregate to i part in 1-5 millions, and 

 this was the base measured with least accuracy. 



-As a result, it appears that along the meridian of 19° 



NT,. 1878, VOL. 72] 



east longitude the curvature of the earth agrees with that 

 given by Clarke's elements, but along meridian 26° east, 

 and more markedly along meridian 30°, this appears not 

 to be the case. A definite settlement of the question will 

 only be possible after the connection of the Rhodesian 

 triangulatlon with that of the rest of South .Africa, a con- 

 nection which will entail a cost of about ibooi. When 

 this has been achieved. Sir David Gill will have made 

 one step more towards the carrying out of his scheme for 

 a great African arc of meridian extending from the Cape_ to 

 Cairo, and by combination with the Russian-Scandinavian 

 arc, a great arc from the Cape of Good Hope to the North 

 Cape. The scheme has the hearty approval of Section .A. 



It Is somewhat remarkable that at Cape Town the 

 section should hear an account of a geodetic survey of a 

 country within the Arctic circle, but the details of the 

 geodetic survey of Spitsbergen given by Its director, Dr. 

 O. Backlund, proved of great interest. It was undertaken 

 by the Swedish and Russian Governments, was carried 

 out on the same lines as that in South Africa, and has 

 given results of a high order of accuracy considering the 

 difficulties of work in such a country. The values of g 

 found at some of the stations in the mountainous parts 

 of the country come out In defect by two or three figures 

 in the fourth place. 



One of the most important communications to the section 

 was that of Prof. Kapteyn on star streaming. Prof. 

 Kaptevn finds that the stars, the proper motions of which 

 relative to the solar system have been determined, fall into 

 two groups, one in which the motions take place in the 

 main parallel to a line joining the sun to a point 7° south 

 of a Orionis, the other with its motions parallel to the 

 line joining the sun to a point 2° south of 7) Sagittarii. 

 If the motions of these two streams be referred to the 

 centre of gravity of the whole of the stars considered, their 

 directions must be diametrically opposite. One of the 

 vertices of these motions in opposite directions Kapteyn 

 finds Is close to (, Orionis, and both lie in the central 

 line of the Milky Way. Prof. Kapteyn does not hold that 

 all motions must be in this line, but that there is a great 

 preponderance of such motions, and that motions oblique 

 to it get fewer the greater the obliquity. .At this stage of 

 the investigation he wishes to stand until further know- 

 ledge of the motions of stars in the line of sight has been 

 obtained spectroscoplcally. 



Dr. A. W. Roberts gave an account of the observations 

 he has made during the past five years on the light fluctu- 

 ations of certain southern binary stars, especially V. Puppis. 

 He has succeeded in reaching a high degree of accuracy, 

 and has determined the orbital elements of six stars by 

 means of his observations, using the relations given by 

 Rambaut. He finds the masses of two of the six systems 

 to be 60 to 300 times, and the densities 000002 to 036 

 time, those of the sun. The large masses are somewhat 

 exceptional, and Mr. Jeans suggested that the light curves 

 of stars of pear shape would be found to agree with the 

 observations made by Roberts. In support of this, Mr. 

 Jeans gave an account of his investigation of the con- 

 densation of a gas occupying initially the whole of space 

 about centres at distances apart approximately equal to 

 that from the solar system to the nearer stars, and with the 

 mass at each centre of the same order as that of the sun. 

 .Any one of these nuclei might take a spheroidal, ellipsoidal, 

 or a pear shape, or separate into two parts, according to 

 its velocity of revolution. 



Mr. R. T. .A. Innes gave an account of the state of 

 double star astronomy In the southern hemisphere, and 

 pointed out the importance of bringing up the observations 

 In the southern to the same state as those In the northern 

 hemisphere. He considers the position and climate of 

 Johannesburg offer exceptional opportunities for the work, 

 and suggested the provision of a telescope by the Transvaal 

 Government. Sir David Gill supported this suggestion. 



Of shorter communications It Is only necessary to 

 mention a few, e.g. Prof. E. W. Brown's on the present 

 state of lunar theory and the necessity of a new set of 

 lunar tables, and Dr. Rambaut's on a new Instrument for 

 measuring stellar photographs, to show that in interest and 

 importance the sectional work in .South .Africa in no way 

 falls behind that of the meetings at home. 



C. H. Lees. 



