I. 2. BY PROFESSOR J. C. BEATTIE, D.Sc., F.R.S.E. Distribution of Magnetic Observatories throughout the world. History of Cape Magnetic Observatory and the attempts to revive it. 3. Purposes served by a Magnetic Observatory. 4. Suggested scheme for Permanent Magnetic Observatories in I.-DISTRIBUTION OF MAGNETIC OBSERVATORIES THROUGHOUT There are at the present day between forty and fifty thoroughly equipped, permanent magnetic observatories in the world. Of these all but five are in the Northern Hemisphere. These five are Batavia in the Eastern Archipelago, Mauritius, Melbourne in Australia, Christchurch in New Zealand, and Córdova in South America. In Melbourne, Christchurch, and Mauritius the capital sum required for building and equipment and the current expenses, are provided by the respective local Governments. On the African Mainland itself the only observatory is Dar-es-Salaam in German East Africa, which is at present in course of erection. There are four in Great Britain, four in France, and four more under construction. Japan has six such observatories, and no European country except Turkey, Servia, Bulgaria, and Greece is without its magnetic observatory. 2.-HISTORY OF CAPE MAGNETIC OBSERVATORY AND THE ATTEMPTS TO REVIVE IT. A permanent magnetic observatory was established at the Royal Observatory at the Cape in 1841. It continued in existence till 1853, when the building was burnt down, and many of the records lost. Since then, despite many attempts to resuscitate it, no fixed magnetic station has been in being. The British Association has several times moved in the matter. In its report of 1887 a committee appointed by the Association for the purpose of considering the best means of comparing and reducing magnetic observations reported "that the establishment of regular magnetic observatories at the Cape of Good Hope and in South America would materially contribute to our knowledge of earth magnetism." (British Association Report, 1887, P. 320). The same resolution was adopted in 1889 (B.A. Report, 1889, p. 50), and again in 1890 (B.A. Report, 1890, p. 173). In 1891 the same committee reported that they had hopes that their recommendation of the establishment of a magnetic observatory was about to be carried out under the direction of Her Majesty's Astronomer at the Cape, and at the expense of the Admiralty (B.A. Report, 1891, p. 150). The negotiations fell through, however. After this date the matter was allowed to rest for some time. It was again brought up in 1898 by the International Meteorological Conference (B.A. Report, 1898, p. 762). By this time electric tramways had been laid down in Cape Town and its suburbs, and the Royal Observatory, which is within a mile of the rails was no longer a suitable place for a permanent magnetic observatory. The Council of the British Association had therefore to make their appeal to the Colonial Government. The Association transmitted the resolution of the International Conference-after having considered and approved of it-to the Cape Government through the then High Commissioner, Sir Alfred Milner. The Cape Premier replied that his Government did not regard as practicable the immediate provision of funds for the carrying out of the scheme (B.A. Report, 1899, p. lxxxv.). 3.-PURPOSES SERVED BY A MAGNETIC OBSERVATORY. The proper carrying out of the magnetic survey of a country requires the presence of such an observatory. By means of the latter corrections can be made on field observations which have been taken at periods of magnetic disturbance. Such an observatory is absolutely essential for the determination of the change in the value of the magnetic elements from year to year. The magnetic chart of the seas neighbouring a land can only be correctly made when this change is known. Observation shows that the secular change of the magnetic elements is not a world-wide progress of the magnetic needle moving regularly in certain directions, but that in addition there are local causes at work in certain regions. In other words, a permanent magnetic station in England is of no help to magnetism in South Africa. In South Africa at present it is impossible to tell what the magnetic state of any part of it will be ten years hence. With the establishment of properly equipped and properly situated observatories in combination with a thorough magnetic survey it will, it is hoped, be possible in a few years' time to tell at least two or three years ahead what will be the value of magnetic elements in and about South Africa. In this way the safe sailing of the shipping frequenting our shores will be greatly facilitated. Captain Creak says, in his address as President of the Geographical Section of the British Association in 1903, after expressing regret that there was still no permanent station at the Cape: "Of the value of magnetic charts for different epochs, I have much to say, as they are required for purely scientific enquiry as well as for practical uses. It is only by this means we can really compare the enormous changes which take place in the magnetism of the globe as a whole; they are useful to the miner, but considerably more so to the seaman. Had it not been for the charts compiled from the results of the untiring labours of travellers by land, and observers at sea, in the field of terrestrial magnetism during the last century, not only would Science have been miserably poorer, but it is not too much to say that the modern iron or steel steamship traversing the sea on the darkest night at great speed would have been almost an impossibility; whereas with their aid the modern navigators can drive their ships at a speed of 26.5 statute miles an hour with comparative confidence, even when neither sun, moon, or stars are appearing." 4. SUGGESTED SCHEME FOR PERMANENT MAGNETIC OBSERVATORIES IN SOUTH AFRICA. In offering any suggestions for remedying the lack of permanent magnetic observatories in South Africa, several points have to be taken into consideration. In the first place, the size of the region demands that several observatories be founded. Secondly, the position of these must be chosen in such a way that their usefulness will not be destroyed at a later period by the magnetic fields due to the use of electricity for lighting or for locomotion. This latter consideration demands that no observatory be founded within a distance less than 10 miles of any town which may have electric trams within the next fifty years. Two such stations should be established in the Cape Colony, one at Matjesfontein, the other at Lovedale, one at Bloemfontein, one at Bulawayo, and one in Natal, somewhere on the coast. In addition to the magnetic instruments, instruments for the regular record of atmosphere electricity phenomena should be provided. The cost per station would be about £3,000; and each station except Matjesfontein would require an observer capable of taking absolute measurement of the various magnetic elements. An The photographic records might be sent to a central office, just as meteorological records are at present; there the work of reduction could be carried out. The establishment of observatories is of no value if the observations are not reduced and published as quickly as possible. Such an arrangement as this would ensure that. additional advantage of such a central office would be to carry out systematic comparisons with the different instruments of the various observatories, thus ensuring comparability. The advantage of having a number of such observatories spread at different heights over such a large surface would be incalculable; many of the outstanding problems in magnetism and atmospheric electricity could be attacked in a rational manner. BY PROF. R. A. LEHFELDT, D.Sc. [ABSTRACT.] The value of gravity has not, so far as I am aware, been measured anywhere on the South African table land. On taking charge of the physical laboratory at Johannesburg I thought it desirable to make a provisional determination with such means as were at hand. There was a pendulum, intended for the use of students, of the usual Borda's pattern, and a cathetometer by Pye of Cambridge, the scale being engraved on the steel upright of the instrument. For time measurements there was a chronometer, which, by the kindness of Mr. Innes, Director of the Government Meteorological Observatory, Johannesburg, could be rated telephonically by comparison with the Observatory standard clock. The dimensions of the pendulum were as follows: Ball diameter 7.783 cm. Mass 1730 gms. Nut at top of Ball, thickness 0.238 cm. Wire (Pianoforte steel), mass 0.011 gm. per cm. Knife edge attachment, mass 53 gms. (radius of gyration about knife edge)213. centre of inertia 2.1 cms. below knife edge. The laboratory possessed a steel beam two metres long, forming part of an optical bench. In this a V groove was cut, and a set of eight steel bars, each 248 mm. long, with rounded ends, made to slide in The the groove. The beam was mounted vertically alongside the pendulum, and a set square used to mark the position of (a) the top of the nut on the ball, (b) the plane (of glass) on which the knifeedge works. These levels were marked by scratches on paper pasted across the steel beam. The length of the suspending wire was so chosen that the distance between the scratches was a few millimetres longer than the length of a whole number of the steel bars. beam was then laid horizontally, the bars placed in the groove, and the short lengths over at the ends measured by a travelling microscope. The lengths of the bars themselves were measured by the scale and vernier of the cathetometer, using the instruments as a pair of calipers. Since the chief error of Borda's pendulum is usually taken to be the uncertainty as to whether the centre of inertia of the ball coincides with its centre of figure, two lengths of wire were used, one about equal to seven rods, the other to four. The length from knife edge to centre of ball was obtained as described, and the length of the equivalent simple pendulum calculated by means of the centre of inertia and moment of inertia of the whole system. The results were: 179.19 cm. 104.07 cm. To observe the time of vibration the chronometer, illuminated by an incandescent lamp, was placed on a table in front of the pendulum, a small mirror fixed to wall behind it, and a telescope arranged to observe the seconds hand of the chronometer reflected in the mirror. The pendulum wire (slightly out of focus, of course) swung in front of the mirror, and could therefore be seen along with the chronometer. A set of five thousand complete oscillations was observed, the time being noted for every hundredth up to 2000, and every five hundredth from there onwards. The period for the first two thousand and for the whole set was calculated in the usual manner (o to 2500, 500 to 3000, etc) and corrected for amplitude. The difference between numbers may be accidental or may be due to an error in the position of the centre of inertia of the ball. I think it is due to the former cause chiefly, and as the measurements cannot claim an accuracy beyond the first place of decimals, I take the result to be : 978.7. The latitude of the physical laboratory is 26° 11' south, and its altitude 1753 metres. Helmert's formula, for a table land theory would give a correction for altitude of only § as great; it is usually stated that the table land formula is inapplicable owing to displacements in the density of the underlying strata, but this would seem not to be the case with the great South African table land, as the formula h g=980.62-2.6 cos 24 8 3300 gives g=978.70 Since there are only two in agreement with the observations. really large table lands in the world-South Africa and Tibetsome geophysical interest attaches to the results, and it is hoped they may be repeated with better apparatus. |