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Other measurements indicated that an audience gave an absorption equal to 44% of that due to an equal area of complete absorber, thus accounting for the improved hearing conditions known to exist in well-filled buildings.

The absorption of sound can thus be adjusted with precision, but it must not be carried too far, otherwise the sound intensity is too much diminished. The ear is able to disregard, or even to take advantage of, reverberation which is not too prolonged, and the extent of absorption has to be adjusted to the appropriate amount. Too many apertures such as open windows or doors must be avoided.

Sabine has also made examination of the exact manner in which sound is reflected in an auditorium by constructing scale models of the latter, and photographing the sound waves at various instants after creation, using the beautiful method due to Toepler (Annalen der Physik, 127, p. 556) and elaborated by R. W. Wood (Physical Optics, 2 ed., 1911, p. 94). By this means the positions of the sound waves, both incident and reflected, are capable of observation at all instants and at all points in the model room, and they provide data upon which can be based correct architectural construction from the acoustic point of view.

Absolute Measurement of Sound. A. G. Webster (Nat. Acad. Sci. Proc.. 5, p. 173, 1919) has advanced to a considerable extent the methods of absolute measurement. For this purpose it is impossible to rely upon audition, handicapped as it is by the vagaries of the ear. What is required is a reliable mechanical device, the performance of which is constant, to record the sound vibrations with sufficient magnification. Webster has made an exhaustive study of the properties of various materials, and has constructed from those most suitable for the purpose two instruments which he has called the phone and phonometer respectively. The phone provides a means of creating a simple tone of intensity and frequency which are under control and capable of exact measurement. The phonometer is an instrument for measuring absolutely the vibrations received by it. It consists of the combination of a diaphragm and a resonator, both of which are adjustable in frequency. The motion of the diaphragm is observed by making it a reflector and part of a Michelson interferometer, so that the amplitude is measured in terms of the wave-length of suitable monochromatic light. In practice the interference fringes are photographed on a moving film upon which they appear as wavy lines. Against this instrument, which is regarded as a standard, other portable phonometers can be calibrated, these depending on the simpler process of the deflection of a beam of light set into angular oscillation by the receiving diaphragm. With such instruments, and also with D. C. Miller's phonodeik (referred to later) L. V. King has carried out an elaborate investigation on the propagation of sound in air and fog-signal efficiency (Phil. Trans.. 218, p. 211, 1919) in the region near Father Point, Quebec. King, in this paper; also describes a modification of the siren called the diaphone, used as a standard source of sound in his research.

Analysis of Sound. Webster's phonometer described above is a resonant instrument, and, therefore, unsuitable for the analysis of mixed sounds. Much progress has been made, however, in the analysis of such sounds, using non-resonant recorders, for example, D. C. Miller's phonodeik. This instrument, which depends on the motion imparted to a tiny mirror by the operation of a vibrating diaphragm, is described in Miller's Science of Musical Sounds (1916), where also will be found the results of the analysis of various sounds. Similar work has been carried out by C. V. Raman, in relation to the vibrations of bowed strings and instruments of the violin family (Indian Assoc. for Cultivation of Science, Bull. No. 15, 1918). In these cases the sound record is of the ordinary type and consists of the trace on a moving photographic film of a spot of light vibrating at right angles to the motion of the film, thus forming a transverse wave. Records of a different type have recently been obtained (A. O. Rankine, Proc. Phys. Soc. Lond., 32, p. 78, 1919) in which the sounds are caused to vary the intensity of a narrow beam of light, which gives on a moving film a line image perpendicular to the motion. The record thus consists of a negative film of varying transparency along its length. It is not so suitable as transverse records for direct analysis of the component frequencies, but it has the advantage that it admits of reproduction of the sound by means of a selenium cell, such as is used in phototelephony. This arrangement constitutes a novel type of phonograph operated by light, first invented by Ernst Ruhmer in 1900, but hitherto little known. (A. O. R.) SOUTH AFRICA (see 25.463). On the conquest of German S.W. Africa by the Union forces in 1915 the whole of S. Africa, except for the Portuguese possessions S. of the Zambezi came under British administration. Excluding the Portuguese territory (for which see and ), S. Africa was in 1921 divided politically as follows:—

1. The Union of S. Africa, a self-governing dominion of the British Empire formed in 1910 and consisting of the former colonies of the Cape, Natal, Orange River (Free State) and Transvaal.

2. The S.W. Protectorate (ex-German S.W. Africa), administered under mandate as an integral part of the Union.

3. The native protectorates of Basutoland, Bechuanaland and Swaziland, administered by the British Colonial Office.

4. Rhodesia, consisting of two separate administrations, S. Rhodesia and N. Rhodesia; both under the rule of the British S.A. Company.

Area and Population—Including both Rhodesias the area of S. Africa is approximately 1,650,000 sq.m., of which some 125,000 sq.m. are Portuguese. The total pop. in 1921 was little over 11,000,000 of whom some 700,000 lived in Portuguese territory.

The following table shows the white population in the Union and in Rhodesia at the censuses of 1911, 1918 and 1921:— The increase per cent, in the Union in the period 1911–21 was 19.23, masculinity (the number of males to 100 females) decreased from 115.92 to 106.14.

The next table gives particulars of area and pop. of British South Africa at the 1911 census:—

Area Sq. m. Pop. 1911 White Native and Coloured Total Union of S. Africa: Cape Province Transvaal Province Natal Province (includes Zululand) Orange Free State Province Total Union Territories: S. Rhodesia N. Rhodesia Total Rhodesia Protectorates: Bechuanaland Basutoland Swaziland Total Protectorates Total British S. A. 276,995 111,196 35,371 50,392 582,377 420,562 98,114 175,189 1,982,588 1,265,650 1,095,929 352,985 2,564,965 1,686,212 1,194,043 528,174 473,954 1,276,242 4,697,152 5,973,394 148,575 290,000 23,606 1,497 747,471 821,102 771,077 822,599 438,575 25,103 1,568,573 1,593,676 275,000 10,293 6,536 1,692 1,411 1,083 123,658 402,434 98,876 125,350 403,845 99,959 291,829 4,186 624,968 629,154 1,204,358 1,305,531 6,890,693 8,196,224 