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Buchan.—The trend of the isobars is the same, but 29:92 in his chart lies close
to and parallel with 29-97, and 29.82 with 29'92. Pretoria in his chart lies on 29:86.
Dec.—The Transvaal is in the southern extremity of a Cyclonic system,
the pressure varying from 29:81 in the N to 29:84 in the S, the centre being somewhere due north of Pretoria. Pressure at Pretoria is 29.825 ins.
Appendix V. Walfish Bay, Lat. S. 22-56°, Long. E. 14:30°.
The area of least rainfall lies between Walfish Bay and Lat. 28°. The rainfall in 1900 at II stations, rendering returns, averaged under
Barometric Pressure, Departure from Average in each Month of
+ *063 The recovery from a
negative wave which - *015
culminated in December, - '049 1902, persisted throughout -'004
1903, until October, when
pressure returned to the +'006 normal. This wave was
the greatest on record, the t*062
defect in pressure for 18 + *002 months being 0*519 in.
The next greatest on re+'033
cord was from Aug. 1875 +043 to Feb. 1876, the total
defect being 0-399 in. - '029
From Oct. 1903 to the middle of 1904, pressure increased steadily relatively to the normal.
+ '050 +018
NOTE.---Barometric pressure.-The Report for 1904 says, “The pressure
abnormalities since the end of the year 1901 have been remarkable both for amplitude and regularity. The curve fell generally from + 0*042-in. in September, 1901 to -- 0*090-in. in December, 1902; rose to + 0*062-in. in June, 1904, and then decreased to –0'036-in. in January, 1905.' Viewed in connection with a progressive movement of the Australian Anticyclone to and from South Africa in each year, an explanation of such abnormalities, or 'negative and positive waves' is given by the theory that has been advanced. The anticyclone does not follow the same path to and fro each year, nor does it pass Mauritius at exactly the same time. This will affect the barometer in the way indicated by these so-called 'waves.' The rainfall is very significant during the period 1880 to 1903, and points to a varying influence from a high pressure system, which is ever altering its relative position with respect to Mauritius, while its movements continue to be progressive and regular. The drought of 1880 was associated with an exceptionally well marked positive pressure wave, and that of 1885-6 with the recovery of pressure after a feeble negative wave, i.e., in 1880 the anticyclonic system was near Mauritius at the time of the rains and influenced the fall, while in 1885-6 it was returning to its normal path after a deviation which allowed the rain-bearing system to come in. The drought of 1889-90 was synchronous with a feeble negative pressure wave, following a well marked positive wave, while that of 1893 was synchronous with a well marked positive pressure wave between two equally well marked negative waves. From 1896 to 1898 the rainfall abnormality curve was slowly recovering from a steep minimum, while the pressure curve was falling steadily. What more suggestive record could there be of a gradual retrocession of the high pressure
system during this period ? The severe drought of 1899-1901 was associated with an irregular pressure curve usually above normal, pointing to a return of the system gradually to its mean path nearer Mauritius. Lastly, during 1903 the rainfall abnormality curve showed no well marked waves though there was a gradual decrease in the amplitude from September to the end of the year, accompanied by a gradual rise in the pressure curve.
The mean monthly rainfall during 25 years has been tabulated, and amounts to 48:36 inches for the year. Only 11 inches of this amount fall from June to November inclusive; 22 inches fall in January, February and March.
Monthly Mean Barometric Pressure, reduced to Sea-level, corrected to
32 F., derived from 9' and 15" readings.
NOTE. - The readings in 1899 were exceptional. The mean pressure for the
year was 30*129 ins.--the record for 33 years. The pressure in March is noted as exceptionally high, but it was surpassed in March, 1901 and 1905. The readings in February and March point to an early establishment of high pressure, which is probably attributable to the approach of the Atlantic system across South Africa.
By R. T. A. INNES, F.R.A.S.
Annual Variation of the Pressure over South Africa.
(a) Royal Observatory, Cape of Good Hope, 1841
1905 (b) St. Helena, 1840-1847. (c) Natal Observatory, etc., Durban, 1873-1905.
ANNUAL VARIATION OF
THE PRESSURE OVER SOUTH AFRICA.
The history of the barometer in South Africa leaves something to be desired. We have to be thankful to the Cape Meteorological Commission for the vast number of monthly means of pressure which it has collected and published in its yearly volumes. Outside of this series, there is not much to work on; in fact, away from the coast, there are practically no useful observations available. The references given in Table I will show the sources of the material that were available. It is well-known that the pressure of the atmosphere (measured by barometers) varies constantly, both irregularly and regularly. The irregular variations are due to the passage of areas of low or high pressure, commonly called cyclones and anticyclones; the regular variations are of an annual and diurnal nature, and may be ascribed to the direct action of the Sun. The regular variations are (1) the double diurnal variation by which, in the mean, the barometer is highest at 10 and 22 hours, and lowest at 4 and 16 hours, and (2) the annual variation, which in South Africa, the barometer is highest in winter and lowest in summer. The regular variations give rise to the diurnal and seasonal winds, but at any particular time, these regular winds may be displaced by the passage of areas of low or high pressure.
To compare readings of barometers taken at different places, we need to know, first of all, the errors of the barometers and their altitudes above sea-level. For many of the barometers near the coast, this information is available, but away from the coast, it is not so. I believe that the only barometer in the interior of South Africa whose height is known by geodetic levelling is at the Johannesburg Observatory. Until we know the
. heights and index errors of many barometers properly distributed over South Africa, it is impossible to compare readings at different places or to draw isobars or lines of equal pressure over the country. The various geodetic surveys carried out under the direction of Sir David Gill make it possible to fix the heights of a considerable number of barometers with precision, and this is a work that must be undertaken at an early date. I can promise to do it for the Transvaal soon, and I hope that my colleagues in the other colonies will do their share as far as possible. We can, in the meantime, say nothing about the