4-— THE BAROMETER IN SOUTH AFRICA. 



By R. T. a. Innes, F.R.A.S. 



1. Annual Variation of tlie Pressure over South Africa. 



2. Movement of Atmospheric Disturbances across South 

 Africa. 



3. Reduced Monthly Readings of the Barometer : — 



(a) Royal Observatory, Cape of Good Hope, 1841- 

 1905. 



(b) St. Helena, 1840-1847. 



(c) Natal Observatory, etc., Durban, 1873-1905. 



I. 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 (i) 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, by 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 



