24. REPORT—1868. 
nine in the morning until three in the afternoon, and rises from three in the after- 
noon until nine in the evening. The diurnal oscillation amounts to 7-hundredths 
of an inch at Port Louis, and to 78-thousandths of an inch at Maritzburg. During 
a period of eight years there were only 217 days on which the diurnal oscillation 
was not distinctly marked at Maritzburg. 
The average yearly range of the barometer is— 
Port Louis 0914 inch. Maritzburg 0:991 inch. 
The highest pressure at 
Port Louis 30-400 inches. Maritzburg 28-714 inches. 
The lowest pressure at 
Port Louis 29:009 inches. Maritzburg 27-215 inches. 
(The height above the sea of the observing station at Maritzburg being 2095-674 
feet.) 
The extreme range of atmospheric pressure during a period of seven years was 
At Port Louis 1391 inch. Maritzburg 1-259 inch. 
The greater range at the Mauritius is obviously due to the occurrence of hurri- 
canes there, which do not extend to Natal. 
The pressure of the atmosphere reduced to the temperature of 32° and to the sea- 
level, derived from a period of eight years, is for 
inches. 
Port Louis, Mauritius.......... 30:056 
Maritzburg, Natal ............ 29:977 
The diurnal and annual oscillations of the barometer are obviously due to the 
rarefying power of day sunshine and summer sunshine. But in Natal there are, 
in addition to these, oscillations averaging about 10 days, for the most part due to 
the alternate predominance of the low polar or higher equatorial current of the air. 
There were in Natal 291 well-marked oscillations of this class in a period of eight 
years. When the south-eastern surface-current (trade-wind) prevails over the 
higher north-west compensatory set of the atmosphere, the mercury of the baro- 
meter goes up; when the upper north-west current predominates, the mercury 
goes down. The thunder-storm rains occur with the troughs of these oscillations, 
and the sea-gales and rains with their crests. Occasionally the upper more rarefied 
current entirely displaces the surface-current for a brief period at Maritzburg, and 
sweeps upon the ground as a strong hot dry wind. This Natalian sirocco blows 
at Menta bntie about twenty-five times in the year. It occurs most frequently in 
the month of September, in which month it may be looked for five times. It 
becomes more frequent during the two months preceding September, and less 
frequent in the two months following September, thus showing that the alternate 
sway of the great antagonistic air-currents is really a seasonal phenomenon due to 
the march of solar influence to and fro over the wide stretch of African land. 
These hot winds do not reach the actual surface of the sea, and are therefore not 
felt in full development, either on the Natal coast or at the Mauritius. They are, 
however, unquestionably connected with the north-west gales of the South Atlantic. 
The gale of the 17th of May, which wrecked the mail steamship ‘Athens’ in 
Table Bay, was an oscillation eleven days long, bursting as a hot wind in the crisis 
of the gale and barometric depression at Maritzburg. 
Abstract of Meteorological Observations made at Pietermaritzburg, Natal. 
By Dr. Mann, F.R.AS., PR.GS., FMS. 
Latitude of observatory ...... 29 36 138. 
Longitude of observatory ...... 30 1 54:5 E. 
Height above the Custom House, near the sea-level at Durban, from a mean 
of eighty barometric observations by standard and compared instruments, 
2095'674 feet. 
