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TRANSACTIONS OF THE SECTIONS. 23 
in February. This agency, however, is somewhat anticipated, and over-ridden, in 
7 a by the establishment of frequent periodic thunder-storms, which begin in 
ctober. 
The thunder-storm is a very much more frequent phenomenon in Natal than in 
the Mauritius. During a series of seven years lightning was seen, or thunder heard, 
on 186 days at the Mauritius. In a series of eight years lightning was seen, or 
thunder heard, on 635 days in Maritzburg. The average number of days of elec- 
trical disturbance in any year is 26 for the insular position of the Mauritius, and 
79 for the continental position of Natal. In the Mauritius lightning is never seen 
in the months of June, July, September, and October; andit is very rarely seen in 
the month of August. 
Insularity of position thus causes greater diversity and irregularity, and more 
widely separated extremes in the deposit of rain, both so far as different seasons of 
the same year and different years of a lengthened series are concerned. But it 
exerts exactly the opposite influence in regard to temperature. The mean tempe- 
rature of the Mauritius is considerably above the mean temperature of Natal ; but 
higher and lower temperatures are experienced in Natal than in the Mauritius. 
The great ocean serves as a ready and exhaustless supply of rain, but as a reserve 
and reservoir of heat. 
The mean annual temperature of Port Louis is 77°1. The mean annual tempe- 
rature of the sea-coast of Natal (Durban) is 68°-2, and of Maritzburg, 2095 feet 
above the sea, 64°:7. 
In a series of six years the highest annual mean at Port Louis was 78°-2, and 
the lowest 77°-0. The highest at Maritzburg was 65°-80, and the lowest 64°-27. 
The mean temperatures of the several months of the year, derived from a series 
of seven years, were, for the two stations,— 
Port Louis. Maritzburg. Port Louis. Maritzburg. 
January ...... 82°30 71:4 Ap \eo rhe Agaric 72°52 55:2 
February .... 82°16 71:8 August ...... 72°83 59°7 
March © .4).:. 81:53 69°7 September.... 74°14 65:1 
24) 1) | Snag 80°63 64:8 October...... 75°78 66'6 
1.15) eee 77:12 59'3 November ,. 79:22 67:1 
JTS! ites ss 74:00 55:2 December .... 81°44 70°4 
The range of monthly mean temperature is 9°°78 at Port Louis, and 16°6 at 
Maritzburg. 
Comparison of Temperature of Port Louis and Maritzburg during a 
series of eight years. 
" Largest 
Highest | Lowest oe d ee a monthly 
tempera- | tempera- tempera- 6 ners range of 
ture. ture. tempe- 
ture. perature. sit) 
PortLouis| 900 | 628 97-2 13-0 18 
Maritzburg 97°6 29:0 68°6 43-0 27 
The extremes in Natal are much less than they would otherwise be, on account 
of the summer being the period of predominant cloud and rainfall, and the winter 
the period of eens ey sunshine. The air-temperature was only below the freez- 
ing-point at Maritzburg five times during eight years. 
The annual and diurnal oscillations of the barometer are well marked at both 
the Mauritius and Natal. The mean pressure of the atmosphere falls from August 
to February, and rises from February to August. The daily pressure sinks from 
