ON THE METEOROLOGY OF PORT LOUIS. 117 



creases with a decreasing temperature. But from April to June it again in- 

 creases with a decreasing temperature, and stands high in the latter month, 

 and in July and August, owiog probably to the high temperature in the 

 northern hemisphere causing an influx of air (S.W. monsoon) towards the 

 heated regions, and thus exciting the S.E. trade-wind in the southern hemi- 

 sphere. From August to November the mean maximum force decreases. 



Secular Variation. — The last Unes in Tables XXVIII. and XXIX. show 

 that 1860, 1861, and 1863 were the years in which the force of the wind 

 was greatest, and we shall presently see that these were the years in which 

 hurricanes were most frequent and violent. The years 1862 and 1864 were 

 remarkable for an absence of hurricanes, and these were the years in which 

 the mean force of the wind was least. 



Extreme Annual Force. — Table XXX. shows the greatest force of the wind, 

 and the epoch, for each year. 



VIII. Cloud. 



Tables XXXI. and XXXII. exhibit the mean amount of cloud for each of 

 the four daily observation hours, each month and each year. The nights 

 and mornings are comparatively cloudless. Towards 10 a.m. the clouds be- 

 gin to gather, by 2 p.m. the sky is often overcast, and in the evening the 

 weather usually clears up. The mean amount of cloud for the year is 

 47, 100 denoting completely overcast. February is the cloudiest and June 

 the least cloudy month, the means being 59 and 40 respectively. The last 

 column in Table XXXII. points to a connexion between the amount of cloud 

 and the temperature. From Jfovember to February the amount of cloud in- 

 creases, and fi'om February to June it decreases. From June to November, 

 however, there is a tendency to a second progression. 



IX. Eainpail. 



Table XXXIII. gives the amount of rainfall for each month and year. 

 The greatest fall ia any one month during the seven years was 46'57 inches in 

 February 1861. In September 1861 and November 1866 thei'e was no 

 ramfaU sufficient to affect the gauge. The greatest mean monthly fall is 

 14-23 inches for February, and the least 0-39 inch for September. From 

 September to February the rainfall increases ; from February to June it de- 

 creases; from June to August it increases again, and then falls in September, — 

 showing, upon the whole, a double progi'ession, having its maxima in Febru- 

 ary and August, and its minima in June and September. The mean annual 

 fall is 37'87 inches, and the mean monthly fall 3-16 inches. The greatest fall 

 in any one year was 68-76 inches in 1861, and the least 20-56 inches in 1866. 

 The principal rain-bearing wind is the trade-wind from E.S.E. to E.N.E. ; 

 but at times, during the summer months, torrents of rain descend with north- 

 erly and north-westerly winds, and on those occasions the mountains become 

 enveloped in dense mist. The greatest rainfall on any one day, in each year, 

 with the date, is shown in Table XXXIV. 



There is reason to fear that the rainfall is decreasing : the fall during the 

 first three years was considerably greater than that during the last four years 

 of the period of seven years. 



In some parts of the island the rainfall is much greater than at Port Louis, 

 as will be seen from Table XXXV., showing the rainfall at nineteen stations 

 for periods ranging from two to five years. Of these stations, Gros Cailloux 

 and Port Louis, both on the coast, and not many feet above the sea-level, are 



