ON THE METEOROLOGY OF PORT LOUIS. 119 



as it comes up from the sea. Comparing tlie rainfall at Beau Vallon, Cluny, 

 Beau Sejour, and Gros Cailloux, which lie nearly in a S.E. and N."W. direc- 

 tion, we find similar relations. 



For some years past many parts of the island, particularly on the western 

 and northern coasts, have been suffering from drought ; the rivers have been 

 gradually diminishing, and the lakes and marshes in the interior been drying 

 Up. As we have already seen, last year (1866) has been the driest of all, 

 the rainfall ia some places having been little more than half the average fall. 

 The consequence is that this year's crop wiU be very much reduced. 



The evil which is thus pressing on the colony is generally attributed to 

 the extensive clearings which have been carried on in all directions during 

 the last fifteen years. The primeval forests with which this little island was 

 at one time clothed have to a great extent been replaced by the sugar-cane, 

 and now the cauo languishes and dies for want of moisture. It would be 

 satisfactory to those interested in the welfare of Mauritius to have the 

 opinions of men of science as to the probable effect of the destruction of 

 forests on the rainfall and humidity, and I am glad of having an opportunity 

 of bringing the subject before the Association. Given a small mountainous 

 island in the trade-wind region, covered with dense forests, and surrounded 

 by a tropical sea : what eflfect, if any, with respect to rainfall and humidity, 

 would be produced by stripping that island of its forests, and exposing 

 soil and rocks to the sun's rays ? It seems to me that, whether the annual 

 rainfall would diminish or not, the air would become drier, as the greater 

 portion of the rains would be speedily carried away to the sea, and the re- 

 maining portion speedily evaporated. This last year, however, shows a very 

 marked decrease of rain, and if the previous six years do not so to the same 

 extent, they show a tendency to a recurrence of floods and droughts — that 

 is, to a disturbance in the distribution of the rainfall. The humidity of the 

 air also has, as we have seen, been upon the whole decreasing at Port Louis 

 since 1860. In that year it was 73-6, while in 1866 it was only 66-4. 



X. Thunder and Lightnikg. 



Table XXXVI. shows that in the course of the seven years no lightning was 

 seen between May and November, except on one day in August 1864. Ja- 

 nuary, March, February, and April are the months in which thunder-storms 

 prevail ; they generally occur in the afternoon, but occasionally at other 

 periods of the day, or iu the night. Some are local, and others travel over a 

 considerable extent of ocean. The average number of days per annum on 

 which lightning was visible is 26-4. The greatest number of days in any one 

 year on which lightning was observed was 40 iu 1863, and the least 19 in 

 1862. 



XI. Gales and Httreicanes. 



Mauritius, as is well known, is subject to hurricanes. The hurricane 

 months are December to April inclusive, but more especially January, Feb- 

 ruary, and March, particularly February. Strong gales occur also in June, 

 July, and August. I will present a few of the leading facts connected with 

 the gales and hurricanes which took jAace during the period under review. 



1860. — Four gales occurred in 1860. The first took place between the 

 11th and 17th of January. The barometer fell to 29'6S0 inches. The vidnd 

 veered from S.E. to S., S.W., and W., and attained a maximum pressure 

 of 10 lbs. on the square foot. The rainfall was 7 inches. This was a 



