4 FLORA OF ST. CROIX AND THE VIRGIN ISLANDS. 



tent, and it is therefore tlie variable degree of moisture at different times 

 which chiefly produces any variation in the development of vegetable 

 life at the different seasons. 



The lowest temijerature observed at the sea-level, in the shade, is 18.1°; 

 the highest, 35.5'^. In the sun, the mercury will sometimes rise as high 

 as 51°, but as a rule does not surpass 40°. Observations made in St. 

 Thomas by Knox * and myself show a decrease of about 2° for an eleva- 

 tion of every 800', which gives to the highest ridges in St. Thomas and 

 Tortola an annual mean temperature 3^0-4° lower than that of the coast, 

 a difference sufficient to produce some variation in the flora of these 

 parts. The northern sloi^e of the hills, from being the greater part of 

 the year, viz, from August to May, less exposed to the rays of the sun, 

 are generally also somewhat cooler and more moist than the southern 

 ones, the consequences whereof are also felt in the life of plants to a con- 

 siderable extent. 



An equal regularity, as observed in the temperature, manifests itself 

 with regard to the pressure of the atmosphere, the daily variations of 

 the barometer being only about 0.05", and the maximum yearly differ- 

 ence only 0.2". It is only during strong gales and hurricanes that the 

 barometer is more seriously affected, it then falling sometimes as much 

 as 2". These hurricanes, as a rule, occur only during the months from 

 August to October, at which period the trade-winds from the northeast, 

 which otherwise blow most part of the year, generally become unsteady 

 and uncertain. These constant winds, combined with the high tempera- 

 ture, no doubt are the reason why the moisture of the air is compara- 

 tively small, being on an average only 73 per cent, of the possible maxi- 

 mum, thus exciting a constant evaporation in plants, and rendering it 

 necessary for them to obtain a greater supply of water through the soil 

 than in more moist climates. For this reason a considerable quantity of 

 rain becomes of the highest importance to the vegetable life, as being the 

 only form in which plants can obtain a sufficient amount of water neces- 

 sary to their existence, even dew being very rare on account of the trade- 

 winds blowing also during the night the greater part of the year. 



Neither of the islands in question is of sufficient elevation above the 

 sea to cool and condense the atmospheric moisture brought on by the 

 trade-wind, nor is their configuration favourable for detaining the clouds, 

 their greatest extent being parallel to the direction of the wind. Thus, 

 for the greater part of the year they receive only a small quantity of 



' Kuox : An Historical Account of St. Thomas, W. I. (New Yovk, 1852.) 



