FLORA OF ST. CEOIX AND THE VIRGIN ISLANDS. 5 



rain, falling chiefly in the form of short, rapid showers of only a few 

 minutes' duration, and it is not till the warmer part of the year that 

 heavy and general rains become possible in these regions. During this 

 latter time, the trade-winds become irregular and slight, or are even 

 entirely suspended, as stated before; hence the moisture generated by 

 the daily evaporation from the ocean is not carried off as soon as formed, 

 but is allowed to gather into rain-clouds, and finally to precipitate itself 

 again as rain nearly on the same spot where it was formed. 



From observations made in various islands for a period of more than 

 twenty-five years, the ajmual mean quantity of rain seems to be about 

 the same in all the islands, averaging 42"-44"; the eastern parts of all, 

 as being more exposed to the direct action of the winds, always show- 

 ing a considerably smaller quantity than the central and western ones. 



Although no month of the year is without rain, yet from the above it 

 will be easily concluded that there is a remarkable difference between 

 the various months in this respect: the driest, February, having only an 

 average of 1.5"; the wettest, October, of 1.0"; and to this difference, at 

 the various periods of the year, it is chiefly due, that notwithstanding 

 the uniform temperature all the year round, yet some variations in the 

 aspect and intensity of vegetable life are observed in the various sea- 

 sons. 



Both the annual and the monthly quantity of rain are subject to vary 

 considerably, one year showing 23", or in some places 18" only, another 

 again 70" or 78". A still greater difference may be observed between 

 the same months of different years : thus, February having had one year 

 0.19" only, another, on the contrary, 3.75"; May 0.47" the one year and 

 1G.81" the other. These excessive variations must, no doubt, materially 

 affect vegetable life, indicating at the same time a considerable degree 

 of hardiness in respect to drought in the perennial plants indigenous to 

 the islands, and as alluded to above, acting upon them in a similar way 

 as the variations in temperature in colder climates. 



The number of days on which rain falls averages for the period from 

 1852-73, 161 a year, giving a mean fall of rain of 0.27" per diem : April 

 showing the lowest nuuber, 9 ; October the highest, 16.- From what has 

 been said before, it is evident, however, that the small monthly quantity 

 of rain during the dry part of the year, viz, January to April, divided 

 even over a great number of days (so as to amount to 0.11" or 0.18" 

 only a day), can be of no great importance, as it is precipitated in a short 

 shower, is insufficient for penetrating into the soil, and so is very soon 



