146 TRINIDAD. 



certain permanent or only transient modifications : such as 

 light, the amount of electric tension, and barometrical pressure, 

 the temperature resulting from the geographical position, as also 

 humidity, and those emanations or exhalations which are desig- 

 nated by the names of effluvia, miasms, or malaria. 



Of light, I will say nothing, except that it has a powerful in- 

 fluence on the healthy development of vegetable and animal life, 

 on the greater or less amount of colouring matter in the leaves of 

 plants, and in the complexion of man. 



The appreciation of the influence of electric tension on the 

 human body is a matter of greater difficulty ; but its indirect action 

 must be powerful, since it is evidently connected with all atmo- 

 spherical phenomena. 



The variations of the barometrical pressure being indicative 

 of the density of the air, and, consequently, of the mass of re- 

 spirable principle therein contained, are an important element in 

 the appreciation of climatic influence. On this point, I may re- 

 mark that, between the tropics, the horary oscillations of the 

 barometer are very regular, and present two maxima at 9 or 9J 

 A.M., and atlO| or 10 J P.M. ; and two minima at 4 and 4J P.M., 

 and 4 A.M. " Their regularity is so great," says Baron von 

 Humboldt, " that in the day-time especially the hour may be 

 ascertained from the height of the mercurial column, without an 

 error, on the average, of more than fifteen or seventeen minutes. 

 I have found the regularity of the ebb and flow of the aerial 

 ocean undisturbed by storms, hurricanes, rain, and earthquakes." 



Amongst the various and numerous causes which modify the 

 mean annual temperature of the island, and all tending to its 

 depression, the following may be taken into consideration : its 

 insular position, its peculiar disposition into two grand sectional 

 valleys, running east and west, between three almost parallel 

 ridges ; and the extensive woods, which covering nearly the whole 

 of its surface, influence the temperature by acting as a shield 

 against the direct bearing of the sun's rays by radiation, and 

 even by augmenting the atmospherical humidity. 



Two series of thermometrical observations, made at different 

 intervals and localities, give 80 50' as the mean annual tempera- 

 ture of Trinidad ; they each comprise a period of five years. In 

 Captain Tulloch's " Statistical Report," &c., I have found the first 

 series of observations : they were probably taken at St. James's 



