CLIMATE. 119 



which actually produce, or are capable of producing, in our 

 organs certain permanent or only transient modifications : such 

 as light, the amount of electric tension and barometrical pres- 

 sure, the temperature resulting from the geographical position, 

 as also humidity, and those emanations or exhalations which 

 are designated by the names of effluvia, miasma, or malaria. 



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

 influence 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 atmospherical phenomena. 



The variations of the barometrical pressure being indicative 

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

 respirable principle therein contained, are an important element 

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

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

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

 9 J a.m., and at 10 J or 10| p.m. ; and two minima — at 4 and 

 4i 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 



j depression, the following may be taken into consideration : its 



i insular position, its peculiar disposition into two grand sectional 



valleys, running east and west, between three almost parallel 



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



I 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. 



Three series of thermometrical observations, made at different 

 periods and localities, give 80 *7 3° as the mean temperature of 

 Trinidad. Two of those series comprise a period of five years 



