THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



No. 57. MAY 1842. 



XXII . — The Physical Agents of Temperature, Humidity, Light, 

 and Soil, considered as developing Climate, and in connexion 

 with Geographic Botany. By Richard Brinsley Hinds, 

 Esq., Surgeon R.N. 

 It is our present intention to institute some inquiries into the 

 circumstances of climate and physical agents in connexion 

 with the distribution of the vegetation of our globe ; and as 

 these are the results of several agents acting in co-operation 

 as well as individually, and their mutual influence embraces 

 much complexity, it will be advisable to regard them sepa- 

 rately under the heads of, 1. Temperature, 2. Humidity, 

 3. Light, 4. Soil. 



I. Temperature. 



Climate is the great presiding agent over the flora of the 

 world, and, as modified by external circumstances, stamps its 

 characters on the productions. Climates vary a good deal in 

 circumstances, according to the latitude. In the belt which 

 borders on the equator, and is confined within the tropics, the 

 annual climate is of the simplest kind, and is divided into a 

 wet and a dry season. The temperature throughout the year 

 varies but little, and a very trifling range takes place in the 

 barometer. The seasons alternate with surprising regularity, 

 the inhabitants looking forward to the accession or departure 

 of the rains almost to a day. In receding north and south 

 from the equator, the wet and dry seasons take place at dif- 

 ferent periods of the year ; when the sun enters the northern 

 hemisphere, the wet or rainy season of that side commences, 

 and it is then the time of the dry season in the southern he- 

 misphere. The reverse happens as the sun occupies the other 

 side of the equator. Thus two tropical climates exist, very 

 similar to each other, and chiefly differing in the circumstance 

 that the seasons occur at opposing periods. These are the 

 outlines of tropical climates as existing over continents ; some 

 modifications take place over the large oceans. Near the 

 equator, and to about 7° N. lat., a peculiar region exists ; the 

 trade winds do not advance so far, and light baffling winds, 



Ann. % Mag. N. Hist. Vol.ix. N 



