GENERAL FACTS RELATING TO THE PRODUCTION, 
DISTRIBUTION, AND EXCHANGE OF COMMODITIES 
I. CLIMATE. UNDERGROUND WATER AND 
TEMPERATURES 
50. Under the head of climate we have to consider here only 
the main climatic factors affecting the production and distribution of 
articles of commerce. The commodities whose production is most 
immediately affected by climatic conditions are those derived from the 
vegetable kingdom ; but those of animal origin, being directly or in- 
directly dependent on vegetation, are subject to the same influences. 
It is, however, climate as influencing vegetation, and more particularly 
as influencing cultivation, or the bestowal of human labour in pro- 
moting vegetation, that we have to keep chiefly in view in considering 
the effect of climate on the production of commodities. 
51. For all kinds of vegetation there is required a certain amount 
of heat and a certain amount of moisture, the laws regarding the 
distribution of which over the globe are explained in text-books of 
geography. In the present work it is enough to call to mind a few 
leading facts. 
52. The great source of heat is the sun, and of moisture the ocean, 
where evaporation is brought about through the heat of the sun. The 
winds, however, are the carriers both of heat and moisture, so that it is 
essential to study the direction of the prevailing winds in order to 
understand the distribution of temperature and rainfall over the globe. 
Temperature decreases on the whole from the vicinity of the equator 
towards the poles, but the rate of decrease is very unequal over land 
and water. Water being more slowly heated and cooled than the land, 
the diminution in temperature towards the poles is more rapid over 
the ocean than over the land in summer, less rapid in winter. The 
vicinity of the ocean for this reason has an equalising effect on the 
temperature of adjacent lands, but this effect is brought about solely 
by the agency of the winds. With reference to. land temperatures 
accordingly 1t is more important to consider the direction of the pre- 
vailing winds than the mere distribution of land and water. Winds 
depend on local differences in the pressure of the atmosphere. They 
tend to blow from regions of high pressure to regions of low pressure. 
Regions of low pressure occur over the warmest parts of the ocean 
near the equator. and in the interior of the great land-masses in