204 “In both hemispheres, the zone in which the temperature decreases most rapidly lies between the 40th and 50th degrees of north latitude. This circumstance must have a happy influence on the culture and industry of the nation inhabiting the neighborhood of that zone. Here is the point where the regions of the vine touch upon those of the olive. Nowhere in the world, do the products of the vegetable kingdom, and the most varied wonders of agriculture, follow with such rapidity on one another. The great variety of products enlivens the commerce and increases the industrial activity of agricultural nations.” (_Humboldt_.) It is true, however, that tropical countries possess, also, in their mountainous parts, the _tierra fria_, _templada_ and _caliente_, superimposed the one on the other. 205 The aggregate coal supply of Great Britain (1869) was 2,180 millions cwt.; of Belgium (1862), 207 millions; of France (1868) 256 millions; of Prussia (1870), 600 millions, of Austria (1870), including brown lignite coal, 158 millions; of Russia (1868), only a little over 9 millions. The great English coal field, in the counties of Durham and Northumberland, embraces 732 English square miles; that of South Wales, 1,200, with a depth of 95 feet, so that the geographical square mile contains here 679 millions of tons, each of twenty cwt. To obtain the same quantity of combustible material as was furnished to Prussia, in 1865, by its coal, it would be necessary to use up 6,331 square miles of forest, (_von Dèchen_, in _Engel’s_ Zeitschrift, 1867, 258.) The supply of coal is, of course, exhaustible while, for instance, turf-fields replace themselves by slow degrees. Compare _Griesbach_, über die Bildung des Torfs, in the Göttinger Studien, 1845, vol. I. The importance of the coal-fields of the United States, which are twenty-two times as large as those of Great Britain, in the distant future, cannot be over-estimated. 206 I need only call attention to the earth-fire (_Erdbrand_) for the purpose of forcing the growth of garden plants in the neighborhood of Zwickau, which is said to have existed since 1505. 207 Thus, in Watt’s steam engines of the larger kind, an hourly consumption of ten pounds of coal is needed to produce a force equivalent to that of one horse, while in the smallest machines of only one horse power, twenty-two pounds are needed. See _Prechtl_, Technolo. Encyklopädie, III, 669.