It Is Thus, That, On A Principal Vein, The Appearance Of A New Substance In The Gangue Indicates To The Miner The Neighbourhood Of A Secondary Vein That Joins The First.

On recapitulating the observations here recorded, we see, that within the tropics, the mosquitos and zancudos do not rise

On the slope of the Cordilleras* toward the temperate region, where the mean heat is below 19 or 20 degrees (* The culex pipiens of Europe does not, like the culex of the torrid zone, shun mountainous places. Giesecke suffered from these insects in Greenland, at Disco, in latitude 70 degrees. They are found in Lapland in summer, at three or four hundred toises high, and at a temperature of 11 or 12 degrees.); and that, with few exceptions, they shun the black waters, and dry and unwooded spots.* (* Trifling modifications in the waters, or in the air, often appear to prevent the development of the mosquitos. Mr. Bowdich remarks that there are none at Coomassie, in the kingdom of the Ashantees, though the town is surrounded by marshes, and though the thermometer keeps up between seventeen and twenty-eight centesimal degrees, day and night.) The atmosphere swarms with them much more in the Upper than in the Lower Orinoco, because in the former the river is surrounded with thick forests on its banks, and the skirts of the forests are not separated from the river by a barren and extensive beach. The mosquitos diminish on the New Continent with the diminution of the water, and the destruction of the woods; but the effects of these changes are as slow as the progress of cultivation.

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