"The statements so frequently advanced, although unsupported by measurements, that since the first European settlements in New England, Pennsylvania, and Virginia, the destruction of many forests on both sides of the Alleghanys, has rendered the climate more equable--making the winters milder and the summers cooler--are now generally discredited. No series of thermometric observations worthy of confidence extend further back, in the United States, than seventy-eight years. We find, from the Philadelphia observations, that from 1771 to 1824, the mean annual heat has hardly risen 2 deg..7 Fahrenheit--an increase that may fairly be ascribed to the extension of the town, its greater population, and to the numerous steam-engines. This annual increase of temperature may also be owing to accident, for in the same period I find that there was an increase of the mean winter temperature of 2 deg. Fahrenheit; but, with this exception, the seasons had all become somewhat warmer. Thirty-three years' observation, at Salem, in Massachusetts, show scarcely any difference, the mean of each one oscillating within 1 deg. of Fahrenheit, about the mean of the whole number; and the winters of Salem, instead of having been rendered more mild, as conjectured, from the eradication of the forests, have become colder, by 4 deg. Fahrenheit, during the last thirty-three years." The facts hereinbefore stated show that there is nothing like a _regular_ amelioration; that the seasons differ during the same decade, and different decades. The cold decade, from 1811 to 1820, has not been reproduced. But it may be, and we know not how soon. Since that period there has certainly been a change--for even the cold period from 1835 to 1840 did not equal that from 1815 to 1820, nor indeed those of 1775 to 1780 or 1795 to 1800. But as these variations, so far as we are enabled to judge, depend upon the varying influence of the sun's rays, and of volcanic action, it is impossible to say that equally cold periods will not return, during the latter half of this century. If the influence of the sun was constant, and volcanic action regular, two causes would tend to modify the seasons: 1st. The exposure of the surface to a more effective action of the solar rays, by a removal of the forests, and by drainage. That such action would be more effective upon a surface thus uncovered and drained, can not be doubted.