To the difference in the magnetic intensity of the eastern portion of this continent, compared with Europe and our western coast, very much of the difference of climate, so far as temperature is involved, may be attributed. We have seen in what manner the iso-thermal lines surround these areas of intensity. So the most excessive climate--that is, the climate where the greatest extremes alternate, other things being equal, is upon or near the line or area of greatest magnetic intensity. I say other things being equal, because large bodies of water modify climates by equalizing the seasons--making the summers cooler and the winters warmer than the mean of the parallel. Thus, our great interior lakes modify the climate in relation to temperature in their vicinity. Their summers are cooler and their winters warmer; but westward of them the same line of equal summer temperature, or iso-thermal line, rises with considerable abruptness, and the winter, or iso-cheimal line of equal temperature, falls in a similar manner. Thus, the range of the thermometer, from the highest elevation to the lowest depression, for the year, is very great, while in the tropics the range is comparatively small. From observations made at the military posts of the United States, Dr. Forrey deduced summer and winter lines of equal temperature, starting from the vicinity of Boston and running west, which showed most remarkably the rise of the summer lines as intensity increased, and the fall of the winter lines in like manner. The influence of the lakes was also most obvious. The elevation of the earth increases, going west, to about 700 feet at the surface of the lakes, and to nearly 4,000 feet at the eastern base of the Rocky Mountains; and, although temperature does not decrease to as great a degree when the elevation above the level of the sea is _gradual_, yet some allowance should doubtless be made for that elevation on this line. When that allowance is made, the ascent of the summer line, to the north, over the area of greatest intensity, is strikingly apparent. Dr. Forrey also instituted a comparison between Fort Snelling, where the climate is as excessive, and the range of the thermometer as great, as in any portion of the continent in the same latitude, with Key West, and I copy his diagram. It is very instructive, showing the gradual mean rise of the temperature, from January to December, inclusive, while the cross lines show the _extremes of each month_.