The summer climate of the United States is remarkable in many respects. The mean temperature of its southern portion exceeds that of the tropical region to the south of it. The Isotheral line, or Isothere (that is, the line of equal summer temperature) of 80°, follows the coast of the Gulf of Mexico and the Atlantic, from the southern point of Texas to that of North Carolina. It then, in latitude 34°, sweeps round to the W. and S. W., bending very gradually to the N., till, in the N. E. part of Texas, it again approaches latitude 34P. It is then bent to the S. by the high table-lands of Texas, crosses the Pecos in latitude 29 1/2, and sweeps to the N. W., till, in the neighborhood of Lake Humboldt, it reaches the parallel of 40°; and is then bent suddenly round by the Sierra Nevada Mountains, and sweeps S. S. E. along the eastern coast of the Californian Peninsula.

The space within the Isothere thus described, includes one of the hottest regions of the globe. Its focus is the district from the head of the Gulf of California to the mouth of the Gila, where the mean summer heat is 90°, while that of the West India Islands is 81 1/2. West of the Sierra Nevada, along the valley of the San Joaquim, is another district of intense summer heat, the mean of the thermometer at Fort Miller in that valley being 85°. This belt of intensely heated country follows the southern line of the coast along the Gulf, and is limited in the interior by the Mexican table-lands, which extend to lat. 30°, and of which the height is six or eight thousand feet, and the mean summer heat is 66°. Within this hot belt the mean heat ranges from 82° to 85°. The influences of this intense heat are modified by the moisture or dryness of the climate..

In the southern part of the Peninsula of Florida, twenty-five inches of rain fall in the three summer months; twenty-two inches fall in the rest of the peninsula; twenty inches in the remaining region east of the Mississippi. Through Eastern Texas the summer rains average twelve inches, gradually lessening to ten and eight inches at the W. and S. W. Three inches fail along the valley of the Rio Grande; one and a half inches on the Gila, and the prolonged point of the belt to the N. W., traverses the rainless desert of Utah.

We have, then, in our southern borders, an extensive range of summer tropical climate, capable, in its eastern portion, of maturing all the annual fruits and plants of the hottest regions of the globe; and, wherever it can be irrigated in its western portion, susceptible of the highest fertility; for the Basaltic rocks of that region disintegrate into one of the most genial of soils, which will, at some future day, when science shall direct the industry of its people, rival the ancient fertility of the now desert valleys of Persia and Syria, and "blossom like the rose." It is highly favorable to the future prospects of these sterile regions, that the summer rains on the mountains are copious and frequent; so that, by damming up the ravines and mountain valleys, artificial lakes may be formed for the irrigation of the subjacent districts.

The adaptation of this hot belt to the cultivation of tropical trees, is controlled by its winter climate. The Isooheimal line, or Isocheim (the line of mean winter temperature) of 65o, passes through the peninsula in latitude 27°, say 120 miles from the cape. South of that line, it is probable that neither ice nor frost is ever known, and that the Clove-tree, the Pimenta, the Date Palm, the Coffee-tree, and the Cacao, would thrive luxuriantly. The Isocheim of 60° passes across the peninsula at St. Augustine, and enters Texas in latitude 27°. South of that line, the olive, the fig, the orange, and the lemon, would flourish.

The Isocheim of 550 crosses the mouth of the St. Johns, and passes along the northern shore of the Gulf to Matagorda, thence W. and W. N. W. to the mouth of the Gila. South of this line, winter frosts occasionally occur, and cut off the orange-trees of Northern Florida.

The extreme N. W. point of the hot district under survey, has a mean winter heat of 350; for the Isothere of 83°, and the Isocheim of 350, nearly touch each other at Lake Humboldt, in Utah. It is only, therefore, in the southeastern part of this district that the cultivation of the intertropical fruit-trees can be successful.

It is true that there may be spots found in the interior protected from the cold winter winds, and from frosts, where the hardier trees of the tropics, especially of the hills and table-lands, such as the cinchonas, may be introduced.

If the winter and summer climates of the district round Humboldt's Lake be thus strongly contrasted, the summer climates of the coast, and of the mountain valleys of California, are equally so. The Isothere of Fort Miller, on the San Joaquim, is 850, and that of Monterey, 150 miles to the west, is 57°!

The summer mean of the Pacific coast does not vary from 57°, between Monterey and Sitka, a space of 22o of latitude, and near two thousand miles of coast.

This extraordinary fact is no doubt caused by the cold waters of the Pacific Ocean. Although there is no steady superficial polar current to be traced near the shore, it would seem that such a current prevails in the deep sea beyond, and that the cold waters, as they approach the land, are forced upwards by the shelving of the bottom.

There is at all times a belt of cold sea-water, of the temperature of 57° to 60°, along the northwestern coast of America, extending many hundred miles out to sea, which controls the climate of the coast, and almost equalizes the temperature of the four seasons!

A similar but narrow belt of cold water borders the Atlantic coast, inside of the Gulf Stream, which, though it does not influence the summer climate, is distinctly felt in the cool northeast winds which sometimes prevail with a clear sky for weeks during spring and early summer.

The Isothere of 750 passes through the Bermuda Islands in latitude 32°, and strikes our coast in latitude 39 1/2°. It is bent to the S. W., by the Virginia Mountains, to the parallel of 34°, along the northern line of North Carolina and Tennessee. West of the Alleghanies it again bends to the N., and then to the W., crosses the Mississippi in latitude 39°, near the mouth of the Illinois, and continues west to longitude 103°. It is then deflected to the S., by the Rocky Mountains, to latitude 320, in the valley of the Rio Grande. It then bends again to the N. W., strikes the Great Salt Lake - the Lake Timpinagos of Humboldt - passes W. in latitude 41 1/2o, till it is again bent S. by the cold atmosphere of the Pacific coast, and strikes that coast in latitude 26°.

The Isothere of 7(P crosses the Hudson at West Point, touches the parallel of 40O in Western Pennsylvania, passes through Sandusky and Chicago, thence N. W. to Fort Snelling, crosses the Missouri near Fort Union, reaches the parallel of 490, and is then bent to the E. of S., by the Rocky Mountains, till it reaches latitude 350, in the valley of the Rio Grande. West of the Rocky Mountains it bends to the north, and reaches the parallel of 47 1/2° in longitude 1180. It then bends suddenly to the south, and strikes the coast in latitude 34P.

"There is a great identity," says the report, "of the temperatures of this large area, embraced by the Isothermals (Isotheres) of 70o and 750 east of the plain, including Iowa, Illinois, Indiana, Ohio, Kentucky, Upper Tennessee, Virginia, Maryland, Delaware, Lower Pennsylvania, and New Jersey, excepting from these some points of coast exposure, and of the mountainous districts, the summer temperatures are more nearly uniform than almost any continental area of like magnitude".