We find very important modifications of temperature, occasioned not only by astronomical influences, but by local causes and geographical characteristics.  For while, as a general rule, the nearer we approach the equator, the warmer we shall be, yet temperature is greatly affected by mountains, seas, currents of air or water, by radiation, by forests, and by vegetation.  It is found, in fact, that the lines of temperature, (the happy conception of Humboldt,) when they are traced upon the map, are anything but true zones or circles.

The line of the greatest mean warmth is not coincident with the equator, but falls to the north of it.  This line at 160 deg.  W. Long, from Greenwich is 4 deg. below the geographical equator; at 80 deg. it is about 6 deg. north, sweeping along the coast of New Granada; at 20 deg. it comes down and touches the equator; at 40 deg.  E. Long., it crosses the Red Sea about 16 deg. north of the equator, and at 120 deg. it falls at Borneo, several degrees below it;—­and the points of the greatest heat, in this line, are in Abyssinia, nearer the tropic of Cancer than to the equator.  On the other hand, the greatest mean cold points, according to the opinions of Humboldt, Sir David Brewster, and others, do not coincide, as would seem natural, with the geographical poles, but they are both to be found in the northern hemisphere, in Latitude 80 deg., 95 deg.E.  Long. and 100 deg.  W. Long. from Greenwich.  The western is ascertained to be 4-1/2 deg. colder than the eastern or Siberian.  If this be the fact,—­but it is not positively admitted,—­an open sea at the pole may be considered as probable, on the ground of its having a higher mean temperature than is found at 80 deg..  Kaemptz places one of these cold points at the north of Barrow’s Straits,—­the other near Cape Taimur, in Siberia.  Burghaus, in his Atlas, transfers the American cold pole to 78 deg.  N. Lat.  It is perhaps too early to determine rigorously the true temperature of these points.

A noticeable fact also is this,—­that places in the same latitude rarely receive the same amount of heat.  Quebec, in British America, and Drontheim, in Norway, enjoy about the same quantity, while the former is in 47 deg. and the latter in 68 deg.  N. Lat.  The mean winter temperature of Pekin, 39 deg. 45’ N. Lat., is 5 deg. below the freezing-point; while at Naples, which is north of Pekin, it seldom, if ever, goes below it, and Paris, 500 miles farther north, has a mean winter temperature of 6 deg. above the freezing-point.  The city of New York, about 11 deg. south of London, has a winter temperature of much greater severity.  The mean temperature of the State of New York, as determined by a long series of observations, is 44 deg. 31’.