ISOTHERTI'AL LINES (Gr. isos, equal, and thermos, warm) are lines laid down on maps to connect together places of the same mean temperature.—/sotheral three (Gr. thgros, summer) are those which connect places of equal mean summer temperature. — LivezeinzOnal lines (Gr. cheim5n, winter) connect places of equal mean winter tempera ture.—Alexander von Ihunboldt was the first to lay down these systems of lines on maps in 1817. Their importance in reference to climate, meteorology, and the geo graphic distribution of plants and animals, can hardly be overestimated.—If the whole surface of the earth were uniform, it is evident that isothermal lines would precisely correspond with the degrees of latitude, and there would be no isotheral and isocheimonal lines, as distinguished from the isothermal; but neither would the earth be habitable for man, or suitable for almost any of the animal or vegetable tribes which actually exist upon it. Isothermal, isotheral, and isocheimonal lines are therefore laid down altogether from observations recorded and compared. In laying them down, care must he taken to make allowance for the elevation of each place of observation above the level of the sea, they being all laid down as for that level. Isothermal lines are named according to the mean temperature which they indicate, the line of 50°, the line of GO°, etc. are far from corresponding with parallels of latitude, nor are they parallel with one another. but are curved in such a manner as to indicate two northern and two southein poles or centers of greatest cold. It is in the extra-tropical parts of the northern hemi sphere that these curvatures are greatest. The northern poles of cold are situated in the arctic regions, one to the a. of Siberia, nearly in the meridian of ,Takutsk, and the
other to the n. of America, nearly in the meridian of the most western part of Eudson's hay; and the isothermal lines throughout the greater part of the northern hemisphere descend to a lower latitude in the e..of Asia and in the e. of America than elsewhere, ascending, however, to a comparatively high latitude on the western coasts of both the great continents. Thus, the line of 50' F., which passes through the n. of England and the a. of Ireland, and there reaches its most northern descends below the latitude of New York, on the eastern coast of America. The distances of the isothermal lines are also remarkably various in different parts of the world. Thus, in the e. of North America, from Charleston to Labrador, the mean annual temperature varies more than a degree and a half for every of latitude; whilst in central Europe the variation is only about nine-tenths of and on the western coasts of Europe still less.
The isotheral and isocheimonal lines are neither parallel among themselves nor with the isothermal lines, and it is in this that a chief difference of continental and of insular climates appears, the summers of the former and the winters of the latter enjoying comparatively large proportions of the heat of the year.
• Another interesting system of lines relative to temperature has been laid down by Mr. Dove, which he calls isabnormal term, however, is objectionable, as formed from words of two languages—lines connecting places which have the same excess above or defect below the normal mean temperature of their latitude. Sec CLIMATE, METEOROLOGY, and TERRESTRIAL TEMPF.RATURE.