Humboldt, in the first volume of his 'Cosmos,' expressed a hope that not only a clearer insight had thus been gained into the distribution of heat in the atmosphere, but that this "system of isothermal, Isotheral, and Isocheirnal lines, if gradually perfected by the united efforts of investigators, may prove one of the chief foundations of a Comparative Climatology." In this hope he was no doubt fully justified, but one of the most profound of his followers, Professor Dove of Berlin, was led to the conclusion, as the result of the tabulation of an enormous mass of observations, ('Meteorologische Untersuchungen; Berlin, 1837; Temperaturtafeln; 1849; 'Die Verbreitung der Werme auf der Oberfliche der Erde,' 2nd edit., 1853; `Iilimatologische Beitrlige; 1857, &c.,) that it was necessary, in order to understand the true climate of a:place, to carry this system of lines of equal temperatures much beyond those of annual and semi-annual periods, there being between places of nearly the same mean annual temperature sometimes very remarkable monthly variations, and to which are due the most marked differences in the general character of the climate and natural pro ductions of the respective places. Dove in short concluded that it was necessary to have a complete series of Monthly Isolltermals,a name ho judiciously adopted for the curves connecting places having equal temperatures in the same month, instead of carrying further the terminology of Isothcral and Isochehnal : the lines of equal annual temperatures he proposed to call Yearly Isothrrmals of simply isothermal. with Humboldt. Of these Monthly Isothermals, Dove has given separate maps on tho equatorial projection for each month, and combined polar and equatorial maps for January and July. He has also carefully observed and laid down in a series of maps the monthly thermal anomalies, uniting them by lines which lie terms habnarmal.. Indeed hismaps, tables, and memoirs altogether present a surprising mass of information on this branch of science, with the greatest precision and clearness. In his essay on The Distribution of Heat over the Surface of the Globe,' translated for the British Association, he arranges the different Monthly Isothcrmals which he has established into four classes : " 1. Isothermals which are always found in pairs, intersecting both hemispheres : this class includes all between 32' and 77° Fahr.
"2. Isothermal,' which are sometimes single and sometimes in pairs, that is, sometimes intersecting one or other hemisphere only, and sometimes both : to this class belong many of the isothermals below the freezing point, and some of those of highest tempera ture which do not pass through all meridians, for example, 81°5 Fahr.
"3. Isothermale which always occur singly (or in one hemisphere only), at a particular season, not touching it in other parts of the year : to this class belong the isothermals of lowest, and also those of highest temperature (40° and 90°5 Fehr.) which are only developed at particular seasons, and enclose detached spaces.
" 4. Lastly, the isolated patches of highest temperature which are developed at particular places in the torrid zone, without passing through all meridians, and are enclosed by isothermals that fork, that is, divide into two branches, (7995, and 81'5 Fahr.)." Having thus briefly indicated the character of the several Isother mal lines, we refer back to the articles CLIMATE and METEOROLOOT, where, and in the connected articles there referred to, the circumstances which appear chiefly to regulate the temperature of the earth will be found discussed at length ; also various details illustrative of the temperature of different parts of the earth's surface; together with tables of the mean temperatures in different latitudes, as well as Iormuhm expressive of the moat important general results. We shall
in this place merely add that Dove has deduced as the result of his tabulation of inntunerable observations, that the mean annual tempera ture at the surface of the earth is in January 48°9 Fehr. in the Northern, and 59°5 in the Southern hemisphere, giving a mean of 54°2 for the whole earth ; while in July it is Fehr. in the Northern and 53°B in the Southern hemisphere, or 62°3 for the whole earth, thus showing an increase of 8° Fahr. from January to July. Again if we take "the mean between the temperatures of January and July for the northern and southern hemispheres respectively, wo find for the northern hemisphere 59'9, and for the southern 56°5 Fahr., being 58°.2 for the whole earth." Further he has found that in all seasons of the year there is encompassing the whole earth a zone having a temperature above 77° Fehr. .
The following tables, by Dove, show the mean annual temperatures at different degrees of latitude in the two hemispheres, and the mean annual decrease of temperature from the equatorial to the polar regions : We now turn to the distribution of the atmosphere over the sea. [SEA.] It is a well established fact, that places near the sea have a more uniform clhnate than those which are at great distances from it, though in the same latitude. Inland places experience a much greater degree both of heat and cold than places on the coast, and the difference between these degrees of heat and of cold Increases with the distance of the place from the sea. This phenomenon has been variously explained. The explanation is now pretty clear, since it has been proved by observation that the temperature of the air over the sea is less subject to changes than, or rather does not undergo such groat changes as, that of the air which is over the land. But as the tempe rature of countries situated between the tropics is not subject to so great changes as that of countries in the temperate zone, and these again are less affected by them than the frigid zone, so it. is found to be the case on the sea also. Beginning with the smallest natural division of time, the day, it is found that between the tropics the difference of temperature within 24 hours seldom exceeds, 2 degrees of Fahrenheit, and rarely amounts to more than 3 degrees. The following table, from Ileyen'a Reise um die Welt,' shows the changes of temperature on the sea during 24 hours between the tropics :- The difference between the highest and lowest temperature is only 2:16'. The mean temperature of the day is which is only above the lowest and below the highest temperature. It must, however, be remembered, that during the day to which these observa tions refer, the vessel on which they were made advanced through nearly 14 degrees of latitude, which of course must have had some effect on the temperature.