test and coldest seasons being separated by a period of half a year, differ very considerably from each other in their temperature. between these two tropics is When the sun's distance from

The whole terrestrial zone lying called the hot zone, or torrid zone. the equator north is the greatest possible, i. e., when it is in the tropic of Cancer or at the point VI, see figure, page 80, the north pole of the earth is illuminated, and the south pole in darkness, as represented in the figure, page 109. If we suppose a circle traced upon the earth as shown at c d, it is evident that as the sun now illuminates all within this circle, the day will be to a spectator situated upon it, 24 hours in length, or in other words visible during a complete revolution of the earth on its axis. A similar circle shown at gh, indicates the position in the southern hemisphere where the longest day is 24 hours. These two circles are called, the former the Arctic, and the latter the Antarctic, the former is situated 23 28' from the north pole, and consequently 66° 32' north of the equator, and the latter at the same distance from the south pole, and south of the equator. The terrestrial zones included between the tropics and the polar circles, are called the northern and southern temperate zones. The four seasons of the year are most strongly characterized in these and the general rule for the dimunition of heat is, directly as the distance from the equator. Within the polar circles are As the earth turns upon

zones,

the northern and southern frigid zones. its axis from west to east, the sun is apparently caused to rise in the east, move over the heavens, and set in the west, thus producing the alternation between day and night. During the day, the surface of the earth is warmed by the rays of the sun, but when these are withdrawn at night, the heat is radiated to the heavens and lost, during the night therefore the surface of the earth is cooled. We shall presently see that the vicissitudes in climate varying with the latitude, are mainly due to the unequal lengths of day and night. Under the equator the days and nights are very nearly equal, throughout the year, each lasting 12 hours. As soon, however, as we leave the equator, the length of the day varies according to the season of the year, and the difference between the day and night becomes more striking as we approach

nearer the poles. The following table exhibits the length of the

longest day for different geographical latitudes.

Upon examining this table it will be perceived that within the tropics, the length of the longest day never varies much from that of the night, and hence, as before observed, the temperature is tolerably equal. In higher latitudes the rays of the sun strike more obliquely than within the tropics, yet the day so much exceeding the night, more heat is gained during the day than is radiated during the night, and thus, what is lost in intensity, is gained in the length or duration, and it thus happens that during the summer it may be very hot, even at places far removed from the equator. At St. Petersburgh, for instance, during a hot summer the thermometer frequently rises to 86°. On the other hand, in winter, at the same latitudes, the days become as much shorter than the nights, as the nights were previously shorter than the days; hence, since the sun's rays fall very obliquely, and are therefore very feeble in their action, the earth radiating much more heat at night than it receives during the day, the winter temperature is very low, the difference between winter and summer temperature will therefore, generally be greater, the farther we remove from the equator.

"At Bogota, which is 40° 35' N. of the equator the difference of temperature between the hottest and coldest month amounts to only 30; in Mexico (19° 25' N. lat.) this difference is 14°; at Paris, (48° 50′ N. lat.) 48°, and for St. Petersburgh, (59° 56′ N. lat.) 570."

It appears from what has been said, that within a distance of 10 or 15 degrees of the equator or equinoctial line, the difference between summer and winter temperature is trifling, but when we

get as far north as the tropics, this differencee becomes very sensible, and it has been truly observed, that the torrid zone may be divided in three, viz: the equatorial belt, extending 10 or 15 degrees from the equator, and the two belts north, and south, between this and the tropics. The equatorial belt, properly so called, is temperate compared with the two others, the zone of the tropic of Cancer being the hottest and least habitable part of the globe. The greatest natural heat of which we are aware, has been observed at Bagdad, at 33 degrees of N. lat., being 111° Fahrenheit. There are many reasons why the equatorial belt should have a uniform and somewhat mild temperature; the clouds, the great rains, the nights naturally cool and equal in length to the days, and the great evaporations. As we go farther from the equator the difference between the summer and winter temperature becomes more marked, the summers being, on account of the protracted heat of the day, very warm even in high latitudes, and the winters extremely cold. Thus, even as far from the equator as the 65th parallel of latitude, the power of the solar beams accumulating through the long days, produces an effect which might be expected only in the torrid zone. There have been examples of forests having been set on fire, and of the pitch melting on the sides of ships. Notwithstanding the general law of the decrease of mean temperature as we recede from the equator, yet it is impossible to draw any conclusion as to the climatic relations of a place from its geographical latitude. If the earth's surface was entirely homogeneous, either covered by water, or by land, possessing the same capacity for heat, then the geographical latitude of a place would determine its climate, and all places having the same latitude would have a similar climate. This however, is not the case, for although the local temperature of a country depends very much upon its latitude, yet the nature of its surface, the proportion of humidity, the distance from the sea, or from lakes or mountains, and its elevation above the ocean, and the nature of the prevailing winds, all have a share in determining the climate. The decrease of heat as we recede from the equator follows different laws in the two hemispheres, being greater in the southern than in the northern, and is also affected by the longi

tude. The true distribution of heat over the earth's surface can therefore only be determined by a long series of observations. Baron Humbolt with unwearied zeal, has collected the data for, and laid the foundation of, a scientific meteorology. The instrument employed to measure the intensity of heat, called a thermometer is to well known to need any description here. The thermometer in ordinary use is what is called Fahrenheit's, the scale being graduated to show 2120 for the heat of boiling water, and 320 for the temperature of melting ice, or freezing water. The zero or commencement of the scale, is the temperature of a mixture of salt and ice, or snow, and which was once supposed to be the greatest artificial cold. The thermometer called Reaumer's is used in some parts of the continent of Europe, the freezing point of water being zero, or the commencement of the scale, and the space between this and the boiling point of water is divided into 80°. The thermometer now used in France, aud the greater part of the continent of Europe, is called Centrigrade; the scale of this thermometer is graduated into 100 degrees from the freezing, to the boiling point of water; this division of the scale appears the most natural, and has been adopted by law in the state of New York.

In employing the thermometer to observe the general temperature of the air at any particular season of the year, it will generally be sufficient to make two observations in the morning, viz: at 4h, and 10h, and two in the afternoon at the same hours, the mean of the observations will give the mean temperature for the day very exactly; thus, suppose the observations made at these hours to be 50°, 80°, 90°, and 60°, adding these all together, and dividing their sum 280°, by 4 gives 70° for the mean tem-. perature of the day. When we know the mean temperature of all the days of a month, we can in like manner determine the mean temperature of that month. We can likewise determine in a similar manner the mean temperature of the year, or of summer, and winter. The mean annual temperature, of a place not subject to very great local changes, such as the clearing up of forests, or drying up of streams and rivers, is very nearly constant. Thus, the extreme difference of mean annual tempera

ture of Paris for a series of 16 years was only 40. We can thus by a series of well directed observations, determine the general' climatic relations of various continents, and the result of such observations are in some instances very different from what would be inferred from mere theoretical considerations. It is found that the decrease of heat as we recede from the equator, follows dif ferent laws in the two hemispheres. The subjoined table shows the mean annual temperatures of Western Europe and North America, continued from the equator.

From this table it appears that the decrease of temperature, or increase of cold is much more rapid in America than in Europe. Baron Humbolt, who has added more to our knowledge of the distribution of temperature over the globe, than any other who has labored in the same boundless field, has proposed a system of isothermal lines connecting different places having the same mean annual heat. The differences between the mean annual temperature of places upon the same parallels of latitude are thus presented to the eye in a very striking manner. On the next page will be found a little chart of isothermal lines for every 50 in Mercator's proportions. It will be seen that the mean annual heat of Eastern Asia and Eastern America, are much nearer than of Eastern America and Western Europe. A simple inspection of this map will give a clearer idea of the variation of isothermal lines from the parallels of latitude. Thus, for instance, the mean annual heat at the North Cape, is 32°; whilst Nain on the coast of Labrador, 14° south of the North Cape, has a mean annual heat of 25°. The table which we give contains a general summary of Baron Humbolt's observations deduced from a very great number of observations. The locality of a place very.