« PreviousContinue »
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 Fahrenheits, the scale being graduated to show 2120 for the heat of boiling water, and 32° 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, and 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 rOh, 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 700 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
153 ture of Paris for a series of 16 years was only 4o. 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 different 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 25o. 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 .
155 inuch affects the climate, and as a general rule the western sides of continents and large islands, are warmer than the eastern. Certain portions of the globe, which from their nearness to the equator would be extremely warm, are rendered tolerably cool by their elevated situations. This is the case with much of the tropical land in America, which is so raised that it rivals even European climates in mildness and agreeable temperature. The air of these elevated tropical districts is remarkably pure and transparent, and the winds which sweep over the plains, are cooled by their passage down the snow-capped mountains, which rear their bright summits to the skies. The vast expanse of table-land, forming the empire of Mexico is of this character, being elevated 7000 feet above the level of the ocean. This land in many parts has the fertility of a cultivated garden. The plains of Columbia in South America, and indeed all along the ridge of the Andes, are similarly situated. The chart which we have given represents the direction of the isothermal linés, or lines connecting places which have the same mean annual heat. It will be evident that places may thus be situated on the same isothermal line, which have very unequal mean temperatures of summer and winter. We need only refer to the table on page 157, to be convinced of this. Thus, the mean annual temperature of London, and Cambridge, Mass. is the same, 50036'; 'bắt the mean temperature o the warmest month at London is 64°40', while at Cambridge it is 72086', and of the coldest month, at London 37076, at Cambridge 29.84, London therefore has a colder summer and a warmer winter than Cambridge. The reason of this, is undoubtedly, the insular situation of the former, for as a general rule the extremes of temperature are experienced in large inland tracts, and little felt in islands remote from continents. The difference between the mean temperature of summer and winter is nothing at the equator, and increases continually with the latitude. When the mean annual temperature is low the difierences between the extremes of the seasons is great, and the contrary.
The effect of climate upon the geographical distribution of plants and animals is very marked. Each, generally has its pe
culiar climate where it thrives best, and beyond certain limits it ceases to exist. The successive zones of vegetation, as we recede from the equatorial regions, have sometimes been supposed to be represented by the different altitudes upon the mountains under the equator, as it is evident we have in ascending from the valleys to their snow-capped summits, every variety of temperature. The analogy fails however in one essential point, for as we ascend the mountains the pressure of the atmosphere is continually diminished and it is evident that less nutriment is thus afforded for the growth of the plant. The influence which the variations of climate alluded to, must have upon vegetation is very evident, thus in many parts of Siberia, wheat and rye are raised upon a soil which is constantly frozen at a depth of three feet, while in Iceland, where the mean temperature of the year is much warmer, and the winter's cold but inconsiderable, it is not possible to raise any of the ceralia or common grains, as the low summer temperature does not suffer them to ripen. It is for the same reason that the vine does not flourish in England, for although it can endure a tolerably great degree of cold, yet it requires a hot summer to make the fruit ripen, and yield a drinkable wine. There is no subject connected with meteorology which requires a more careful, and studied investigation than that of climates. So many causes influence the temperature of the air, and some of them are so variable, that no labor short of a well conducted series of observations, extending through a long course of years can give a satisfactory result. In the brief account we have given, we have been able to present little else than the leading facts, and must refer the reader to the writings of Leslie, De Candolle, Mirbel, and Humbolt, for further information.