CHAPTER V.

INTRODUCTION TO PHYSICAL GEOGRAPHY.

Is the preceding chapters we have stated simply the principles of Mathematical Geography, by means of which the relative positions of places on the earth's surface are ascertained, and laid down on maps. We have now to consider the several physical agencies at work at different places on the earth's surface, which influence the living beings, both vegetable and animal, which cover profusely that surface. This branch of science is called Physical Geography, and its importance cannot be overlooked, because its laws regulate the climate and natural productions of every part of the globe.

A systematic study of Physical Geography embraces the following subjects:

1. The distribution of Heat on the surface of the earth. 2. The distribution of Moisture on the surface of the earth.

All organic life depends, primarily, on heat and moisture; each individual organic group requiring its own special conditions of heat and moisture, and perishing at once if these conditions cease to be fulfilled. We can see, therefore, that as Commercial and Political Geography depend upon a knowledge of the productions, animal and vegetable, of the various countries of the globe, we can make no rational progress in their study without first laying down the general principles upon which heat and moisture (on which animal and vegetable organisms depend) are distributed upon the surface of the globe.

In discussing the laws of distribution of heat and moisture, we shall find that they are much influenced by the following circumstances, which will require a general comprehensive description:

3. The distribution of land and water, and their arrangement into mountains and plains, shallow seas, and deep ocean-valleys.

4. The circulation and currents of the atmosphere and

ocean.

5. The laws of production of rivers and lakes.

We propose to give a general outline in the present chapter of the foregoing subjects, and to discuss them more in detail in the succeeding chapters.

1. DISTRIBUTION OF HEAT ON THE SURFACE OF THE EARTH. The heat of the surface is derived from two sources, viz., the heat which proceeds from the interior of the earth itself, and the heat which it derives from the influence of the sun. In former geological periods, the heat derived from the interior of the earth was much more considerable than at present, for it has been ascertained that the earth has undergone a secular cooling, and has parted with much of its original heat by radiation into surrounding space. It is probably owing to this cause that plants and animals, which resemble those now found in tropical climates, formerly lived in high latitudes, where at present only scanty traces of organic life are to be found. At the present time, the heat obtained from the interior of the earth must be regarded as very unimportant when compared with that derived from the The heat derived from the sun is estimated as sufficient to melt a coating of ice forty-six feet in thickness, spread over the entire surface of the globe; whereas the heat derived from the interior is calculated to be sufficient to melt a similar coating of ice only one quarter of an inch in thickness. This result shows us that the heating power of the sun exceeds that of the interior of the earth 2,208 times; so that in considering the distribution of heat on the globe, we may wholly disregard the heat of the earth itself. If the earth were composed of uniform materials, of uniform height, and not divided into land and water, the heat derived from the sun at each place would depend solely on its latitude, being greatest at the equator and least at the poles. Owing, however, to the inequalities in height, the

sun.

* If we neglect the sun's declination, the mean temperature at each latitude would be nearly proportional to the length of the parallel of latitude, which is greatest on the equator, and becomes zero at the poles. If we take the mean temperature at the equator to be 80°, the temperature at any other latitude, up to 60°, or 70°, could be found from the table (p. 53) by increasing the numbers given in the table in the proportion of 80 to 60, or of 4 to 3. Thus, the latitude of Dublin being 53° 21', the tabular number is found to be 35.83, which number increased in the proportion of 4 to 3, gives 470.77 as the mean temperature of Dublin. The actual mean temperature of Dublin is 50°, and the difference between the calculated and observed temperatures is due to the disturbing influences summed up in the laws of climate.

irregular distribution of land and water, the various kinds of surfaces presented to the sun, and the action of warm and cold currents of air and water, the original distribution of heat just mentioned is very considerably modified, and is found to hold true only for places within 15° to 20° north or south of the equator.

The deviations from the primary distribution of heat, depending upon latitude, are summed up in several laws, called laws of climate. The first of these laws depends on the distribution of land and water, and may be thus expressed :

First Law of Climate.-A mass of land in low latitudes raises the mean temperature of the place, and a mass of water lowers the mean temperature. On the other hand, a mass of land in high latitudes lowers the mean temperature of the place, and a mass of water raises the mean temperature.

It would not be possible in so brief a sketch as this to give the reasons for this remarkable law, which depends on the different absorbing and radiating powers of land and water; but it is easy to illustrate the truth of the law by various examples. The mean temperature of the equator is found to be as follows:-Africa, 85°.10; Asia, 82°.94; America, 80°.96.

The reduced temperatures of Asia and America, as compared with Africa, are due to the cooling effect of the Indian Ocean, Gulf of Mexico, and Pacific. In the Nubian desert, where the influence of the mass of African equatorial land is probably greatest, the thermometer (in the shade) has been observed to stand at 133°, and in the valley of the Euphrates, where the influence of the central mass of Asia is very great, the thermometer (in the shade) has been read at 132°.

If, on the other hand, we turn our attention to high northern latitudes, we find that the coldest spots are not the places nearest to the pole. There are, in fact, two places called poles of maximum cold, one in the north-east of America (long. 100° W.), and the other in Northern Siberia `(long. 90° E.)

Mean temperature of American pole of cold=37 degrees below the freezing point of water; of Asiatic pole of cold= 31 degrees below the freezing point of water.

The American pole, which is the coldest spot on the surface

of the earth, lies between the parallels of latitude 80° and 85°, and is actually colder than the north pole itself. These poles of cold illustrate the effect of the large masses of land formed by America and Asia, in high latitudes, in lowering the mean temperature.

The lowest recorded temperatures in America and Siberia have reached in America=102 degrees below the freezing point of water; Siberia=120 degrees below the freezing point of water.

We thus see that the difference between the highest temperature in Africa, and the lowest temperature in Siberia amounts to 221 degrees, which is considerably greater than the difference between the freezing and boiling points of

water.

The height of the place above the sea level modifies its mean temperature, which decreases with its height for the first 5,000 feet, at the rate of 1° for each 240 feet. The decrease above 5,000 feet is less rapid.

In consequence of the preponderance of land in low latitudes in the Northern Hemisphere as compared with the Southern, and also in consequence of the fact that large masses of water occur in the Arctic regions, while large masses of land are found in the Antarctic regions; the mean temperature of the Northern Hemisphere is higher than that of the Southern.

Difference in mean temperature of corresponding latitudes, North and South :

The second and third laws of climate depend upon the fact that in latitudes above 30° North and‍20° South, the prevailing winds are south-westerly in the Northern Hemisphere, and north-westerly in the Southern Hemisphere.

Second Law of Climate.-In the Northern Hemisphere, in latitudes above 30°, the mean temperature of places on the same parallel of latitude is higher on the west coast of a continent than on the east coast.

The prevailing S.W. winds of the Northern Hemisphere, and N.W. winds of the Southern Hemisphere, when blowing

on the western coasts of a continent, bring with them large quantities of vapour formed by the heat of the sun in the tropical waters of the globe, and this vapour being carried by the prevailing winds into the colder regions of the higher latitudes, is there condensed as rain, and parts to the air with its latent heat, thereby raising its mean temperature. On the contrary, when the prevailing S.W. and N.W. winds reach the east coast of a continent, they have already parted with most of their vapour, and appear as dry winds raising the temperature of the air but very little. The effect produced by the prevailing winds is aided in many cases, particularly in the Northern Hemisphere, by the existence of westerly currents of warmer water in the ocean.

The following examples illustrate well the truth of the second law of climate:

EXAMPLE 1.-Pekin, on the east coast of Asia, has about the same latitude as Lisbon in the west of Europe, but their mean temperatures are very different.

Latitude.

39°

40°

EXAMPLE 2.-Dublin has nearly the same latitude as Nain in Labrador, on the east coast of North America, while their mean temperatures are as follow:

EXAMPLE 3.-Sitka, on the west coast of North America, has nearly the same latitude as Nain, on the east coast of the same continent, and their mean temperatures are

EXAMPLE 4.-If we follow the Arctic Circle round the pole we find the following changes in the mean temperatures of places situated upon this parallel of latitude.