nitrogen is drawn directly from the air by the leguminous plants: elovers, peas and beans. Thus the fertility of the soil is materially enhanced. Large amounts of lime will keep out chestnut, rhododendron or laurel, but will give a splendid growth of grass as is seen in the blue grass region of Kentucky. When grass fails and moss grows the wise farmer knows that lime is needed to correct the acidity of the soil. It is sour, as he says.

From the standpoint of the farmer, the soils have other important features. Some of the richest alluvial soils are so level that adequate natural drainage does not exist. Other districts are so hilly that cultivation is difficult and the land washes badly. Yet others are stony. Some retain moisture and may be heavy and cold. Others permit rapid evaporation and are light and warm. Whatever be the soil, the wise farmer knows the wisdom of using the crops adapted thereto and the methods of cultivation necessary under the given conditions for the best result. He knows that alfalfa will send its roots ten to twenty feet to water, while to produce corn it is necessary to have the water close to the surface.

With the exception of the tides, and possibly radioactive substances, the sun is the sole source of energy on this planet. Even the coal and petroleum, on which we depend for fuel and artificial light, are little more than stored up sun rays of bygone days. We may consider the influence of the sun in several aspects as: (1) controller of the orbit of the earth; (2) the source of light; (3) the source of heat; (4) the cause of the distribution of rain, and (5) the cause of wind and storm.

The rotation of the earth on its axis in its great course about the sun gives us the phenomena of day and night with its variation from twelve hours each at the tropics

to six months each at the poles. It gives us further the procession of the seasons with their varied characters. Here we have the basis for our measurements of time and the start of the mathematical sciences.

Light enables us to see and direct our movements. To a large degree it controls the growth of plants and animals. The construction of the substances we call proteids by the chlorophyll-holding plants takes place only in the light. The lower the temperature the greater the amount of light needed. The influence of light on the leaves causes them to draw many needed elements from the air. Thanks to the spectroscope we now know the spectrum of colors from red at the lower end to violet at the upper. Experiment has revealed that red and yellow light promote the assimilation of carbon dioxid. Under such light plants prosper as the following shows:

(Mimosa pudica) Sensitive plants, seeds planted in May 3

So necessary is the sunlight that few plants will survive if the tops are repeatedly cut, no matter how sturdy the roots.

In the Northern Hemisphere, the sun, shining longer and more directly upon the southern slope of hills, makes them much warmer than those of the northern slope. From the surface of water, or from cliffs and buildings, so much heat is often reflected that adjoining slopes are frequently much warmer than those removed from such influences. Reflection from the snow will sometimes blister one's face or produce the dreaded snow-blindness. 3 HANN, J. Handbook of Climatology, p. 36.

The shady sides of forests or wind breaks are correspondingly cooler. Thus plant growth is hastened or checked. The temperature determines the character of the vegetation. In the tropics one finds the characteristic plants with broad leaves and naked buds, of great size and continuous growth — a veritable riot of profusion, with woods so full of undergrowth that travel is difficult. Further north come the deciduous trees with protected buds, accustomed to endure both heat and cold, or spinyleaved evergreens. Other things being equal, the nearer the pole the scantier and more dwarfed the vegetation. Periods of growth are followed by months of rest when the sap no longer runs. The woods no longer teem with underbrush save where the older trees have been destroyed. A similar change may be noted among the animals. Lizards and snakes gradually disappear along with the thick-skinned but practically hairless elephants and hippopotami. In their stead wolves, bears and fur-bearing animals appear. Gaudily colored birds are replaced by those of simpler dress; toucan and parrot yield to sparrow and grouse. In the world of water, sponge and coral are replaced by oyster and lobster; the ocean ferns by kelp. Hence we divide the plants in our gardens into three groups: tender, half-hardy, hardy. The first are peculiarly susceptible to frost (such as cannas); the second will endure moderate cold and occasionally survive if left in the ground over winter (gladioli); while the last are rarely injured, no matter how severe the cold (peonies). We may grow tropical plants out-of-doors during our hot summers, but we do not change their nature. With the advent of cold weather we must take them indoors or let them perish.

This succession of the seasons produces less effect upon

the temperature of the ocean than on land as may be illustrated by the following diagram.*

The extremes of temperature are much greater on continental areas than on the ocean. In continental areas the maximum temperature is attained about one month after the sun reaches its highest altitude; the lowest about one month after its lowest declination. In marine areas the highest temperature comes a month or so later (August), while the lowest is not reached before March. In the interior of the United States the temperature of the winter months fluctuates more than 4.5° about the mean; in Russia, 6.3°; but in the coast climate of England only 2.7°. The ocean climate, therefore, changes less from day to day and its extremes are less. The temperature in large bodies of water varies to some extent with the depth. The temperature in Lake Michigan at the surface in summer is put at 64.9°; at a depth of 18 feet, 64.9°; 74 feet, 44.9°; 436 feet, 39.5°.

The different response of land and water to the rays of the sun sets up the movements we call winds or currents. In older times man thought: "The wind bloweth where 4 HANN, J. o. c., p. 141.

it listeth," but now we have come to understand that these movements of air and water are in response to definite laws. To a large extent we have charted the ocean streams and understood why the Gulf Stream flows north, modifying the climate of the Atlantic Coast, while from the north comes a cold stream. The circulation in large bodies of water is significant inasmuch as the water absorbs oxygen at the surface. This must be carried to lower levels if fish are to survive. These currents are not well known today. The air currents are less thoroughly understood, though great progress has been made in mapping them since the middle of the nineteenth century. The United States is in the zone of prevailing west winds, which in large measure accounts for the cold climate of the eastern portion of the country in winter and its heat in summer. The wind, moreover, determines the distribution of the water taken from the ocean through evaporation and carried by the clouds. The presence of a high mountain range on the west coast stops the moistureladen clouds from the west and creates a semi-desert to the east. Barring a little water from the north, all the rest for our great country must work north and west from the Gulf of Mexico and the Atlantic. The result is that west of the 100° meridian the supply is inadequate for ordinary crops. Another result is that the west coasts of America, as well as of Europe and Asia considered as a whole, are much warmer and subject to fewer extremes than the east coasts.

We may compare two

The city of London is covered with fog from 50 to 75

51.08°

35.06°

75.38°

54.9°

49°

59°