Besides, the rays, thus falling less obliquely and in greater number on a given surface, continue so to fall for a longer time, because the days are then longer.

5. Q. When are the days longer than the nights?

A. From the Spring Equinox (March 21st) to the Autumnal Equinox (September 23rd) the sun is north of the equator, and the days are then longer than the nights in the northern hemisphere; but for the rest of the year, the sun, being south of the equator, the days are longer than the nights in the southern hemisphere. At the equinoxes the days and nights are everywhere of equal length.

6. Q. Why are the days and nights everywhere of equal length at the equinoxes?

A. Because then the boundary of the earth's enlightened hemisphere passes through the poles of the earth and divides every parallel of latitude into two equal parts, each of which is exposed to the sun's rays and withdrawn from them for the same length of time. At all other periods that boundary divides the parallels unequally, and the days and nights are unequal in length.

7. Q. Where are the days longer than the nights?

A. They are longer on that hemisphere over any part of which the sun is vertical at the time -the difference of length increasing as we approach the pole. At the Summer Solstice (June 21st) the sun is in the Zenith at the Tropic of Cancer 23° 28′ N.L., and does not set at the Arctic Circle for 24 hours; but at the Winter Solstice (December 22nd) the sun is in the Zenith

F

at the Tropic of Capricorn 23° 28′ S.L., and then the night at the Arctic Circle is 24 hours long.

8. Q. How is noon distinguished from midnight when the sun is 24 hours above the horizon?

A. Noon may then be distinguished from midnight by the greater height to which the sun rises in the sky at noon, and by the part of the horizon above which he is seen. At the poles the year consists of one day and one night-each lasting 6 months; but twice a year there is Twilight, for about 50 days, which relieves the long darkness, and reduces the length of the actual night to about 2 months.

9. Q. Why is not summer hottest in polar latitudes where the days are longest?

A. Because the temperature is much less affected by the length of the day than by the elevation of the sun. Though the days are longer in the polar regions, the temperature there, even of summer, is usually below the freezing point, because the sun never rises high in the heavens.

10. Q. Why does not the heat constantly received by the earth as constantly raise its general temperature?

A. Were the earth to retain all the heat it receives its temperature must constantly rise. This, however, is not the case, but a general equilibrium of temperature is maintained, because the earth parts with its heat by radiation-the amount thus lost balancing the amount received.

11. Q. Why is not Midsummer the hottest period?

A. Though most heat is received about the 21st of June, the hottest period is later in the season, because the excess of heat received over that

radiated continues to accumulate so long as the earth receives more heat than it parts with during each 24 hours. When the sun is above the horizon more than 12 hours the temperature is generally above the average, and below it when the sun is less than 12 hours above the horizon.

12. Q. How does the excess above the average amount of heat, received by the earth, vary?

A. So far as the temperature depends on the position of the sun, the excess of heat above the average increases from the Vernal Equinox to the Summer Solstice, and decreases from the same Solstice to the Autumnal Equinox. The temperature, however, rises after the excess above the average begins to decrease, because for a time each day adds to the heat accumulated about the earth.

13. Q. Why is not the shortest day the coldest?

A. For some time after the shortest day the earth loses more heat by radiation than it receives from the sun during each 24 hours, and as every day's loss reduces still lower the temperature, the weather generally becomes colder till the heat gained balances the loss during the 24 hours.

LESSON 17.-LENGTH OF THE SEASONS: SOLAR HEAT.

1. Q. When do the Four Seasons begin?

A. Spring may begin either on the 20th or 21st of March; Summer on the 21st or 22nd of June; Autumn on the 22nd or 23rd of September; and Winter on the 21st or 22nd of December. The Summer Season reckoned from the Vernal

to the Autumnal Equinox comprises over 186 days, and is therefore fully 7 days longer than the Winter Season, which numbers less than 179 days.

2. Q. Why is Summer the longer season?

A. Because (1) the earth's orbit being an ellipse with the sun in one of the foci, the summer segment of the ellipse, or half of the orbit, is longer than the winter half, and (2), the earth is nearer the sun in winter, and therefore moves more rapidly in the less arc than in the larger summer arc (Lesson 6).

3. Q. How does the earth's orbital velocity vary?

A. The velocity is greatest when the earth is nearest the sun, and from that point it constantly decreases till the greatest distance is attained. The earth's angular velocity in its orbit increases in the same proportion as the square of the earth's distance from the sun decreases, and this velocity is about 1-15th part greater at the perihelion than at the aphelion.

4. Q. Why is it winter when the sun is nearest ?

A. On the whole the earth actually receives more heat, in a given time, in winter when the sun is nearest than in summer when he is farthest off; but this effect of the decreased distance in winter is not perceived by us, because at that time our northern hemisphere is turned from the

sun.

5. Q. How does the heat received by the earth vary?

A. The intensity of radiated heat increases as the square of the distance decreases; and the earth being about 1-30th of its mean distance

nearer to the sun, must therefore receive about 1-15th more heat, when at its perihelion than when at its aphelion. In equal latitudes and under equal circumstances of exposure the sun's noonday heat, in the height of summer, is greater in the southern than in the northern hemisphere. 6. Q. What is meant by this 1 15th more heat?

A. The excess of solar heat received in a given time by the earth when nearest the sun, is not 1-15th of the number of degrees above the zero of any ordinary thermometer, but 1-15thof the number of degrees above what would be the zero point if there were no sun. This zero point, it has been calculated, would not be less than 239° below the zero of Fahrenheit.

7. Q. What is supposed to be the actual excess of heat received by the earth, in a given time, during the southern summer?

A. If an average temperature of 100° Fahr. be assumed, 1-15th of the interval, between this and a zero point 239° below that of Fahrenheit, would amount to nearly 23° Fahr. Though the variation in the sun's distance should thus cause a difference of temperature of 23°, for a given time, yet the whole earth receives as much heat in one half of its orbit as in the other.

8. Q. How can the earth receive as much heat in our summer if the sun is more powerful in winter?

A. The earth receives equal quantities of solar heat in describing equal angles in any part of its orbit, because the earth's angular velocity and the sun's heating power vary in the same proportion, viz., in the inverse ratio of the square of the sun's distance. Now, from either equinox