ANNUAL DISTRIBUTION OF LIGHT AND HEAT. 113

and what will be the difference of effect of his declination in these latitudes ?

8. On the 3d of February, in what latitude north does the sun set 20° south of the true west; and whether does the north point of the magnet vary eastward or westward in a certain longitude on that parallel of latitude, if the sun, on that day, is found to be setting 30° south of the west of the compass ?

ANNUAL DISTRIBUTION OF LIGHT AND HEAT.

The distribution of the light and heat* of the sun over the surface of our earth, is unequal and variable from several

causes.

B First.-Its globular form, as its surface, in the higher latitudes, curves off and retires, receiving the sunbeams obliquely.

A board twelve inches square will entirely intercept the rays of light and heat which, at a corresponding distance from a lamp, exactly spread themselves over the more extensive half-surface of a twelve inch globe. The pupil can readily conceive over which portions of the globe-surface these rays are cast most thinly.

с Secondly. "The nature of the surface whether water or land; and the situation, whether at a greater or less height above the level of the ocean. To such circumstances may be added, the particular configuration and geographical relations of places; as their aspect to the north or south; their being sheltered or exposed; the composition and nature of the soil, particularly its colour and state of aggregation, on which depends its power* of absorbing and of radiating heat and light, and of retaining or of parting with humidity, &c.; the proximity, or absence of seas; the predominancy of certain winds; the frequency of clouds, fogs, &c."-Prout's Bridgewater Treatise, p. 216.

D

Thirdly. The obliquity of the earth's axis of rotation to the plane in which it performs its annual revolution.

* See articles Light and Heat (Index).

The pupil is requested to observe, particularly, the latitude in which the vertical ray falls, as indicating the declination caused by the relative position of the axis.

We hope the explanation given on pages 106—8, have fully prepared the pupil for a just idea of the circumstances illustrated above. He will recollect that the axis (our button peg) maintains its parallelism during its 366 rotations, (otherwise our equinoctial plane would be a varying one); and he will now perceive that the consequence to our globular earth, is an ever-varying distribution of the sun's influence as it respects the latitudes of its surface: the sun (represented above by the taper) becoming, by this wondrous ordination of the Creator, a star of ever-changing declination; and, in the course of the year, or period of complete revolution, shedding its vertical rays in a range of latitude from 23° 28′ north, to 23° 28' south of the equator. Hence the seasonal changes of summer, autumn, winter, and spring.

Our purpose will be answered by referring to the five positions particularly noted above: we begin with the posi tion on the right, representing the extreme circumstances of the

21st OF JUNE, (our SUMMER SOLSTICE.)

* Here it may be seen that the northern extremity of the axis, or arctic pole, is bowed towards the sun to the exact extent of its inclination to the plane of the orbit. The

ANNUAL DISTRIBUTION OF LIGHT AND HEAT. 115

taper flame, or sun, is therefore vertical in latitude 23° 28' north; and affording, notwithstanding the earth's daily rotation, continuous light to the whole segment within the arctic circle; i. e., to the whole of the north frigid zone, leaving a corresponding segment (the south frigid zone) continuously unenlightened.

Referring to Problem VI. of Section I., it will readily be understood that the equatorial inhabitants, as those of Quito or the north of Celebes, have an invariable period of twelve hours' light; but that of all the places which, at any instant, are rising into the sun's beams along the western edge of the terminator, those most southern will first arrive at its eastern edge to lose his light; whilst those most northern will enjoy it longest. And thus, since these are the extreme circumstances of our earth's position in sunlight, we have a representation of the longest day of the northern hemisphere, and the shortest day of the southern

one.

AUGUST, (SECOND WEEK OF).

G We pass towards the left to view the condition of the earth in the early portion of August; and we remark that, in consequence of the curvature of its path, the unvaried direction of its axis is now, so to speak, considerably on one side of the sun; and the latitude now receiving his vertical beams about 15°. Hence his rays reach over the north pole to that extent only, and a considerable portion of the arctic or frigid zone is spending a part of the diurnal rotation of twenty-four hours out of their direct influence.

The equator still retaining its period of twelve hours, the summer days of the northern hemisphere are now considerably shortened, whilst the winter ones of the southern hemisphere are lengthening.

SEPTEMBER 23, (our AUTUMNAL EQUINOX).

H The earth has now attained a position with regard to the sun, at right angles* with that which it had at the solstices.

* As may be seen in the cut, at the solstices there is a common plane passing through the axis, the polar point in the heavens, and the sun. This plane is represented on the celestial globe by the

solstitial colure.

Its poles now are neither inclined towards the sun nor away from it hence, its equator is receiving his vertical rays, and the terminator or boundary of his light, passing through the poles, coincides with the whole meridian of any place rising or setting, and divides every parallel of latitude into two equal parts. The sun now appears in the heavens without declination, coinciding, like y of Virgo, or the star in the belt of Orion, with the equinoctial, and gives the name to that line, because when he is thus circumstanced, the nights and days of every latitude are of equal duration.

NOVEMBER, (FIRST WEEK OF).

K The circumstances of the southern hemisphere are now precisely those of the northern hemisphere in the position marked out for August. The north pole is inclined 15° from the sun; and that portion of the arctic zone which is not farther than 15° from the pole, or has above 75° of latitude, cannot rise into sun-light; a similar portion of the antarctic zone having it continuously. Hence, too, the sun is vertical in latitude 15° south; and appears in corresponding south declination. The inhabitants of the northern hemisphere have a shortness of day depending on their distance from the equator, whilst those of corresponding latitudes in the southern hemisphere, have corresponding shortness of night.

21st DECEMBER, (our WINTER SOLSTICE).

The position of the earth on the day of the winter solstice, may be explained by referring to that of the 21st of It will be seen that the circumstances of the two

June.

hemispheres are now completely reversed. The arctic zone, the whole of which, on that occasion, had continuous light, is now entirely enveloped in darkness; whilst to the whole of the antarctic zone, the sun is constantly above the horizon. He is shedding his vertical rays in the latitude answering to the declination of the southern tropic, and the inhabitants of the southern hemisphere have now attained their longest day; the inhabitants of corresponding northern latitude having their night of corresponding length, and their day the shortest.

LATITUDE AND LENGTH OF DAY.

FEB.

MAR. 21.

MAY.

117

"The positions for these three occasions may be understood by reference to those which are opposite. It must be remarked, that whilst the consequences to the two hemispheres in February and May respectively, are the reverse of those in August and November, the circumstances of the 21st of March, as far as it respects the distribution of light, are precisely those of the 23d of September.

PROBLEM IV.

TERRESTRIAL GLOBE.

The day of the year being given, to find the length of that day in a certain latitude, and the altitude of the sun at any distant place, when he is setting at the place of observation.

Repeat the following:

Heteroscii, (def. 74). Read and describe pp. 114-117.

Rule.-1. Find the sun's declination for the given day, and rectify the globe for that declination; the wooden circle of the globe will then represent the " terminator,' or boundary of his direct beams; and the brass meridian, the plane in which the sun lies. Bring the given place to the brass meridian, and set the index to twelve, or noon. Turn the globe eastward, and count the hours passed over by the index until the place coincides with the eastern edge of the terminator; this will show the time from noon until sun-setting, or half the length of the day.

Subtract those hours from twelve or midnight, and it will show half the length of that night, or the time of sunrising; since the sun rises so much after midnight as it sets

It represents the horizon, only of any place having latitude corresponding exactly to the declination of the sun, at the instant of

noon.