8.The Interior of the Earth. -Whenever borings have been made into the interior of the earth it has been found, after a depth had been reached where there was no effect from the heat of the sun, that the temperature rose as the boring increased. From this gradual increase of temperature, it must be that far down within the earth the tempera- ture is very high. The pressure within the earth is so great, however, that there are probably no liquid rocks at great depths. If the earth had a liquid interior the attraction of the bodies about it in space would cause changes in its shape, but it is as rigid as steel.
The outside cold part of the earth is called its crust. How thick thisis no one knows. This is the part that is of particular interest to us, for it is the only part which we are able to observe and study.
9.The Cause of Day and Night.
Experiment 8. -(a) In a darkened room place a globe a short distance from a small but strong light. Rotate the globe with its axis at right angles to the line which joins the centers of the globe and light. How much of the globe is illuminated by the light? Is the same part of the globe illuminated all the time? Does any place on the illuminated part receive light for a longer time during a rotation than any other place? Remove the globe to the opposite side of the light without changing the direction of its axis. When rotated, is there any change in the globe"s illumination? If so, what?
(b)Now make the axis on which the globe rotates parallel to the line joining the centers of the globe and light. Rotate the globe. How much of the globe is illuminated by the light? Is the same part illuminated all the time? Does any place on the illuminated part receive light for a longer time during a rotation than any other place? Remove the globe to the opposite side of the light without changing the direction of its axis. When rotated is there any change in the globe"s illumination? If so, what?
(c)Place the axis of the globe so that it is inclined to the line joining the centers of the globe and light. Rotate the globe. How much of the globe is illuminated? Is the same part of the globe illuminated all the time? Do any places in the illuminated part receive light for a longer time during a rotationthan other places? Remove the globe to the opposite side of the light without changing the direction of its axis. When the globe is rotated is there any change in the length of time of illumination of the places before noted? If so, what?
MEDIEVAL IDEA OF THE UNIVERSE.
From a fourteenth-century manuscript. Above the earth are the clouds and the moon; then the rays of the sun; next various planets; above these the stars; and finally the signs of the zodiac.
As has already been stated, the ancients considered the earth as the center of the universe and thought that the sun and stars revolved around it. We of the present day, however, know that it is the rotation of the earth from west to east that causes the appearance of the rising and setting sun and thus makes day and night.
Of course it makes no difference about a person beginning to see a light whether the light is brought toward him or whether he goes toward the light. We are turnedinto and out of the sunlight by the rotation of the earth. We speak of the sun as rising high in the sky, but what really happens is that we are turned so that the center of the earth, our heads and the sun come nearer and nearer toward a straight line.
When we say down we mean toward the center of the earth, andwhen we say up we mean in the opposite direction. These are the only two directions that we could be easily sure of, if it were not for the rotation of the earth. This gives the direction of the rising sun, which we call east, and of the setting, which we call west. A line which runs at right angles to the one joining east and west, i.e. one running parallel to the axis of the earth, is said to run north and south. Thus the points of the compass, as well as day and night, are determined for us by the earth"s rotation. The north star, which is so important to the sailor in determining his direction, is simply a star which is almost in line withthe axis of the earth. It is the rotation of the earth which gives us also our means of measuring time.
As was seen in the previous experiment, the direction of the axis of a rotating globe has much to do with the light which different parts of it will receive from a luminous object. The hemisphere which is inclined toward the luminous object will have a larger part of its surface illuminated and therefore each place on it will be longer in the light during a rotation than when the hemisphere is inclined in the opposite direction.
As the axis of the earth is inclined to a line drawn from the earth to the sun, the light the earth receives is similar to that received by the globe in the last part of the experiment. Thus the days and nights vary in length during the year, because in summer the northern hemisphere is inclined toward the sun and in winter away from it.
10.The Movement of the Earth around the Sun. -The earthnot only turns on its axis every day, but it travels around the sun, con- tinually changing its position in relation to the stars. It moves with the tremendous average velocity of about 19 miles a second. It is this revo- lution around the sun which gives us our measure of time which we call a year. It takes 365 days and a fraction to complete this revolution, so we consider 365 days to be a year, and add a day practically every fourth year to make up the fraction.
In the journey around the sun the earth does not move in a circle but in an ellipse, which is a figure something like acircle but having one of its diameters longer than theother. This figure can be drawn by sticking two pins into a piece of paper, a little distance apart, and tying to each pin one end of a string, thelength of which is several times the distance between the pins. Then puta pencil into the loop of the string
Fig. 8.
and draw the curve which will be formed on either side of the pins by the pencil being moved over the paper in the extended loop.
