THE INTERIOR OF THE EARTH 21 



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 tempera- 

 ture, it must be that far down within the earth the tem- 

 perature 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 this is 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. Revolve 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 il- 

 luminated all the time? Does any place on the illuminated part re- 

 ceive light for a longer time during a revolution than any other place ? 

 Remove the globe to the opposite side of the light without changing 

 the direction of its axis. When revolved, is there any change in the 

 globe's illumination? If so, what? 



(6) Now make the axis on which the globe revolves parallel to 

 the line joining the centers of the globe and light. Revolve 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 revolution than any 

 other place? Remove the globe to the opposite side of the light 

 without changing the direction of its axis. When revolved 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. Revolve the globe. How 

 much of the globe is illuminated? Is the same part of the globe il- 

 luminated all the time ? Do any places in the illuminated part re- 



