MAGNETISM OF THE EARTH 37 



extends through the Pacific Ocean, is the accepted line 

 which separates one day from the next. Thus any one 

 traveling around the earth must drop a day from his 

 calendar if crossing this line toward the west, and repeat a 

 calendar day if crossing the line toward the east. 



In practice, the International Date Line, where this 

 arbitrary change of day occurs, does not quite coincide with 

 the 180th meridian. A glance at the accompanying map 

 will show why it is convenient to vary the Date Line from 

 the meridian line. 



Daylight Saving. In midsummer the sun rises between 

 4 and 5 o'clock in middle latitudes. Thus it is well up in 

 the heavens before the average citizen is astir. On the first 

 of April, 1918, the United States Government decided to 

 set the clock ahead one hour. This gave more daylight in 

 the ordinary waking hours, and thus effected a saving in 

 the cost of lighting. On the 27th of October, when the 

 long days were past, the clock was set back one hour, and 

 normal time was resumed. Many countries did this during 

 the War. 



Magnetism of the Earth. There is a peculiar prop- 

 erty of the earth which has been of the greatest assistance 

 to geographical explorers and without which it would be 

 very difficult to find a way over the sea. This property 

 is called terrestrial magnetism. In very ancient times 

 pieces of iron ore were found which had the property of 

 attracting iron. Such pieces of ore are called loadstones. 

 Artificial loadstones are called magnets. 



Experiment 8. Having pushed a long cambric needle through 

 a small disk of cork so that it will float horizontally, carefully 

 place the disk and needle upon the quiet surface of a large dish 



