34 FIRST TEAR SCIENCE 



position and Greenwich is to determine when the sun has 

 reached its highest point and to multiply by fifteen the 

 difference in hours between the time as shown by the 

 chronometer and twelve o'clock. If the chronometer is 

 too slow he is east, and if too fast, west, of Greenwich. 



The determination of latitude is more difficult, but can 

 be easily done by one knowing how and having the proper 

 instrument. The manner of its determination is described 

 in the appendix. 



17. 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 load- 

 stones. Artificial loadstones are called magnets. If a bar 

 of loadstone or a magnetic needle is floated in a basin of 

 water, or if freely suspended, it will invariably assume a 

 definite position. 



This discovery was made in the far east at a very early 

 date, but it was put to no particular use in the sailing of 

 ships until about the middle of the thirteenth century. 

 Since then it has enabled sailors to go far out from the 

 sight of land and yet always to know the direction in which 

 they are going. It was supposed even up to the time of 

 the first voyage of Columbus that the magnetic needle 

 always pointed toward the north star or perhaps at some 

 places a little to the east of it, and the sailors of Columbus 

 were greatly alarmed when they found as they sailed west 

 that the needle swung off to the west of the true north. 



This difference in the direction of the needle f rom-a true 

 north and south line is called the declination. The west- 

 ward declination was one of the great discoveries of 



