16.Determination of Longitude. -Since the earth turns on itsaxis once in 24 hours, the interval between the passage of successive meridians under the sun will be four minutes (24 × 60 ÷ 360 = 4). At a point one degree east of us the noon by local time is four minutes ear- lier than it is with us, and at one degree west it is four minutes later. If an accurate clock were set to twelve o"clock when the sun was nearest vertical at a certain place, and were then carried to a place 15° west, it would indicate one o"clock at the more western locality when it was high noon at this place. Or, changing the statement, if the clock indi- cated one o"clock when the sun reached the highest point, the place must be 15° west of the place from which the clock started.
Thus we see that by means of an accurate timekeeper we can tell difference of longitude between different places. Every sea captain is provided with one or more accurate clocks called chronometers, whichare usually set to the time at Greenwich. Thus all he needs to do to get the difference in degrees of longitude between his 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 property of theearth which has been of the greatest assistance to geographical explor- ers 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 attract- ing iron. Such pieces of ore are called loadstones. 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 from a true north and south line is called the declination. The westward declination was one of the great discoveries of Columbus. We know now that the reason for the declination of the needle is that the north end of it does not point toward the north geographical pole as was at first supposed but toward a point in the southwestern part of Boothia Felix which is called theLINES OF EQUAL MAGNETIC DECLINATION IN THE UNITED STATES.
north magnetic pole. The south magnetic pole as recently determined is a little to the east of Victoria Land.
These magnetic poles do not remain in the same place all of the time but swing slowly back and forth, so that the declination changes for the same place. On account of this it is necessary for surveyors, who use the compass, to find out the declination each year. The annual change in the United States varies from 0 to 5 seconds. In 1910 the declination at Eastport, Maine, was 19.4° west and for Seattle, Washington, 23.5° east. For intervening places there were intervening values. Maps are now made with lines upon them connecting places of equal declination. These lines are called isogonic lines.
18.Magnetism.
Experiment 11. -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 of water. Does the needle assume any definite direction? Taking the needle from the water stroke one end of theREGION AROUND THE MAGNETIC POLE.
needle from the cork out with the north end of a magnet and the opposite end with the south end of a magnet. When the needle is again floated on the water is it indifferent about the direction in which it points? What has caused the change, if there is any?
Experiment 12. -Suspend by a string a short bar magnet in a sling made from a bent piece of wire. Turn it around in several different directions. After each change allow it to come to rest in whatever position it will. Does it prefer any one position to all others?
Experiment 13. -Suspend a bar magnet horizontally in a sling and bring one of the ends of another bar magnet towardit. What is the effect? Reverse the ends of the magnet; is thereany change in the position of the suspended magnet? Bring a large soft iron nail toward either end of the suspended magnet. What is the effect? Reverse the ends of the nail. (Be careful that the nail has not become permanently affected by the magnet.) Is the effect the same as when the ends of the magnet were reversed?
Fig. 13.
Bring pieces of copper, zinc, and other substances toward the magnet. Do these affect it? Notice that the ends of the bar magnet are marked. What can you state about the attraction or repulsion of similar ends of magnets? Of opposite ends? Does it make any difference in its effect on the suspended magnet toward which end the nail is brought? What substances do you find attracted by the magnet?
To the end of a small nail hanging by attraction to a magnet bring another nail. How does the first nail act in respect to the second?