Armed
with this apparatus, scientific travellers and navigators have
visited various points of the earth in order to determine the dip.
It is thus found that there is a belt passing around the earth
near the equator, but sometimes deviating several degrees from it,
in which there is no dip; that is to say, the lines of magnetic
force are horizontal. Taking any point on this belt and going
north, it will be found that the north pole of the magnet
gradually tends downward, the dip constantly increasing as we go
farther north. In the southern part of the United States the dip
is about 60 degrees, and the direction of the needle is nearly
perpendicular to the earth's axis. In the northern part of the
country, including the region of the Great Lakes, the dip
increases to 75 degrees. Noticing that a dip of 90 degrees would
mean that the north end of the magnet points straight downward, it
follows that it would be more nearly correct to say that,
throughout the United States, the magnetic needle points up and
down than that it points north and south.
Going yet farther north, we find the dip still increasing, until
at a certain point in the arctic regions the north pole of the
needle points downward.
Pages:
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207