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MARYLAND GEOLOGICAL SURVEY 459
How the daily range varies with magnetic dip is shown by the
next table. The best indication of approach to the magnetic pole is
given by the increase in the value of the magnetic dip or inclination,
and hence the column of dip values has been added. For Fort Conger,
where the dip is 85°, the diurnal range amounts to 1° 39'.
That the diurnal range increases as we proceed towards the mag-
netic pole is due to the fact that the magnetic force which acts on the
compass needle diminishes with approach to the pole, and hence the
deflecting forces which cause the diurnal variation have a more pow-
erful effect near the pole than farther away.
TABLE IV.
Showing the variation in the diurnal range of the declination with approach to magnetic pole.
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Station.
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Lat.
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Long.
W. of Gr.
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Magnetic
Dip.
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Range.
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Years of
observations.
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Key West................
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24° 33'
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81° 48'
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54° 32'
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4'. 7
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1860. 2-'66. 2
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Los Angeles..............
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34 03
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118 15
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59 30
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5. 8
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1882. 8-'89. 8
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Washington..............
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38 54-
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77 01
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71 19
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7. 5
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1840. 5-'42. 5
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Philadelphia.............
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39 58
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75 10
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71 58
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7. 8
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1840. 5-' 45. 5
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Madison.................
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43 04
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89 24
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73 56
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6. 7
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1877. 2-'78. 2
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Toronto.................
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43 39
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79 24
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75 15
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8. 8
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1842. 5-'48. 5
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Sitka.....................
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57 03
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135 20
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75 55
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10. 6
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1848-' 62
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Uglaamie Point..........
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71 18
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156 40
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81 24
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40. 1
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1882. 7-'83. 6
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Plover Point.............
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71 21
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156 16
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81 36
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38. 6
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1852-' 54
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Fort Rae................
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62 39
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115 14
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82 54
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41. 4
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1882. 8-'83. 7
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Kingua Fiord............
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66 36
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67 19
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83 51
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43. 7
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1882. 8-'83. 7
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Fort Conger..............
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81 44
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64 44
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85 01 1
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98. 8
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1881. 7-'82. (i
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The times when the declination reaches its extreme values, or when
it reaches its average value, as will be evident from Table I, are sub-
ject to fluctuations in the course of the year, being generally retarded
about a half-hour or more during the months when the sun is south of
the equator. These changes, which undergo a complete cycle in the
course of one year, likewise manifest themselves in the magnitude
of the diurnal range as already pointed out.
The approximate local mean time when the mean declination is
reached in the morning is, on the average for the year, as follows:
H. M.
At Toronto.................. 10 17 A. M.
Philadelphia............... 10 20
Washington............... 10 25
Key West................ 10 51
Madison.................. 10 43
Los Angeles............... 10 35
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