AND THE INTENSITY OF MAGNETICAL FORCE. 285 
Of the Dip or Inclination.—The ordinary compass needle is so pivoted, above the 
centre of gravity, that it is compelled to move in the horizontal plane; but if it could 
move freely in a vertical plane, as a cannon can be elevated or depressed on its trunnions, 
it would not, in our latitude, remain horizontal, but the north end would descend, 
making an angle of about seventy degrees with the horizon. This angle, which a 
free needle would make with the horizon, is called the Dir or Inctination. At some point 
near the equator, the dip is nothing, and beyond that, to the south, the south end of the 
needle dips or descends below the level. 
Of the Total Intensity of Force.—When a free needle has taken its position in the true 
line of the dip, it is held in that position by the whole force of the earth’s magnetism; 
and this force, which may be determined by means of the dip and the horizontal force, 
is called the ‘Toran Inrensiry or Force. 
Observations.—N ow all these four quantities are found to vary, in passing, in certain 
directions, from one place to another, but there is usually some line along which each 
quantity is nearly the same. 'The lines, along which the variation is the same, are called 
lines of equal variation, as that from Lake Superior to North Carolina, along which the 
variation is nothing. ‘Those lines along which the dip is equal, are called isoclinal lines, 
or lines of equal dip; as that on the chart, traced from the north-west corner of Illinois, 
through Indiana, the middle of Ohio, along the south part of Pennsylvania, through 
Philadelphia, to the Atlantic, marking the dip of 72°. Those lines, along which the 
total intensity is equal, are called isodynamic lines, or lines of equal force, as the several 
elliptical curves marked around Lake Superior, more or less parallel to each other. It 
will be seen that the development of these last lines has been the chief object of this 
paper. 
A map of a country, with a delineation of these several lines, constitutes a magnetic 
chart of that country; a thing which cannot be made without much expense and labour, 
in the necessary experiments and calculations. 
None of these quantities remain constant at the same place, but are undergoing a slow 
revolution; or, in other words, the several lines are gradually moving. Thus, the line of 
no variation, (and, with it, all lines of equal variation,) is gradually travelling west- 
ward. ‘The city of Pittsburgh, Pennsylvania, which, a few years ago, had no variation, 
the line passing through it, has now a variation to the westward, the line of no variation 
being now found to the west of that city. The lines of equal dip have been said to be 
travelling slowly to the north, and the dip at any given locality to be, consequently, 
diminishing. Continued observations will determine this and other similar points. 
Some Account of the Instruments, and of the Mode of Odservation. 
The following observations were made with instruments manufactured by the late 
Mr. Robinson of London. The dipping compass was furnished with two needles, each 
of six inches in length, and with the means of reversing their polarity. The results here 
tabulated, are, in each instance, the mean of sixteen readings, including all the usual 
