284 OBSERVATIONS ON THE MAGNETICAL DIP, 
the distance increase. This simple law, it is commonly known, lies at the foundation of 
the whole of physical astronomy, and is that law which renders it a perfect science. 
The objects of the “ magnetic crusade,” as it has been called, have been: 
1. ‘To examine, as far as possible, all of the circumstances and conditions of this peculiar 
force, called magnetism, in all parts of the globe; and, 
2. To endeavour to deduce, from the results, some law or laws which shall, like the 
above laws of gravitation, embrace all the phenomena, and, like that law, enable us 
to predict, not only the present condition of that force at any particular place, but its 
condition at any future time, and all of the intermediate revolutions which it may have 
undergone. 
Magnetism, as a force, is identical with any other force, but it differs from gravitation 
in regarding, chiefly, one kind of matter only, and that matter in a peculiar condition, 
namely, ferruginous matter, possessing polarity. 
The earth, all over its surface, and to the greatest heights and depths accessible to 
man, has the property of attracting a magnetized body, and of directing its polar axis in 
a particular course, and this property is called terrestrial magnetism. ‘The elements of 
terrestrial magnetism, considered as a simple force, are only two; namely, the quantity 
or intensity of that force, and the direction in which it acts. But, in reference to other 
fixed lines and planes of position, we enumerate four elements of terrestrial magnetism. 
The planes to which these elements are referred are astronomical ones, the horizon and 
the meridian, and the line which they form by their intersection, the meridian line of 
any place. These four elements of magnetism are, 
1. The variation or declination of the compass. 
2. The horizontal intensity of force. 
3. The dip or inclination. 
4, The total intensity of force. 
Of the Declination.—It is well known that the needle of the common compass does not, 
in all places, point to the true north, but makes an angle with the meridian greater or 
less, in one region, declining to the east, and, in another, to the west. This angle which 
the needle makes with the meridian line, is called the declination. A line on the chart 
accompanying this paper, called the “line of no variation,” is traced from the lower end 
of Lake Superior, along the western shore of Lake Huron, across Lake Erie, and thence 
to the coast of North Carolina, passing through the places at which the needle will coin- 
cide with the meridian, and point to the true pole. In the parts of the United States to 
the eastward of this line, the declination or variation is westward, and, in parts to the 
westward of said line, it is eastward. 
Of the Horizontal Intensity of Force.—The common compass needle is moved to the 
position which it finally occupies, by a certain force not in all places the same; which 
force, if it be drawn out of the line which it inclines to occupy, will bring it back again 
with a velocity proportionate to that force. This is called the horizontal intensity of 
force. It may be measured and compared by means of the number of vibrations which 
the same needle, deflected a few degrees from its course, will perform in a given time. 
