SUPPLEMENT. 
19 
The line of no declination, which, in this ease, represents the 
equator, would also be proportionably nearer to the poles which 
are nearest one another. 
If the magnetic poles were situated in opposite meridians, 
and opposite parallels upon those meridians, which pass through 
the magnetic and true poles, there could be no declination, for 
the reasons mentioned in the former cases; but, upon the 
equator eastward and westward to the distance of 90° in longi¬ 
tude, the declination would actually increase, so as there to be 
equal to the angle, which measures the distance between the 
true and magnetic poles, and would thence in the same manner 
decrease for the other 90° to the opposite meridian. 
The Halleyan lines of 10° 20°, &c., as far as the greatest de¬ 
clination upon the equator, in this case become arcs or curves, 
which conform themselves as nearly as may be to the course and 
direclion of the lines of no declination, and are called by mag- 
netists, the lines of the first order, but the lines of the greatest 
equatorial declination cross one another at the distance of 90° in 
longitude, from the meridian or circle of no declination, some¬ 
thing in form like the letter X,or like two Gothic arches joined 
at the vertex. They are termed lines of the second order, and may 
very properly be considered as the boundary between the lines 
of the first and third order; as the lines of no declination are 
always boundaries between the lines of the east and west de¬ 
clination. In this case, these lines of no declination, including 
the arcs of 189°, form only one great circle along the meridian^ 
dividing the surface of the globe into two hemispheres, in 
one of which there is east declination, and in the other west 
declination. 
From the greatest equatorial declination to the arcs of 180°, 
the Halleyan lines of the third order are curves returning into 
themselves, and in shape nearly resembling parabolas erected 
upon the arcs of 180°. As a variety of these cases also, we have 
only to add, that if the magnetic poles w ere situated in opposite 
meridians, but in parallels which are not opposite, then in that 
hemisphere, in which the true and magnetic poles approach nearest 
to one another, the figures formed by the Halleyan lines would 
