254 
MAGNETIC SURVEY IN NORTH AMERICA. 
cannot be understood unless the magnetic state of both hemispheres is taken into the 
account. 
The coincidence of two or more points of maxima, (whether in the same or in 
different hemispheres,) under one and the same geographical meridian, may constitute 
magnetic epochs, which in the future history of the science may create an interest 
which can be very little conceived at present. The first conjunction of this kind, 
which our present purely empirical knowledge permits us to anticipate, is that of the 
two minor maxima, which, if the same progress of translation should continue that 
appears to have taken place in the last two and a half centuries, will hereafter be 
found on the same geographical meridian, and on the same side of the globe. Ac- 
companying the movement of the two minor maxima of Force, the remarkable closed 
systems of the Declination lines, which are now found respectively in Siberia and in 
the Southern Pacific, by a movement of translation corresponding to that which they 
have undergone in the last two centuries, will also be found hereafter in the same 
geographical meridian, and will then doubtless have experienced a considerable 
modification of their form. 
If we connect the two points of 90° of Inclination, the one in the northern and the 
other in the southern hemisphere, by an arc of a great circle crossing the terrestrial 
equator in the Atlantic Ocean, and if we examine the Inclination and Force along this 
arc, we shall find that the portions in which the Force decreases whilst the Inclination 
increases, amount to nearly a third of the whole distance between the points of 90° 
thus measured along the surface of the globe. Not only therefore is the once-sup- 
posed law, according to which the magnetic Force should everywhere increase in a 
certain expressed ratio with the increase of the magnetic latitude, inapplicable to the 
phenomena, but the modification which has latterly been substituted, — “ the law," as 
it has been lately expressed, “of the general increase of the magnetic Force with the 
magnetic latitude,” — seems scarcely justified by the facts; and it may be doubted 
whether this expression is not more likely to mislead, by perpetuating the erroneous 
hypothesis in which it first took its rise, than to be of advantage as an empirical law, 
where the exceptions are so considerable. It is obvious that a too hasty generaliza- 
tion from observations made in those regions of the globe where the Force decreases 
with the increase of the dip, (as for example in the twenty degrees of latitude, or there- 
abouts, in North America, comprised between the point of maximum of the Force, and 
the point of 90° of Inclination,) might have appeared to justify an inference which would 
have been the direct contrary of the above-mentioned law. There is in fact no such con- 
nection between the Inclination and the Force as will justify the one being spoken of 
as an immediate function of the other, or will sanction a general statement, that the 
increase of the one is to be looked for from the increase of the other, or vice versd. 
I have generally preferred, in these Contributions, the employment of the expres- 
