Page:A Voyage to Terra Australis Volume 2.djvu/542

528 needle, and the same with the upper parts of magnetic lands in the southern hemisphere. But it is an universal law in magnetics, that powers of the same quality repulse, and dissimilar powers attract each other; therefore the upper parts, both of the iron in the ship and of the land, should, like the north end of the dipping needle, repulse the north and attract the south end of the compass needle. Now the compass in, or upon the binnacle of a ship is raised above the greater part of the iron, and therefore more in a situation to be attracted by the upper, than the lower parts of the different pieces. The same will generally be the case with respect to the land; its southern polarity must often be lower than the depths of the sea, whilst the upper part, which attracts the south end of the compass needle, will be nearly on a level with, sometimes a few degrees above the ship.

This reasoning from abstract principles is consistent with my observations on and near the coasts of Terra Australis; and if it be just, the contrary effects must take place in the northern hemisphere, at least in high latitudes: the upper parts, both of the iron in a ship and of land possessing magnetism, will attract the north end of the compass needle. That it is the north point of a compass on the binnacle which is attracted by the iron of a ship in the northern hemisphere, has already been shown; but whether the land do generally attract the same point, I have no knowledge from experience: answers to the following queries would probably be useful in the determination.

Is the west variation on the coast of Holland and Germany considerably less than on the east coast of England and Scotland, in the same latitude?

Is it sensibly less at Holy Head than at Dublin; at Port Patrick than at Caricfergus?

Is the variation as much, or greater on the Yorkshire, than on the Lancashire coast?

And generally in the northern hemisphere, is the west variation greater, or east variation less on the east sides of islands and projecting points than on the west sides?

Observations made on ship-board for determining this or any other general question of magnetism, will require, when the head is not at North