It must be observed, in the use of this table, that though I name particular cities, for the sake of exciting attention, yet the tables are adjusted only to longitude and latitude.  Thus when I predict that, at Prague, the variation will in the year 1800 be 24-1/4 W. I intend to say, that it will be such, if Prague be, as I-have placed it, after the best geographers in longitude, 14 30’.  E. latitude 50 40’. but that this is its true situation I cannot be certain.  The latitude of many places is unknown, and the longitude is known of very few; and even those who are unacquainted with science will be convinced that it is not easily to be found, when they are told how many degrees Dr. Halley, and the French mathematicians, place the cape of Good Hope distant from each other.

Those who would pursue this inquiry with philosophical nicety, must, likewise, procure better needles than those commonly in use.  The needle, which, after long experience, I recommend to mariners, must be of pure steel, the spines and the cap of one piece, the whole length three inches, each spine containing four grains and a half of steel, and the cap thirteen grains and a half.

The common needles are so ill formed, or so unskilfully suspended, that they are affected by many causes besides magnetism; and, among other inconveniencies, have given occasion to the idle dream of a horary variation.

I doubt not but particular places may produce exceptions to my system.  There may be, in many parts of the earth, bodies which obstruct or intercept the general influence of magnetism; but those interruptions do not infringe the theory.  It is allowed, that water will run down a declivity, though sometimes a strong wind may force it upwards.  It is granted, that the sun gives light at noon, though, in certain conjunctions, it may suffer an eclipse.

Those causes, whatever they are, that interrupt the course of the magnetical powers, are least likely to be found in the great ocean, when the earth, with all its minerals, is secluded from the compass by the vast body of uniform water.  So that this method of finding the longitude, with a happy contrariety to all others, is most easy and practicable at sea.

This method, therefore, I recommend to the study and prosecution of the sailor and philosopher; and the appendant specimen I exhibit to the candid examination of the maritime nations, as a specimen of a general table, showing the variation at all times and places for the whole revolution of the magnetick poles, which I have long ago begun, and, with just encouragement, should have long ago completed.

[1] An account of an attempt to ascertain the longitude at sea, by an exact theory of the variation of the magnetical needle; with a table of variations at the most remarkable cities in Europe, from the year 1660 to 1860.  By Zachariah Williams.

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