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ferred into my ordinary practice, that I cannot remember when I was without them.

When Sir Isaac Newton had declined the office assigned him, it was given to Mr. Molineux, one of the commissioners of the Admiralty, who engaged in it with no great inclination to favour me; but however thought one of the instruments, which, to confirm my own opinion, and to confute Mr. Whiston's, I had exhibited to the Admiralty, so curious or useful, what he surreptitiously copied it on paper, and clandestinely endeavoured to have it imitated by a workman for his own use.

This treatment naturally produced remonstrances and altercations, which indeed did not continue long, for Mr. Molineux died soon afterwards; and my proposals were for a time forgotten.

I will not however accuse him of designing to condemn me, without a trial; for he demanded a portion of my tables to be tried in a voyage to America, which I then thought I had reason to refuse him, not yet knowing how difficult it was to obtain, on any terms, an actual examination.

About this time the theory of Dr. Halley was the chief subject of mathematical conversation; and though I could not but consider him as too much a rival to be appealed to as a judge, yet his reputation determined me to solicit his acquaintance and hazard his opinion. I was introduced to him by Mr. Lowthorp and Dr. Desaguliers, and put my tables into his hands ; which, after having had them about twenty days under consideration, he returned in the presence of the learned Mr. Machin, and many other skilful men, with an entreaty that I would publish

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them speedily; for I should do infinite service to mankind.

It is one of the melancholy pleasures of an old man to recollect the kindness of friends, whose kindness he shall experience no more. I have now none left to favour my studies; and therefore naturally turn my thoughts on those by whom I was favoured in better days: and I hope the vanity of age may be forgiven, when I declare that I can boast among my friends, almost every name of my time that is now remembered : and that in that great period of mathematical competition scarce any man failed to appear as my defender, who did not appear as my antagonist.

By these friends I was encouraged to exhibit to the Royal Society, an ocular proof of the reasonableness of my theory by a sphere of iron, on which a small compass moved in various directions, exhibited no imperfect system of magnetical attraction. The experiment was shown by Mr. Hawkesbee, and the explanation, with which it was accompanied, was read by Dr. Mortimer. I received the thanks of the society: and was solicited to reposit my theory properly sealed and attested among their archives, for the information of posterity. I am informed, that this whole transaction is recorded in their minutes.

After this I withdrew from public notice, and applied myself wholly to the continuation of my experiments, the confirmation of my system, and the completion of my tables, with no other companion than Mr. Gray, who shared all my studies and amusements, and used to repay my communications of magnetism, with his discoveries in electricity. Thus I proceeded with incessant diligence; and perhaps in the zeal of enquiry did not sufficiently reflect on the silent encroachments of time, or remember, that no man is in more danger of doing little, than he who flatters himself with abilities to do all. When I was forced out of my retirement, I came loaded with the infirmities of age, to struggle with the difficulties of a narrow fortune, cut off by the blindness of my daughter from the only assistance which I ever had; deprived by time of my patron and friends, a kind of stranger in a new world, where curiosity is now diverted to other objects, and where, having no means of ingratiating my labours, I stand the single votary of an obsolete science, the scoff of puny pupils of puny philosophers.

In this state of dereliction and depression, I have bequeathed to posterity the following table: which, if time shall verify my conjectures, will shew that the variation was once known; and that mankind had once within their reach an easy method of discovering the longitude.

I will not however engage to maintain, that all my numbers are theoretically and minutely exact: I have not endeavoured at such degrees of accuracy as only distract enquiry without benefiting practice. The quantity of ihe variation has been settled partly by instruments, and partly by computation : instruments must always partake of the imperfection of the eyes and hands of those that make, and of those that use them: and computation, till it has been rectified by experiment, is always in danger of some omission in the premises, or some error in the deduction.

It must be observed, in the use of this table, that

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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 244 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 enquiry 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 inconveniences bave 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, shewing the variation at all times and places for the whole revolution of the magnetic poles, which I have long ago begun, and, with just encouragement, should have long ago compleated.

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