relate have been completely elucidated. There can be no doubt that the tendency of different classes of scientific truth to diffuse themselves first throughout educated society, and ultimately to some extent over the length and breadth of every civilised community, would be found to be governed by certain definite laws; and our proposed chart, if its construction were practicable, would probably show some singular relations between the character of scientific discoveries and their power of penetrating through the different strata of society. As a first guess it would be naturally surmised that those philosophical inquiries which had the most important practical bearings would have the largest measure of this penetrating power, and that while the curiosities of science remained a sealed book to all but a few, the processes which directly influence every-day existence would be so far familiar as almost to form an essential element of a liberal education. As a matter of fact, it would be easy to multiply examples of exactly the opposite tendency. For one person who could give a clear account of the action of a common clock, or the process by which electricity is made to convey messages across the globe, there are scores who could explain the principle of that ingenious plaything the stereoscope, and hundreds who could pronounce with confidence, if not with accuracy, on the precise antiquity of a flint hatchet or a Celtic arrow-head.

There are some subjects, indeed, which seem to be doomed to general neglect, almost on account of their peculiar claims to universal attention. A seafaring people might be supposed, as a matter of course, to take a deeper interest in the theory of the mariner's compass than in almost any other branch of science, and yet it is scarcely too much to say that the investigations which have completely revolutionised this department of magnetical science have awakened no interest in the general public, and have scarcely been mastered by more than a few of those whose lives are hourly risked upon the supposed accuracy of compass indications. As a rule, most people who are not sailors believe in the compass imagined by poets as a marvellous and unerring guide, but for which the most adventurous navigator would scarcely trust himself out of sight of land. Among seamen a very different estimate prevails. Although there are many who in spite of the proved errors of the compass in all iron and in most wooden vessels persist in blindly pinning their faith upon a guide which without extreme care is as likely to lead them to destruction as to safety, a still larger proportion of the masters of merchant vessels have learnt to distrust and disregard the indication of the needle altogether, and to rely exclusively on the famous three L's-'lead, latitude, and look-out,' to keep them clear of rocks and shoals. A few, and only a few, outside of the

British

British Navy have accustomed themselves to extract from the seemingly capricious readings of the compass-card the information which is veiled and distorted by a host of disturbing influences.

One reason why the practical application of the laws which govern the action of ships' compasses is so imperfectly understood may be gathered from a cursory glance at the publications to which we have referred at the head of this article. They include works of every degree of merit, from the most elaborate investigations of our foremost men of science down to the crude absurdities of would-be discoverers, who have thought to repeal the laws of nature by the force of patents and specifications. But we do not find a single attempt since 1824 at a popular explanation of a branch of science which, though somewhat complicated, is quite capable of being made intelligible to any man of ordinary capacity. Such a work is much needed. The Admiralty Manual of 1863 is almost perfect in its way, but it contains little more than dry facts and rigid rules for the guidance of practical navigators, followed by an extremely elegant investigation of the scientific theory, addressed exclusively to mathematicians. There is nothing in it for the general reader, nor, indeed, could there be without a departure from the immediate purposes of the publication. But a popular treatise is not the less a desideratum; and until the gap is filled up, it matters not whether by private hand or in an official publication, there is no prospect of seeing the principles of this branch of science as generally understood as a matter of so much practical importance ought to be.

The history of the mariner's compass is singular enough. That it was used in the East long before it was known to European navigators seems to be a well-established fact, and whether the Chinese discovery was or was not as early as the date assigned to it in their annals (about 2600 B.C.), there is no doubt that the properties of the magnetised needle were utilised in that strange country at a very early period. At the present day the evidence of an independent origin may be seen in the peculiar form of the Chinese compass, which is a needle without the compasscard that we employ in Europe, and is read from the South instead of the North Pole, as with us. The introduction of the compass into Europe has been traced to an earlier date than was formerly ascribed to it, but we do not seem to have possessed for more than five or six centuries the knowledge with which the Chinese have been familiar for perhaps 4000 years. Even after the European rediscovery of the polarity of the magnetic needle little was done to perfect the theory of the compass until the commencement of the present century.

The variation of the compass from the true north differing in different

different parts of the earth's surface was much too considerable and too remarkable to pass unobserved, and the fact that the north end of a balanced needle was drawn downwards in northern, and the south end in southern latitudes, was of necessity perceived as soon as the compass came into general use. Maps of the lines of equal variation and equal dip were constructed by the illustrious Halley, but no progress was made in the theory of compass disturbances before the time of Captain Flinders. Men of the stamp of Cook, and the other great navigators of the last century, were not likely to neglect the observation of the compass, and they did in fact ascertain that after making due allowance for the variation there remained an appreciable amount of error in the indications of their compasses. It is singular that the idea of some calculable disturbing force should not have suggested itself, but they seem scarcely to have thought of accounting for the discrepancies they noticed except by the assumption that they were wholly due to the imperfect manufacture of the instruments they employed. Under this impression it was natural enough that the compass should fall into disrepute, and that the most scientific sailors should come to rest their sole confidence on astronomical observations and to abandon almost entirely the trust which had once been placed in the magnetic needle.

With Flinders a new era in the history of the compass commenced. In his famous voyage to Australia, in 1801, he observed, as others had observed before him, that the direction of the compass-needle frequently wandered from that which the known variation due to the geographical position of the ship assigned to it. Not content with barely noting the fact of an apparently capricious deviation, or ascribing it to unexplained errors of construction, Captain Flinders set to work to seek for the cause of the phenomenon, and it was not long before he found that the error was most considerable when his ship's head pointed about east or west, that it disappeared when the line of the keel was made to coincide with the actual direction of the needle, and that the whole phenomenon amounted merely to this, that the north end of the compass-needle was drawn by some unknown force towards the ship's bows. The most elementary of the known facts of magnetism sufficed to suggest an explanation. Knowing that when two magnets are brought within range of each other's influence the opposite poles attract and the similar poles repel each other, Captain Flinders concluded that the mysterious force which drew the north pole of his compass towards the bows could only be the south pole of a magnet lying somewhere between the binnacle and the ship's head. This was the result of observations made in northern latitudes. But when the ship was taken as far south as Bass's Straits, the phenomena

were

were entirely reversed. Now it was the south end of the needle which was drawn towards the bows, and the hypothesis that the mass of the ship within reach of the compass acted like the south end of a magnet had to be replaced by assuming the vessel to be endowed with exactly the opposite quality of magnetism. The problem, therefore, resolved itself into this simple question: Was there any known cause by which a portion of the ship lying (as nearly the whole ship did) before the compass could be converted when in the northern hemisphere into the south pole, and when in the southern hemisphere into the north pole of a magnet? To ask the question was in fact to answer it. It was well known, long before Captain Flinders's time, that if a bar of soft iron were placed near to the north pole of a magnet it would instantly be magnetised itself, the end of the bar nearest to the north pole of the magnet becoming a south pole and the opposite end a north pole. This magnetism of soft iron by induction, as it is termed, is, however, only transient. It ceases when the dominant magnet is removed, and is restored when the magnet is brought near again. If the magnet is turned into the opposite direction, so as to bring its south pole where the north pole was before, the induced magnetism of the iron bar is reversed also, and what was previously a south pole now becomes a north. It occurred to Captain Flinders that this was precisely what happened to his ship when she passed from northern to southern latitudes. The earth was the dominant magnet, and the magnetism which she induced in the iron stanchions and bolts in the ship's hull was the influence which disturbed the needle.

It was obvious that in northern latitudes an upright iron bar would have its lower end nearer to the Arctic regions which contain the earth's south pole,* than its upper end. The lower end would therefore, become by magnetic induction an opposite, or north pole, and the upper end a similar, or south pole. Every piece of iron in the ship would thus have a tendency to south polarity at its upper end; and, as the deck compass would be nearer to the upper than the lower part of the ship's hull, the effect would be a resultant attraction throughout the ship upon the north pole of the compass needle. As the compass was placed near the stern, and as any iron in the ship might be assumed to be symmetrically placed, the broad result of this attraction would be a tendency in all positions of the ship's head to draw the north end of the needle towards the bows-the very phenomenon which was actually observed. So, again, in southern latitudes, where the influence of the earth's north

That is, the pole similar to the south pole, and capable of attracting the north pole of an ordinary magnet. This is always spoken of as the earth's south pole, although north in geographical position.

magnetism

magnetism predominated, the upper portion of the iron in the hull would acquire by induction northern polarity, and would tend to attract the south pole of the compass towards the bows; and this, again, was in exact accordance with observation. It followed, moreover, from the hypothesis, that if the ship's head were so pointed as to bring the needle fore and aft, the attraction, being also fore and aft, would act in the line of the needle, and leave its direction unchanged, though it would increase or decrease the intensity of the force by which it was drawn into its position of rest-the horizontal directive force as it is technically termed. Accordingly it was observed that in any such position the compass was true to the direction corresponding to the known variation, and that the greatest error occurred when the needle was at right angles to the ship's keel, that is, when the vessel was sailing to the magnetic east or west.

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This explanation, offered by Captain Flinders, has been the basis of all subsequent investigations; and, though another independent cause of disturbance has since been discovered to be extremely powerful in iron ships, the theory of Captain Flinders accounts for almost the whole of the deviation which is met with in wooden ships, where there is little magnetic material besides upright stanchions, generally of wrought-iron. From the detection of this defect to its remedy was an easy step. It occurred at once to Captain Flinders that the error might be entirely corrected if the compass were placed in such a neutral position as to have the iron in the ship equally distributed before and abaft the binnacle; and, as this might be an inconvenient position for the steering compass, he suggested the introduction of upright iron stanchions a little abaft the compass, for the express purpose of compensating the ship's attraction. Various modes of compensation have since been devised, but, so far as the particular source of error detected by Flinders is concerned, none of them are better than the upright iron bars which he proposed for the purpose.

It was long before any further steps were taken to examine more closely the facts of compass deviation. It happened, however, that among the captains engaged in whaling expeditions was one who brought to the task an unusual amount of scientific information and acuteness. Captain Scoresby, or to use the title by which he afterwards became better known in the world of science-Dr. Scoresby had made the phenomena of magnetism his special study; and his voyages to the neighbourhood of the earth's magnetic pole afforded him peculiar opportunities for the investigation. The same object was still more carefully pursued in the exploring voyages of Sir John Ross and Sir Edward Parry, by Major-General (then Captain) Sabine, who was deputed to accompany these expeditions as astronomer.

Vol. 118.-No. 236.

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