THE DEVELOPMENT AND PURPOSES OF MAGNETIC SURVEYS.

THE MAGNETIC DECLINATION.

"True as the needle to the pole" is an old and familiar saying. How untrue it is even within the borders of our own country may be judged from the fact that in the extreme northeastern part of Maine the compass needle points twenty-one degrees to the west of north, while in the extreme northwestern part of the state of Washington it points twenty-three degrees to the east of north; hence a change of forty-four degrees from one end of our country to the other! And even over so comparatively small a territory as that of Maryland the pointing of the needle varies from six degrees west to three and onehalf degrees west.

There are portions of the earth's surface where the needle points due east and west and still others where the north end actually points south. We are thus made acquainted with one element involved in a magnetic survey, viz., the magnetic declination, or "variation," as the mariner and the surveyor are accustomed to say. Scientifically defined, the magnetic declination is the angle between the true north and south line and the magnetic north and south line as pointed out by a compass needle, i. e., a magnetized needle so mounted as to swing freely about a vertical axis or pivot.

It was several centuries after the introduction of the compass in Europe before this deviation of the magnetic meridian from the true meridian was discovered, and the discoverer was no less a man than Christopher Columbus. We all doubtless remember reading of the consternation caused on board ship when it was observed that the compass had shifted its direction from east of north to west of north. Columbus had, in fact, crossed the line, on September 13th, 1492, along which the needle pointed true north, i. e., the line of no magnetic declination or variation, or the so-called agonic line. To the east of this line the needle pointed east, and to the west the needle bore west.

This line lay a little to the west of Fayal Island of the Azores. It will be remembered that this line figured quite prominently for many years in political geography as the line of demarcation between the kingdoms of Portugal and Castile.

It is only lately that Columbus has been properly credited with this discovery. Generally he is given credit merely for the discovery of the line of no declination. This is due to an error made by an Italian, Formaleoni, who declared that the compass charts in Bianco's famous atlas of 1436 contained on them values of the magnetic declination. Humboldt, relying on Formaleoni, repeated the error in his " Cosmos." Later researches have shown beyond a doubt that Formaleoni was wrong, having misinterpreted a diagram on one of the charts. So likewise has it been conclusively proven that the value of 5° E. for the magnetic declination at Rome in 1269, which had been ascribed to one Petrus Peregrinus, had been inserted in the Leyden manuscript in the early part of the sixteenth century.

An examination of the early compass charts made by the writer would indicate that during Columbus' time and a century or two before, the needle pointed approximately to the true north or by a small amount east over the entire Mediterranean. For this reason, probably, the magnetic declination was not discovered for so long a time after the European mariner had begun to rely on the bit of magnetized steel to guide the wanderings of his ship. Frequently statements are seen, with deductions based on them, that the needle pointed by a large amount (15°-20° and more) west in the Mediterranean in about the 14th or 15th century. This, however, cannot be the case.

It was not until near the middle of the sixteenth century that the fact of the "misdirection" of the needle-to translate literally the German word, missweisung, for magnetic declination-received general acceptance. It was believed, namely, that the needle's deviation from the true north was due to mechanical imperfections, and the compasses of

1 According to Mr. Schott's computations, the place on Columbus' voyage where the needle pointed true north was in north latitude 28° 21′ and in longitude 29° 16′ west of Greenwich.-Coast and Geodetic Survey Report for 1888, Appendix 7, p. 305.

this period were frequently corrected for this deviation so that they would point to the true north.

The earliest land observation which has thus far come to our notice was made at Rome by Georg Hartmann, vicar at Nuremburg, of whom we shall again have occasion to speak, in the early part of the sixteenth century, probably in the first decade. The needle bore 6° E. In Paris in 1541, according to Bellarmartus, the declination of the needle was 7° E. and, in London, in 1580, William Boroughs found 111° E.

Observations now began to multiply, and the year 1600 marks a distinct epoch in terrestrial magnetism, for in this year, nearly three centuries ago, appeared one of the most remarkable books ever written. It was epoch-making not alone for its contents and the marvellous conclusions reached, but also for the truly philosophical art of reasoning employed. Not a conclusion was reached without being subjected to a rigorous experimental demonstration. In this respect it stood notably apart from similar books of its day, and in many respects it is still a standard work. The book to which I refer was none other than the great treatise on magnetism, "De Magnete," by Dr. William Gilbert of Colchester, physician-in-ordinary to Queen Elizabeth.'

The final conclusion reached by Gilbert with regard to the cause of the magnetic phenomena observed up to that time was summed up in the following sentence: "Magnus magnes ipse est globus terrestris” The terrestrial globe itself is a great magnet."

This was the first rational explanation of the action of a compass needle. Before that all sorts of fanciful theories were in vogue. Thus, for example, it was said that it was the attractive influence of the pole star which made the needle point approximately to the north. The only modification that modern science could make with regard

1 This was written in Latin, and strange to say, was translated into English only within the last few years. This translation was performed by P. Fleury Mottelay and published by Wiley and Sons, New York. It is entitled: WILLIAM GILBERT OF COLCHESTER, Physician of London, On the Loadstone and Magnetic Bodies and on The Great Magnet the Earth. A new Physiology, demonstrated with Many Arguments and Experiments.

to Gilbert's dictum would be that the earth acts like a great magnet, if it is not in itself a magnet. We do not as yet know definitely whether the earth is a magnet or an electro-magnet, i. e., whether the earth acts upon a freely suspended magnetic needle as a permanently magnetized body, with definite magnetic poles or centers of attraction, or whether it acts like a soft piece of iron rendered temporarily magnetic by a current of electricity circulating around it. To put the matter tersely, no satisfactory answer has as yet been given to the question: "Is the earth's magnetism permanent or induced?" We know that the earth possesses magnetism in some form, but the how and the whence, in spite of innumerable attempts of some of the most brilliant minds to solve the riddles, are still mysteries.

Helmholtz characterized the earth's magnetism as one of the most puzzling of natural forces.

Since it can be mathematically demonstrated that it is always possible to distribute electric currents within the earth's crust in such a way that the external magnetic effect of these currents will be precisely the same as that due to a system of permanent magnets embedded in the earth, it follows that we can equally as well satisfy the magnetic phenomena observed on the earth's surface, on either hypothesis.

The electric current theory has many points in its favor, but thus far no adequate cause has been found, that is, one that would explain not only qualitatively but also quantitatively the currents and the direction in which they would have to proceed, i. e., roughly from east to west around the earth. And Professor Schuster's question as to whether every large rotating mass is a magnet has not yet been experimentally attacked. There is a strong suspicion that the earth's rotatory motion has an important share in the production of the earth's magnetism.

Furthermore, we as yet have no knowledge whatever with regard to the actual distribution of magnetism within the earth's crust; nor shall we ever have so long as we confine our observations entirely to the surface of the earth. For an infinite number of distributions of magnetism can be found which will satisfy surface phenomena. What is needed is observations in the region above us-in balloons

Let us hope

and in the depths below us at the bottom of the sea. that the means will soon be forthcoming to permit the making of such observations, so that continued progress, another step forward, may be made in the subject of the earth's magnetism!

THE SECULAR VARIATION OF THE MAGNETIC DECLINATION.

The year 1634 witnessed the discovery of the next remarkable fact with regard to the magnetic declination, the so-called secular variation, whereby the earth's magnetism suffers most remarkable changes in the course of time. It was an accepted fact by this time that the needle's deviation from the true north was not the same at all places, but varied with geographical position. But now an entirely new fact was clearly demonstrated. Henry Gellibrand, a professor of mathematics at Gresham College, after a careful determination made on June 12, 1634, at Deptford, about three miles southeast of London Bridge, found that the needle pointed 4° 6′ east, while Gunter, on June 13, 1622, had found the value of 5° 56′ east, and, as above cited, Boroughs found the declination to be in London in 1580, 11° 15′ east. It was therefore clearly apparent that the declination or variation was changing and getting smaller. Gellibrand gives his conclusion thus:

"Hitherto, according to the tenets of our magnetical philosophers, we have supposed the variation of all particular places to continue one and the same. . . . But most diligent magnetical observations have plainly offered violence to the same, and proved the contrary, namely, that the variation is accompanied with a variation."

..

He embodied his observations in a little treatise published in 1635.' This fact of the "diminution of the variation" was likewise shown by the Paris observations. It seemingly was ascribed, however, to

The title was, Henry Gellibrand: A Discourse Mathematical on the Variation of the Magneticall Needle, together with its admirable diminution lately discovered. London, 1635.

This interesting book has just been reprinted in facsimile under the editorship of the well-known meteorologist and bibliographer, Prof. Hellmann, of Berlin. It constitutes No. 9 of Hellmann's Neudrucke von Schriften und Karten über Meteorologie und Erdmagnetismus." A. Asher & Co., Berlin. Price, 3 marks.