sage from thence to the Cape; in the course of which, the point of least intensity will, almost of necessity, have to be crossed, or at least approached very near.

Nor is the theoretical line indicated by Gauss as dividing the northern and southern regions, in which free magnetism may be regarded as superficially distributed, undeserving of attention. That line cuts the equator in 6° east longitude, being inclined thereto (supposing it a great circle) 15°, by which quantity it recedes from the equator northward in going towards the west of the point of intersection. Observations made at points lying in the course of this line may hereafter prove to possess a value not at present contemplated.

As a theoretical datum, the horizontal intensity has been recommended by Gauss, in preference to the total, not only as being concluded from observations susceptible of great precision, but as affording immediate facilities for calculation. As it cannot now be long before the desideratum of a chart of the horizontal intensity is supplied, the maxima and minima of this element may also deserve especial inquiry, and may be ascertained in the manner above pointed out.

The maxima of horizontal intensity are at present undetermined by any direct observation. They must of necessity, however, lie in lower magnetic latitudes than those of the total intensity, as its minima must in higher; and from such imperfect means as we have of judging, the conjectural situations of the maxima may be stated. as occurring in

Observations have been made of the horizontal intensity in the vicinities of II. and III., and are decidedly the highest which have been observed anywhere.

In general, in the choice of stations for determining the values of the three magnetic elements, it should be borne in mind, that the value of each new station is directly proportional to its remoteness from those already known. Should any doubt arise, therefore, as to the greater or less eligibility of particular points, a reference to the existing magnetic maps and charts, by showing where the known points of observation are most sparingly distributed, will decide it.

For such magnetic determinations as those above contemplated, the instruments hitherto in ordinary use, with the addition of Mr. Fox's apparatus for the statical determination of the intensity, will suffice; the number of the sea observations compensating for their possible want of exactness. The determinations which belong to the second branch of our subject,-viz. those of the secular changes, of the diurnal and other periodical variations, and of the momentary fluctuations of the magnetic forces,-require, in the present state of

our knowledge, the use of those more refined instruments recently introduced, and to be presently described.

The variations to which the earth's magnetic force is subject, at a given place, may be classed under three heads, namely, 1. the irregular variations, or those which apparently observe no law; 2. the periodical variations, whose amount is a function of the hour angle of the sun, or of his longitude; and 3. the secular variations, which are either slowly progressive, or else return to their former values in periods of very great and unknown magnitude.

The recent discoveries connected with the irregular variations of the magnetic declination, have given to this class of changes a prominent interest. In the year 1818 M. Arago made, at the Observatory of Paris, a valuable and extensive series of observations on the declination changes; and M. Kupffer having about the same time undertaken a similar research at Kasan, a comparison of the results led to the discovery that the perturbations of the needle were synchronous at the two places, although these places differed from one another by more than forty-seven degrees of longitude. This seems to have been the first recognition of a phenomenon, which now, in the hands of Gauss and those who are labouring with him, appears likely to receive a full elucidation.

To pursue this phenomenon successfully, and to promote in other directions the theory of terrestrial magnetism, it was necessary to extend and vary the stations of observation, and to adopt at all a common plan. Such a system of simultaneous observations was organized by Von Humboldt in the year 1827. Magnetic stations were established at Berlin and Freyberg: and the Imperial Academy of Russia entering with zeal into the project, the chain of stations was carried over the whole of that colossal empire. Magnetic houses were erected at Petersburg and at Kasan; and magnetic instruments were placed, and regular observations commenced, at Moscow, at Sitka, at Nicolajeff in the Crimea, at Barnaoul and Nertschinsk in Siberia, and even at Pekin. The plan of observation was definitely organized in 1830; and simultaneous observations were made seven times in the year, at intervals of an hour for the space of forty-four hours.

In 1834 the illustrious Gauss turned his attention to the subject of terrestrial magnetism; and having contrived instruments capable of yielding results of an accuracy before unthought of in magnetic researches, he proceeded to inquire into the simultaneous movements of the horizontal needle at distant places. At the very outset of this inquiry he discovered the fact, that the synchronism of the perturbations was not confined (as had been hitherto imagined) to the larger and extraordinary changes; but that even the minutest deviation at one place of observation had its counterpart at the other. Gauss was thus led to organize a plan of simultaneous observations, (not at intervals of an hour, as before, but) at the short intervals of five minutes. These were carried on through twenty-four hours six*

* Recently reduced to four.

times in the year; and magnetic stations taking part in the system were established at Altona, Augsburg, Berlin, Bonn, Brunswick, Breda, Breslau, Cassel, Copenhagen, Dublin, Freyberg, Göttingen, Greenwich, Halle, Kasan, Kracow, Leipsic, Milan, Marburg, Munich, Naples, St. Petersburg, and Upsala.

Extensive as this plan appears, there is much yet remaining to be accomplished. The stations, numerous as they are, embrace but a small portion of the earth's surface; and what is of yet more importance, none of them are situated in the neighbourhood of those singular points or curves on the earth's surface, where the magnitude of the changes may be expected to be excessive, and perhaps even their direction inverted. In short, a wider system of observation is required to determine whether the amount of the changes (which is found to be very different in different places) is dependent simply on the geographical or on the magnetic co-ordinates of the place; whether, in fact, the variation in that amount be due to the greater or less distance of a disturbing centre, or to the modifying effect of the mean magnetic force of the place, or to both causes acting conjointly. In another respect also, the plan of the simultaneous observations admits of a greater extension. Until lately the movements observed have been only those of the magnetic declination, although there can be no doubt that the inclination and the intensity are subject to similar perturbations. Recently, at many of the German stations, the horizontal component of the intensity has been observed, as well as the declination; but the determination of another element is yet required, before we are possessed of all the data necessary in this most interesting research.

The Magnetic Observatories about to be established in the British Colonies, by the liberality of the Government and of the East India Company, will (it is hoped) supply in a great measure these desiderata. The stations are widely scattered over the earth's surface, and are situated at points of prominent interest with regard to the Isodynamic and Isoclinal lines. The point of maximum intensity in the northern hemisphere is in Canada; the corresponding maximum in the southern hemisphere is near Van Diemen's Land; St. Helena is close to the line of minimum intensity; and the Cape of Good Hope is of importance on account of its southern latitude. Again, in India, Madras and Singapore are in the neighbourhood of the two lines of minimum intensity and of no dip, which in this region of the globe approach one another; and Simla, in the Himalaya mountains, is a station of interest and importance on account of its great elevation. At each observatory the changes of the vertical component of the magnetic force will be observed, as well as those of the horizontal component and declination; and the variations of the two components of the force being known, those of the inclination and of the force itself are readily deduced. The simultaneous observations of these three elements will be made at numerous and stated periods, and there is every reason to hope that the directors of many of the European Observatories will take part in the combined system.

But interesting as these phenomena are, they form but a small part of the proper business of an observatory. The regular changes (both periodic and secular) are no less important than the irregular ; and they are certainly those by which a patient inductive inquirer would seek to ascend to general laws. Even the empirical expression of the laws of these changes cannot fail to be of the utmost value, as furnishing a correction to the absolute values of the magnetic elements, and thereby reducing them to their mean amount.

The hourly changes of the declination have been frequently and attentively observed; but with respect to the periodical variations of the other two elements, our information is as yet very scanty. The determination of these variations will form an important part of the duty of the Magnetic Observatories; and from the accuracy of which the observations are susceptible, and the extent which it is proposed to give them, there can be no doubt that a very exact knowledge of the empirical laws will be the result.

With respect to the secular variations, it might perhaps be doubted whether the limited time during which the observatories will be in operation is adequate to their determination. But it should be kept in mind that the monthly mean corresponding to each hour of observation will furnish a separate result; and that the number and accuracy of the results thus obtained may be such, as fully to compensate for the shortness of the interval through which they are followed. A beautiful example of such a result, deduced from three years' observation of the declination, is to be found in the first volume of Gauss's magnetical work, of which a translation is published in the fifth number of Taylor's Scientific Memoirs.

Allusion has been made above to a different system of magnetic elements from that usually chosen. Before proceeding further, therefore, it is necessary to state more fully what those elements are which have been taken as the immediate objects of research; and to describe the instruments which have been adopted for the purpose.

The elements on which the determination of the Earth's Magnetic Force is usually based are, the declination, the inclination, and the intensity. If a vertical plane be conceived to pass through the direction of the force, that direction will be determined when its inclination to the horizon is given, as well as the angle which the plane itself forms with the meridian; and if, in addition to these quantities, we likewise know the number which expresses the ratio of the intensity of the force to some established unit, it is manifest that the force is completely determined.

For many purposes, however, and especially in the delicate researches connected with the variations of the magnetic force, a different system of elements is preferable. The intensity being resolved into two portions in the plane of the magnetic meridian, one of them horizontal and the other vertical, it is manifest that these two components may be substituted for the total intensity and the inclina

tion; while, at the same time, their changes may be determined with far greater precision. The former variables are connected with the latter by the relations

in which R denotes the intensity, X and Y its horizontal and vertical components, and 0 the inclination; and the variations of and R are expressed in terms of the variations of X and Y by the formulæ :

As the instruments furnished to the Magnetic Observatories, and to the Naval Expedition, for the observation of these elements are, for the most part, novel in form, it will be useful to give a somewhat detailed account of their construction and various adjustments, before entering on the plan of observation to be pursued.

DECLINATION MAGNETOMETER.

Construction.-The essential part of the declination magnetometer is a magnet bar, suspended by fibres of untwisted silk, and inclosed in a box, to protect it from the agitation of the air. The bar is a rectangular parallelopiped, 15 inches in length, 7ths of an inch in breadth, and 4th of an inch in thickness. In addition to the stirrup by which the bar is suspended, it is furnished with two sliding pieces, one near each end. One of these pieces contains an achromatic lens, and the other a finely-divided scale of glass; the scale being adjusted to the focus of the lens, it is manifest that the apparatus forms a moving collimator, and that its absolute position at any instant, as well as its changes of position from one instant to another, may be read off by a telescope at a distance. The aperture of the lens of this collimator is 14 inch, and its focal length about 12 inches. Each division of the scale is 35th part of an inch, and the visual angle under which it is seen in the telescope is so considerable, that it may be readily subdivided into tenths by estimation. The corresponding angular quantity is about 43 seconds, and the readings may therefore be made to four seconds nearly.

To the suspension thread is attached a small cylindrical bar, the ends of which are of smaller diameter, and support the stirrup which carries the magnet. The apertures in the stirrup, by which it hangs on the cylinder, are of the form of inverted Y s, so that the bearing points are invariable. A second pair of apertures at the other side of the magnet, serves for the purpose of inversal; and care has been taken to render the lines connecting the bearing points of each pair of Y s parallel, so that there may be no difference in the amount of torsion of the thread in the two positions of the stirrup. The two pairs of apertures are at different distances from the magnet, in order that the line of collimation may remain nearly at the same