Local Attraction.

10. It has been found that the results of magnetic observations, whether of the declination, inclination, or the intensity of the magnetic force, are liable to be influenced by local attraction proceeding from the rocks or soil in the vicinity of the instrument, and particularly so at stations where the rocks are of igneous character, such as traps, basalts, granites, &c. As a precautionary measure, therefore, magnetical instruments should always be used on stands which raise them 3 or 4 feet above the ground; and those stations are to be preferred of which the geological character is sedimentary or alluvial. Stations of igneous character, though less eligible for obtaining results which show the correct magnetical elements corresponding to the geographical position of the station, may nevertheless be serviceable as stations of comparison between the land and sea instruments; but for this purpose it is essential that the different instruments to be compared should be used precisely on one and the same spot at the station, in which case the local attraction may be supposed to be a constant quantity. And if the station be one frequently resorted to by vessels from which magnetic observations are made, it is desirable that the spot should be susceptible of a definite and well-recognisable description.

At sea, from the quantity of iron which a ship contains, it is scarcely possible that its influence on the instruments should be altogether avoided; but from the circumstance that the greater part of the most influential iron is in fixed positions in the ship, it has been proved by sufficient experience, that by a proper selection of the place in which a magnetic instrument is used on board ship, and by a certain process of observation (repeated whenever the ship has undergone any considerable changes of geographical position), the influence of her iron is susceptible of a sufficiently approximate calculation, and of being eliminated accordingly. Directions for the observations proper to be made for this purpose have been published by the Admiralty.

Summary of the Observations to be made.

11. An officer, therefore, who purposes to make magnetic observations, or to cause them to be made on board his ship, has to attend to the following points :-He must take care that he obtains the instruments some days before the ship is ready for sea, in order that he may assure himself that they are all complete, and that, if inexperienced in their use, he, or the observer whom he selects, may have some preliminary practice with them. He will then have to determine the constants, index corrections, &c. (unless these shall have been furnished with the instruments), and to make the observations required for a base station, with the needles which are to be employed in the relative determinations of the magnetic force and dip at sea. Positions will then have to be selected on board for the standard compass, and for Fox's apparatus, and the pillar for the one, and the gimball stand for the other, fitted accordingly. When the ship is ready for sea, the observations which are directed in the Admiralty Instructions for ascertaining the deviations of the compass caused by the iron of the ship are to be made on board; and when the ship is swung for this purpose, the deviations of the dip and of the force must be also observed on sixteen, or at least on eight, principal points of the compass, with Fox's apparatus used at the spot selected for it.

This completes the preparations to be made before the ship's departure. Whilst at sea, the observations of dip and intensity described in Appendix No. 3, as well as those of the declination or variation by the standard compass, should be made daily, whenever the weather and other circumstances permit. Whenever the ship is in harbour, and time and opportunities are suitable, it is desirable that the instruments should be taken on shore, and used at a spot selected as least likely to be influenced by any local attraction; and that the declination, inclination, and absolute horizontal force should there be determined, and the comparative observations made with Fox's apparatus. If the

ship has materially changed her geographical position since the last occasion when the deviations were ascertained, or if changes have been made in her equipment by which the deviations may have been affected, it is desirable that the process for their examination should be repeated; and lastly, the harbour observations here described should not fail to be repeated whenever the ship finally returns to England.

Record and Transmission of the Observations.

12. Blank forms are supplied for the entry of observations of all classes, and for the first or uncorrected calculation of those which require that process to be gone through at the time. It is desirable that the forms should be filled up in duplicate, and that one copy should be retained, and the other sent to England from time to time, as soon as circumstances make it convenient. On their arrival they should be immediately examined, and any suggestion to which they may give rise communicated at once to the observer.*

Application of the Results.

13. The observations when thus received require that the several corrections arising from the influence of the iron, the variations of temperature, the changes in the magnetic force of the magnets, and from various other sources, should be sought out, computed, and applied, and the true or corrected results finally derived. These form the materials from which it is intended to construct maps, showing the variations of the magnetic force, and of the magnetic direction in its two co-ordinates of inclination and declination, corresponding to the present epoch, over the whole surface of the globe. The variations of the three elements are shown on these maps, by lines connecting, for example, in the maps of the magnetic force, those points where the intensity is observed to be the same;-in the

* This has hitherto been done on all occasions when practicable, and it is very desirable that it should always continue to be done.

maps of the inclination, those points where the inclination. is observed to be the same;-and in the maps of the declination, those points where the declination is observed to be the same. These lines are known by the names of Isodynamic, Isoclinal, and Isogonic lines. The Isogonic lines, which form the maps of the declination (or variation charts), have a direct practical importance and value in navigation, which in a notice addressed to naval officers needs not to be dwelt on. In theoretical respects, the Isodynamic and Isoclinal lines are not less essential; the three form the basis of a systematic view of terrestrial magnetism, as it manifests itself to us on the surface of the globe.

The mode in which the results are made to contribute to the formation of these maps is the following:-The results of the three elements finally corrected are entered, each in its proper geographical position, on maps on a large scale, severally appropriated to the force, the inclination, and the declination. Each result has a small characteristic mark denoting the observer. When any portion of the globe is sufficiently covered by the results of observations in proper distribution, the isophænomenal lines are drawn for that portion of the globe in correspondence with the observations, with a free hand, but with a careful judgment, aided occasionally by a process of calculation which it is not necessary here to describe. From these maps tables of double entry are formed, having the latitude at the side, and the longitude along the top of the page, and the values of the magnetic elements corresponding to the several latitudes and longitudes are placed at the points of intersection. By proper care in the process, the step of forming the tables from the maps need involve no additional uncertainty whatsoever. Maps and tables thus prepared will, when completed, form an experimental theory of terrestrial magnetism, in which the facts of nature will be shown with greater or less exactness, in proportion as the observations are numerous, correct, and suitably distributed, and as they are more or less correctly represented in the maps. Mathe

H

matical formulæ, based on general mathematical views, having numerical coefficients of which the values are derived from these maps, may also serve for the computation of the magnetic elements at any geographical position on the surface of the globe; and if the points taken from the maps to serve as the basis of the numerical values of the coefficients are sufficiently numerous, and have a proper distribution over the surface of the globe, and if the formulæ are carried to a sufficient number of terms, it may be expected that the elements computed from them will have the same degree of exactness as the maps from which their coefficients are taken.

It may be natural at this stage to inquire what prospect exists of being able to complete a work of this magnitude within a reasonable time; and to this question a more satisfactory reply can be returned than may perhaps be generally anticipated. Nearly three-fourths of the surface of the globe being covered by the ocean, it is chiefly by naval surveys that the materials for such a work can be collected. By the zealous and unwearying assiduity of British officers, acting under the sanction and with the approval of the Lords Commissioners of the Admiralty, and in some instances in expeditions specially appointed for the purpose, magnetic observations designed expressly for the object above-mentioned, and conducted upon a uniform system, have been extended in late years over nearly all the accessible parts of the ocean. Of these surveys, the results of some have been already deduced and published in the Philosophical Transactions, the expense of publication having been borne conjointly by the Government and by the Royal Society; and the results of others are undergoing the process of calculation and arrangement for publication. Whilst this more exact and careful elaboration of the results of the great mass of materials which have been accumulated is in progress, provisional maps of each of the three elements corresponding to the epoch of 1840, between the geographical latitudes of 60° N. and 60° S., have been constructed from a preliminary and general examination