expedition under Sir James Clark Ross: those of the expedition under Lieutenant (since Commander) Moore, R.N., and Lieutenant (since Captain) Clerk, Royal Artillery, also to the Antarctic Ocean; of Captain Sulivan, from England to the Falkland Islands and back; of Sir Edward Belcher (in H.M.S. Sulphur), on the north-west coast of America, and in the Pacific and Indian Oceans; of Lieutenant Alexander Smith, R.N., and Lieutenant Dayman, R.N., in voyages between England and Van Diemen Island; together with maps of the inclination and force in the British provinces of North America, chiefly from the observations of Captain Lefroy, of the Royal Artillery; and maps of the declination (or variation) in the North and South Atlantic Oceans, corresponding to the year 1840, from various sources. In the second class, viz. surveys completed and in progress of reduction but not yet published, may be enumerated the remaining portion of the observations made in Sir James Clark Ross's Antarctic expedition; a small but valuable collection by the Niger expedition; another by Sir Robert Schomburgk in Guiana; two series on the coast of New Holland, one by Captain Wickham, and the other by Lieutenant (since Captain) Stokes; the observations made in a special expedition to Hudson's Bay and back by Lieutenant (since Commander) Moore; and three extensive series, one by Captain Shadwell, in the hydrographical expedition of Captain Blackwood to Torres Strait and round New Holland; one by Sir Edward Belcher, C.B., in H.M.S. Samarang, in the Indian and Chinese Seas; and one by the late Captain Owen Stanley in Torres Strait and the coasts of New Guinea, with a continuation in the homeward

passage across the Pacific and round Cape Horn, by Lieutenant Dayman, R.N. To these must be added observations during an expedition to the shores of the Polar Sea in America in search of Sir John Franklin, by Sir John Richardson, C.B.; and a very valuable magnetic survey amongst the islands of the Indian Archipelago, by Captain Elliot, of the Madras Engineers, at the expense of the East India Company, now (1850) publishing in the Philosophical Transactions.

Amongst the observations now in progress may be named those of the three expeditions to the Arctic Polar Sea under Sir John Franklin, Captain Collinson, C.B., and Captain Austin, C.B.; of the hydrographical surveying expeditions of Captain Kellett in the Pacific; of Captain Bayfield in the Gulf and River St. Lawrence; of Commander Moore in the Pacific and Behring's Strait; and of Captain Stokes and Commander Drury in New Zealand; and the land expedition of Dr. Rae on the northern part of the North American continent.

When it is borne in mind that several of the abovenamed surveys include periods of three or four years, and in some instances not only determinations at the several ports and harbours which may have been visited, but also daily observations (weather permitting) of the three magnetic elements at sea in passages from port to port, the accumulation of materials, and their already extensive distribution over the surface of the globe, may in some degree be judged of. No part of the globe has yet however been considered so far completed, that its magnetic curves have been finally drawn, or the tables of the magnetic elements corresponding to geographical positions finally derived

from them. The results of each individual series are computed and published as soon after they have been received in a complete state, as can be accomplished by the establishment at Woolwich, which is very limited in comparison with the duties it endeavours to perform. The magnetic lines indicated by each series are also provisionally drawn; but the final coordination of the different series, and their combination with the results of the magnetic surveys carrying on at the same time by governments and individuals in various parts of the remaining fourth of the globe which is occupied by land, is deferred until the accumulation of materials in the more frequented portions shall appear sufficient for the purpose, and until in the less frequented portions there shall be no immediate prospect of any further considerable accession. The final period at which the work may be completed cannot but be accelerated by the step which the Lords Commissioners of the Admiralty have taken, in desiring the present notice to be drawn up, and in holding out inducements to naval officers of professional and other advantages, as a recompense for what may in some respects be regarded as extra-professional services.

In prosecuting a work of this general and purely experimental character, unconnected with hypothesis of any sort, the phenomena of all parts of the globe must be viewed in the abstract as possessing an equal importance; and it does not appear desirable, therefore, to name any one of the lines, whether isogonic, isoclinal, or isodynamic, as deserving of special attention in preference to others. There is one direction, however, which may be safely given, and which it may

be well to remember at all times, viz. that "the value of each new station is directly proportional to its distance from those where observations have already been made;" and in this point of view it may be useful to notice, that up to the present time fewer observations have been made in the Pacific than either in the Atlantic or the Indian Oceans.

D

APPENDIX No. 1.

DESCRIPTION AND USE OF THE UNIFILAR MAGNETOMETER.

THE Unifilar Magnetometer consists of two parts-an apparatus for deflection, and an apparatus for vibration: these correspond with the two parts of the process by which the absolute horizontal force is determined; the experiments of deflection consist in observing the angular deflection of a suspended magnet produced by the influence of a second magnet, which is placed on a support at one or more known distances from the suspended magnet, and in a line drawn from its centre perpendicular to its direction: the experiments of vibration consist in suspending the magnet which was used as the deflecting magnet in the experiments of deflection, and observing its time of vibration. By the first part of the process (or the experiments of deflection) we obtain the ratio of the magnetic moment of the deflecting magnet to the Earth's horizontal magnetic force at the place of observation; the latter being to the former as 1 to the sine of the angle of deflection multiplied by half the cube of the distance employed;—or if m denote the magnetic moment of the needle, X the Earth's horizontal force, r the distance apart of the centres of the magnets, and u the angle of deflection, the expression is

By the second part of the process (or the experiments of vibration) the product of the same two qualities is obtained: being the quotient of a constant, which we may call K (see p. 38), divided by the square of the time of vibration: or if T be the time of vibration,

m

obtained separately: for if we call m X =a and =

=

X

ß, m (the

magnetic moment)