III.-Statement abstracted from a Return of the House of Commons, showing the total number of Steam Vessels Registered in the United Kingdom on or before the 1st of January, 1855, and the Per-centage of Deduction from Gross Tonnage, allowed for Engine-room, distinguishing whether Wood or Iron Paddle, and Wood or Iron Screw Vessels.

Notice of the Instruments employed in the Magnetic Survey of Ireland, with some of the Results. By the Rev. H. LLOYD, D.D., M.R.I.A.

THE Irish portion of the magnetic survey of the British Islands was commenced in the beginning of August last, and is now nearly terminated, nothing remaining for its completion but the determination of some of the instrumental constants. The observations in the northern half of Ireland were made by Profs. Galbraith and Haughton, and those of the southern half by Dr. Lloyd and G. Johnstone Stoney, Esq. The instruments employed were similar in the two divisions of the island, and consisted of a theodolite magnetometer, and of a dip circle furnished with an apparatus for the determination of the earth's magnetic force in absolute measure. The comparability of these instruments was established by observations taken at Dublin at the commencement of the survey. The elimination of the magnetic changes will be effected by means of simultaneous observations made at the magnetical observatory of Dublin. The theodolite magnetometer for the measurement of the magnetic declination differs somewhat from instruments of a similar nature hitherto in use, the difference consisting in observing the sun (or other celestial object) by reflexion, and transferring the transit adjustments to the axis of the mirror employed for that purpose. By this arrangement, the observing telescope is always horizontal, and in readiness for the magnetic part of the observation. The instrument is furnished also with the means of determining the horizontal component of the magnetic force in absolute measure; but this was not employed, the determination of the total intensity by means of the dip circle leaving nothing to be desired. The apparatus required for this latter determination consists of two additional needles,

one of which is loaded with a small weight, placed at a fixed distance from the centre on the southern arm. The observation consists of two parts, in one of which the product of the earth's magnetic force into the magnetic moment of the magnet is found by observing the position of equilibrium of the loaded needle, when placed on the agate planes; while, in the other, the ratio of the same quantities is found by employing the same needle to deflect another substituted in its place. The apparent obstacle to the success of this method lies in the smallness of the angle of deflection produced by a dippingneedle, when employed as a deflector at the usual distances; the error of the deduced force being inversely as the sine of the angle of deflection. The equation of equilibrium of the deflected needle involves a quantity, which may be expanded into a series of the inverse odd powers of the distance (beginning with the inverse third), the coefficients of which are functions of certain integrals depending on the distribution of magnetism in the two needles. The law of distribution being unknown, these coefficients can only be determined by repeating the observation at several known distances, and eliminating among the resulting equations. Now, at the distances usually em⚫ployed in observations of deflection, the angle of deflection produced by a dipping-needle would be too small for accuracy; and if the distance be diminished, the number of terms of sensible value will be increased, and - there will arise increased difficulty and uncertainty in the elimination. This difficulty is avoided by availing ourselves of a circumstance which appears to have been generally overlooked. It is not necessary that the usual deflection distance should be one of the series employed in deducing the coefficients of the inverse powers of the distance in the value of the constant; it is not even requisite that the relative positions of the two magnets should be similar in the two cases: for if the value of the corresponding function be found, for any other position, and at any other distance, that of the required function will be known by a comparison of the deflections produced. Accordingly, the principle of the present method (so far as the deflection process is concerned) consists in observing the angle of deflection in the regular series of observations, at a very short distance, the deflecting needle being attached to the moveable arm of the divided circle which carries the verniers, so as to be always rendered perpendicular to the deflected needle in the course of the observation. In the determination of the constant the whole apparatus is to be turned in azimuth, until the deflected needle is vertical. The deflecting magnet is then to be removed from its usual position above described, and placed horizontally on a support outside the box on a level with the agate planes, and the equilibrium produced by turning the apparatus in azimuth as before. This observation having been repeated at several known distances, we have all the data for the determination of the unknown constant. By these means the uncertainty of the result, arising from the smallness of the angle of deflection, is removed from the regular series of observations, and thrown upon the determination of the constant, which may be made at leisure, and repeated as often as is requisite for accuracy. Even when the constant is undetermined, the instrument may be used to observe the total force relatively, the method having this advantage over other relative methods hitherto used, that the results are completely independent of the changes of the magnetic moments of the needles in the intervals of observation. Dr. Lloyd concluded by stating some of the results of the survey, so far as they have been calculated. But as these calculations are still incomplete, the results can only be regarded for the present as provisional.

Report of Dublin Dredging Committee, appointed 1857-58.
By Professor J. R. KINAHAN, M.D., M.R.I.A.

DURING the past year, the following districts, all in the immediate vicinity of Dublin, have been examined:-Killiney Bay, Bray Head, Kish Bank, South Strand, Dalkey Sound, Howth, Malahide, Lough Shinny, Bettystown, and considerable advance made towards the completion of the Report. The following numbers of species have been catalogued :

Fishes, 60; Mollusca, including Polyzoa, 262; Crustacea, 105; Arachnida, 5; Echinodermata, 29; Hydrozoa, 60; Actinozoa, 17; Sponges, 10; and many Annelides not yet identified. Various untoward events prevented the Committee from carrying out fully the dredging arrangements which they had made at the commencement of the year. For the better perfecting of the Report, they have determined to allocate to the several Members of the Committee certain classes of animals as the special object of study, and have divided these as follows:- Professor Kinahan, M.D., Articulata and Sponges; Dr. Carte, Vertebrata; Dr. Wright, Mollusca; and Professor J. R. Greene, Echinodermata, Cœlenterata, &c. They propose also to include in their final report, as complete a list of the fishes of Dublin Bay as practicable, for which, as well as the further prosecution of their dredging researches, they would ask for a grant not exceeding £15 for the year 1858-59, the Committee consisting, as last year, of Professor Kinahan, Dr. Carte, Dr. Wright, and Professor Greene.

Report on Crustacea of Dublin District. By JOHN ROBERT KINAHAN, M.D., M.R.I.A., Professor of Zoology in the Department of Science and Art. - Part I. Decapoda Podophthalmata.

THE marine districts comprised in this Report consist of a series of open bays, into most of which a river-mouth enters; and of one or two extensive sand-banks which lie off the east coast of Ireland.

The chief stations are-Dublin Bay and the estuaries of the rivers Dodder, Anna Liffey, and Tolka; a long lighthouse pier separates this into subdistricts the North and South Bulls. The South Bull is almost exclusively fine sand, containing great numbers of broken shells, being made up from the washings of a cliff of marine drift; ; an extensive strand, left dry here at low water, is dotted over with sand-pools, in which Carcinus Mænas, Mysis vulgaris, Mysis Chameleon, Palæmon squilla, P. varians, Crangon vulgaris, Gammarus locusta, Gammarus palmatus and other species are found. A tidal stream called the Cockle lake, divides the bed of drift already spoken of from the strand proper; this at high tides is in many places from 2 to 3 fathoms deep, but is singularly destitute of crustacea; the bottom is a quick

sand.

Passing along from this station towards Kingstown Harbour, we meet several patches of Zostera, in which Port. holsatus is found in some numbers; having passed Kingstown, the bay becomes rocky, one or two zostera-clad banks, here called Mullet Grass, being interspersed among the rocks; one of these, near Sandycove, furnished me at low water with the followingspecies:Hippolyte varians, H. Cranchii, Pandalus leptorhynchus (new species), Mysis vulgaris and chamaleon, Crangon vulgaris, Crangon fasciatus, Apseudes talpa, Atelecyclus heterodon.