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ends or poles towards the north and south poles of the earth. This directive force (which, when possessed by steel, under ordinary circumstances, is permanent) is disturbed by the presence of iron, the amount of the deviation depending on the distance between the magnet and the iron, and its direction upon the position of the disturbing agent. A permanent magnet, and such the compass needle is, must therefore be acted upon in all directions by an iron ship, but with forces varying with the quantity and distance of the metal. Looking at this fact only, it was supposed that the final result of the local attraction could be counteracted by adjusting the needle, that is to say, by fixing permanent magnets near the compass, so as to bring the needle into its true magnetic position. Supposing the antagonistic force of local attraction and selected permanent magnets, to be in equilibrio, the needle would be at liberty to obey the impulsive force of the terrestrial magnetism. But it did not occur to those who took this narrow view of the difficulty, that the local attraction is not a fixed or permanent quantity. The defleetion of the needle is not the same when the ship is lurching and pitching upon a heavy sea as when she lies moored in dock. But the disturbance might not have been very serious from this cause if it had been the only or even the principal source of derangement. The principal error is in considering the ship as though it were a mass of iron and nothing more. It would be difficult to find a piece of iron which had passed through the hands of the artisan without acquiring some degree of magnetic power. Percussion, contortion, or indeed any mechanical force, gives a directive force, more or less permanent, to iron; the arrangement of the poles being according to the position of the metal at the time. In the very act of constructivg an iron ship, therefore, a magnetic force is communicated to it, and the direction of that force will depend upon her position in the stocks in relation to the magnetism of the earth." Hence it will appear that an iron ship when she is launched, is not merely a large ferruginous mass acting upon a permanent magnet under the ordinary conditions of mass and distance, but a floating magnet, or we should perhaps say a combination of magnets, having but little intensity, and holding the power with feeble tenacity. A few days may altogether change her magnetic condition. At one time we see her sleeping in dock, or rising and falling lazily upon the tide, swinging tardily with the ebb and flow. A few hours after she may be ploughing her way through a stormy sea, trembling under every stroke of the piston ; and as she pitches and rolls each blow and twist disturbs and changes the magnetic direction she received from the shipwright's hammer.
If such be the magnetic condition of an iron ship-if it be as
we imagine, a reservoir of magnetic forces, there can be no adjustment of the compass; for there is no permanent condition to which the adjustment can be adapted. An'arrangement of permanent magnets, which may at one time neutralize the local attraction, and give the needle its proper direction, may at another act as a disturbing force and be the cause of irreparable mischief. The merchant and the ship-owner will do wisely to follow the example of the Admiralty in this matter, and abandon the custom of adjusting ships' compasses ; for it is better to trust to the probable detection of an error in the direction of a compass which is free to move, than to place faith in one which is held in the grasp of so many uncertain forces.
From these remarks it must not be supposed that the compass is of little value to the sailor. In spite of its liability to erroneous action it must still be the mariner's guide, and it is often his only one. But there are some precautionary measures which may be taken. They are pointed out by Dr. Scoresby in the following remarks:
* It is most important for safety in navigating iron vessels, that captains should be made aware of the liability of the compasses to change, and so to mislead them; that they should know the circumstances under which, in accordance with natural laws regulating and applying the earth's inductive action, changes are most likely to occur ; that they should always be watchful of opportunities for determining the true magnetic direction with reference to their compasses, by observation of the sun and stars; and that by providing a place for a standard compass aloft, as far from the deviating influence of the body of the ship as possible, they might have guidance sufficient, with some allowances, for steering a correct magnetic course. With the precautions and means such as might be thus applied, the difficulties in respect of compass guidance in the navigation of iron ships might be mainly and practically overcome.'
But with all the caution that can be used the evil still exists, and many are asking what can science do to correct the error. The appeal now made for assistance is pressing,—the urgency is great. To prove this, we may quote from the address of Mr. Towson, the Secretary to the Local Marine Board of Liverpool :
'In the name of the merchants and shipowners of Liverpool, I implore the attention of the section to this important subject, in the hope and belief that if the members should respond to that appeal, they would be able before the next meeting to confer the benefit they seek, not on their own account alone, nor in consideration of the vast amount of property involved, but for the sake of the vast amount of human life which is continually being jeopardized and lost.'
Upon the methods now used in testing and correcting the
deviation of the needle Mr. Towson speaks with firmness and, from his official position, with authority.
Besides collateral means adopted for correcting the compass, there are two systems in use for that purpose : Captain Johnson's system of swinging the ship and tabulating the results, which is exclusively employed in the royal navy; and the Astronomer Royal's method of compensating the compasses by means of magnets, which is almost exclusively resorted to in the port of Liverpool. The objection to Captain Johnson's plan is, that the corrections are liable to be employed the wrong way. After examining about 2000 masters of merchant vessels, I am convinced of the soundness of this objection. There is a general tendency in practice to come to a wrong conclusion on the subject. The mariner knows that westerly deviations indicate that the north end of the needle is drawn to the west, and comes to the conclusion, that if his compass has a westerly deviation it must cause an object bearing north to appear westerly, whereas it would really appear easterly; and I have known the same mistake made on board ships in the royal navy. The most formidable objection to the Astronomer Royal's system is, that the magnetic poles of the compensating magnets are liable to change or to vary in their intensity. The change of retentive magnetism, deviation from heeling, and the change produced by going into the other hemisphere, are defects common to both systems. I have never met with a captain who could tell me the original deviation of his compass. In the case of the 'Tayleur,' the deviation of her steering compass was 60°; of her compass before the mizen mast 40°. Was there ever such a case in the royal navy ? Lieutenant Pasce, when appointed to the ‘Jackal,' in 1845, was dissatisfied with a deviation of 25°, and obtained permission of the Admiralty to have the compass replaced, when it was reduced to 18°; and no doubt the masters of merchant vessels would, on this point, be equally prudent if they knew the real amount of the original error.'
It is greatly to be desired that some process should be discovered for the correction of the errors to which the mariner's compass is liable; and surely it is not indulging a flattering hope, considering the present state of experimental science, if we venture to anticipate that some correction will be found before another year has passed.
The brief review we have taken of the scientific labours of the past year will amply justify the assertion that they have been eminently practical Science, when pursued in a Christian spirit, is always philanthropic in its objects and results. Its purposes are to protect man from the evils of ignorance, and to give him the security and benefits of knowledge; and in no way can it effect these objects more completely than by supplying him with, if we may so speak, perfect tools for the exercise of his energy and industry. But before we close this sketch of last year's labour, it will be necessary to mention one or two other
subjects, less practical in their character, which have also received some attention from men of science, and are likely to be still further pursued during the present year.
The large increase in the number of private astronomical observatories is doing much to extend our knowledge of celestial objects and to accumulate data, the value of which will be fully recognised by future observers. Four more planets have been discovered, and the same number of comets. Of the four planets, three were first observed in Mr. Bishop's observatory,—two by Mr. Hind and one by Mr. Maith. The fourth was discovered by Mr. Luther, at the observatory of Bilk, near Dusseldorf. The four comets are new to us, if not to our system, for they cannot be identified with any that have been before observed. One was observed at Berlin, two at Gottingen, and the fourth was visible to the naked eye in many parts of Europe, and was, on one occasion, seen in daylight by Mr. Hartnup.
The progress of Stellar astronomy keeps pace with the onward march of discovery in the solar system. The erection of Bessels noble telescope, and the results obtained with it by that lamented astronomer (of which the measure of the parallax of 61 Cygni was the most important), inaugurated a new era.
A large amount of the labour of the astronomers of the two last centuries, long comparatively useless, is now being reduced and catalogued. In some instances this has been already partially done, but much still remains to be done. The British Association volunteered to assist in the accomplishment of the task, and in 1845, published a catalogue of eight thousand three hundred and seventy-seven stars. This catalogue includes many stars of the seventh magnitude; but as these are often calculated from one observation, chiefly by Laland and Lacaille, they are not uniformly correct
. In spite of this, however, although the astronomer does sometimes turn his telescope to the point indicated and does not find the star ;—and although the place of a star employed as a point of reference for some moving body is not always correctly defined ;--the catalogue is valuable, and the errors will be surely, though slowly, corrected. The detection of error is also an excitement to improvement, and the importance of making another attempt to supply the wants of the astronomer is already acknowledged :
The British Association would add greatly to the benefits it has already conferred on astronomical science,' says Professor Challis, “by promoting the publication, when sufficient materials can be collected, of a general catalogue of all stars to the ninth magnitude inclusive, which have been repeatedly observed with meridian instruments. The modern sources at present available for such a work are the reduced and published observations of the Greenwich, Puikowa, Edinburgh,
Oxford, and Cambridge Observatories, and the recently completed catalogue of twelve thousand stars observed and reduced by the indefatigable astronomer of Hamburgh, Mr. Charles Rumker, together with numerous incidental determinations of the places of comparison stars in the “ Astronomische Nachrichten.' To complete the present account of the state of Stellar astronomy, mention should be made of two volumes recently published by Mr. Cooper, containing the approximate places arranged in order of right ascension of thirty thousand one hundred and eighty-six elliptic stars from the ninth to the twelfth magnitude, of which a very small number had been previously observed. The observations were made with the Makree equatorial, and have been printed at the expense of her Majesty's government.'
We cannot mention the subject of astronomy without a particular reference to the application of electricity to the duties of the astronomical observatory. Telegraphs, signal balls, and sympathetic clocks, are now to be classed among the common things, and their operations are understood by all intelligent men who watch the progress of discovery and its influence upon society. But it may not be generally known that in no scientific pursuit or commercial enterprise is the voltaic battery more useful than in an astronomical observatory. There is something apparently fabulous, or it certainly would have been so designated a few years ago, in the statement that an electric clock in Greenwich Observatory 'maintains in sympathetic movement the large clock at the entrance gate, two other clocks in the Observatory, and a clock at the London-bridge Terminus of the South-eastern Railway :-it sends galvanic signals every day along all the principal railways diverging from London :-it drops the Greenwich ball, and the ball on the offices of the Electric Telegraph Company in the Strand, and the correctness of the last of these operations is tested by means of a galvanic signal needle upon the case of the Greenwich transit clock. All these effects are produced without sensible error of time. A time-signal ball at Deal has also been connected with the electric arrangement at Greenwich, and thus the shipping in the Downs is provided with the means of obtaining correct time.
Public attention was sometimes since drawn to an ingenious method of determining the difference of longitude between distant places by voltaic signals, invented and used in America. By this method the Astronomer Royal has determined the difference of longitude between Greenwich and the Observatories of Cambridge, Edinburgh, and Brussels; and in the same manner Professor Encke has obtained the difference of longitude between Berlin and Frankfort-on-the-Maine. As soon as the necessary connexions and turntables had been made for a branch line of voltaic wires from Greenwich Observatory to London-bridge, and those preliminary operations which are necessary to give facility