페이지 이미지

work longer, that is in higher latitudes, than, it would otherwise do. This was observed as an experimental fact by Lieutenant Foster, while the vessel was on the coast of Greenland where it was obvious, that when the plate was removed the needle was wholly inactive, but as soon as the plate was replaced, the needle again became serviceable. At this time the cause of this important effect of the plate was not known; but on the return of the ship, Lieutenant Foster having attentively reconsidered the subject, demonstrated it to be the necessary consequence of the common principle of the composition and resolution of forces; a discovery in the highest degree creditable to the acuteness and ingenuity of this rising young officer. On the coast of Greenland, the effect of the local attraction of the vessel amounted to 45° at east and west, so that the action of the iron at this time on the needle was equal to that of the earth; and the direction which the neede would at any time take up, was in the diagonal of the Parallelograms described on lines representing these two equal sorces; and the intensity of the same would, in like manner, be denoted by the diagonal of that parallelogram. Therefore, with the ship's head towards the north,the intensity would be greater than with the earth alone; with the ship's head at east or west, the intensity would be greater also in the ratio of v2 to 1, ut when the head was towards the south, the iron of the *el being opposed in power to that of the earth, the action of the needle is destroyed, and is wholly inactive and unseroble for the purposes of navigation. Thus, let NE (Fig. 6. Pl. I.) represent the force which the earth exercises on the needle, and NS that belonging to the iron of the vessel, *NL the resultant of the two; it is obvious, after the an: #e SNE exceeds a certain quantity, that the resultant NL is * than either of the single forces, and which soon becomes **mall to bring the needle home to its proper direction. utby opposing to the force NS, another equal force due to the plate, as NP (Fig. 7.), then the resultant of the three forces is the single terrestrial force NE, and the needle will be as *...to move as if no iron were in its vicinity. o role property of the correcting plate leads Lieuin o oster to form the most sanguine expectations, that, c **nsuing voyage, he will have the satisfaction to see the **, during the passage of the Hecla and Fury through i. Sound and Barrow's Straits, perfectly active, where *to it has been stowed below as completely unserviceable. **ust here conclude our account of these important experiments with the following extract from Mr Barlow's reort : “The importance of this principle of correction, even for the purposes of keeping the reckoning at sea, is sufficiently demonstrated in the two cases given by Lieutenants Mudge and Foster, (pages 219, & 224.), where, in the former, the error by the common compass course was nineteen miles in latitude, and twenty-eight in longitude; while, by the corrected compass course, the error was reduced to two miles in latitude, and four in longitude; and in the instance furnished by Lieuten ant Foster, the error in latitude alone was thirty-five miles in a run of only fifty, which almost wholly disappeared on the corrected course.” “I am aware,” says the author, “that seamen depend very little upon the reckoning by compass, while they can make the requisite astronomical observations; but as it frequently happens that many days may pass without their obtaining such observations, it cannot but be of considerable importance to them, in such cases, to possess the means of approximating the nearest possible to their true place. It is not, however, at sea that this method is of greatest use; it is in narrow channels, in piloting ships by means of charts and bearings, and in marine surveying, that it finds its most valuable application: in these instances nothing can supply the place of the compass, and it cannot but be important in such cases that its directive power should be freed from all irregularity.” “Every reader, whether a nautical man or not, must be aware of the great amount of error, and fatal consequences, which might arise in a few hours to a vessel in the channel, in a dark and blowing night, having for its only guide a compass subject to an error of 14 degrees in opposite directions at east and west, the very courses on which she would be endeavouring to steer; and who can say how many of the mysterious wrecks which have taken place in the Channel are to be attributed to this source of error, of which the most recent, that of the Thames, Indiaman, is a serious example. This vessel, besides the usual materials, guns, &c. had a caro of more than 400 tons of iron and steel; and it may easily pe imagined, that such a cargo would produce an effect on the compass at least equal to the Griper and Barracouta; and this alone would be quite sufficient to account for the otherwise unaccountable circumstance, that after having *. head in sight at 6 o'clock in the evening, the vessel should have been wrecked upon the same spot at 1 or 2 o'clock in the morning, without the least apprehension of being at all near shore.


“The subjects are unquestionably deserving of the attention of the first maritime nation in the world; and I am willing to hope, that the labour and attention I have bestowed on this inquiry for the last five years, will be found advantageous to nautical science, and entitled to the favorable consideration of those public boards, who are its natural patrons and protectors.”

We are gratified in having to state, that the Board of Longitude has expressed its opinion of the importance of these discoveries, by conferring on Mr Barlow the largest premium (£500) allowed by the late Longitude Act; at the same time stating, that this sum is not to be considered as any remuneration for the time and expenses bestowed upon the inquiry, which is recommended to be considered by the Navy and Admiralty Boards, as distinct from the above reward. W. doubt not that the other public Boards, independent of Government, but interested in the progress of navigation, will express their opinion of Mr Barlow's labours by similar marks of approbation.

Aster this sheet had actually been prepared for going to . we learnt that Professor Barlow and Lieutenant Foster ad instituted a very careful set of experiments, on board both the Hecla and Fury, first without, and then with, the Plate. We understand that the results give the strongest confirmation of all that has been said above, and that the Plate affords a perfect correction; but we have not succeeded in obtaining the details, which, however, we hope to lay before our readers in the next Number.

We shall wind up with one observation, addressed to the commercial public-namely, that, if any vessel be in future allowed to go to sea, and especially, to high latitudes, without the precautions so clearly pointed out by Mr Barlow, the loss both of property and of lives, in the event of shipwreck, may in most cases, as we conceive, be fairly attributable to the owners.

In answer to a letter addressed to Professor Barlow, on the subject of his Plate, respecting which a great degree of interest has been expressed at Leith, Glasgow, and other ports in

[ocr errors]

Scotland, he has informed us, that at present his Plate, with complete instructions for its use, will soon be ready for sale, at Messrs W. and T. Gilbert's, in London; and that he would recommend to them to correspond with their agents at the different sea-ports, in order to make some arrangements for the supply of all ships. In the mean time, he authorizes us to announce, that he is preparing for publication, in a popular form, a minute description of the method of trying the experiments, and of affixing the Plate. We need scarcely promise our readers to give our pages gladly to the circulation of instructions, contributing so directly to the improvement and security of navigation.

ART. XXIV.-Experiments on the Application of Professor Dabereiner's recent Discovery, to Eudiometry.” By Edward TURNER, M.D., Fellow of the Royal College of Physicians, and Lecturer on Chemistry, Edinburgh. [Edin. Philos. Jour.]

Having observed with equal pleasure and surprise the singular property of spongy platinum, recently discovered by Professor Doebereiner, it occurred to me to investigate whether this discovery was to afford simply an amusing experiment, or to confer a benefit on science, by the addition of a new agent in analytical chemistry. My preliminary trials were flattering, and I therefore resolved to enter on the investigation with all possible regard to accuracy.

Pure platinum was dissolved in the usual way, and precipitated by a solution of sal-ammoniac; the yellow precipitate, on being ignited, gave metallic platinum in the form of a delicate porous mass. When a jet of hydrogen gas was thrown upon this mass, it became instantly red hot, and in a few seconds the heat was so intense as to inflame the hydrogen. The effect was greatly augmented, by throwing a jet of oxygen upon the platinum at the same time. A most intense white heat was then produced, accompanied by vivid combustion of the hydrogen, forming a splendid and striking experiment.

If the platinum be brought in contact with a mixture com.

* Read before the Royal Society of Edinburgh, on the 5th of April and 3d of May 1824.

posed of two volumes of hydrogen to one of oxygen, instantaneous action ensues; the metal becomes red hot, and an explosion follows. Of course, it would be hazardous in the extreme to try this experiment on a considerable quantity of gas. The explosion does not occur immediately on the introduction of the metal into the mixed gases; the combination takes place at first silently and gradually, though rapidly; the metal is in consequence heated, then becomes dull red, at last passes to bright redness, and at the same moment the remainder of the gases unites with an explosion. This succession is commonly so rapid as to escape observation, but I shall afterwards relate an experiment which shows the order distinctly. It hence appears that the platinum differs somewhat in its mode of action from electricity; the latter agent causes the whole mass of gases to unite at once, and an explosion is the necessary consequence; whereas the platinum causes them to j. not by its primary action, but through means of the heat disengaged by that action. Thus, although platinum, in its state of greatest activity, will generally cause an explosion, when brought in contact with 4th of a cubic inch of a very pure explosive mixture; yet, if the quantity be still less, there is no explosion, since, before heat enough is developed for this effect, the whole gas is consumed. When spongy platinum is put into a mixture of hydrogen and atmospheric air, a copious dew condenses on the surface of the mercury, and this effect was observable, even when the hydrogen bore a small proportion to the whole quantity of gas operated on. Several very interesting points of inquiry were suggested by this fact; but, before proceeding to their investigation, I found it necessary to avoid certain inconveniences attached to the employment of spongy platinum, as prepared by the ordinary process. Its delicacy, for instance, is such, that it often falls to pieces from the pressure necessary to pass it under the mercury; or, at all events, its Porous texture is so much injured from this cause, as to render its action irregular. Another serious objection to its employment in this state, is the facility with which the mercury acts upon it, particularly when heated. I observed this fact in a very marked manner on the following occasion. I had warmed the platinum gently with the view of increasing its energy; but it was no sooner plunged under the mercury than an amalgam formed, which sunk instantly to the bottom of the trough. On driving off the mercury by heat, l obtain"d the platinum in a very altered condition; for it was firm,

« 이전계속 »