lished, it occurred at once, that a thin iron plate of 5 lbs. or 6 lbs. weight, might be made to represent and to counteract the whole amount of the attraction of the vessel, and thereby leave the needle perfectly undisturbed; the action of the ship and that of the plate, as regards their action on the needle, neutralizing each other. Mr. Barlow having laid his proposition before the Admiralty, orders were given for experiments being made in different parts of the globe, it having been expected by some, that the laws which had been laid down were of a local kind, and that they would be found to change, like every thing else which had yet been observed of a magnetic needle in different parts of the earth. Those, however, who could appreciate the investigations of the author, felt no doubt on this point. The correcting plate was accordingly placed under the superintendance of captain Bartholomew and captain Baldy, on the east coast of Africa; under that of captain Basil Hall, in his long and interesting voyage to the western parts of South America; under captain Clavering, in his voyage to Spitzbergen; under captain Lyons, in Hudson's bay; and under sir Edward Parry, in his voyage to Lancaster sound; stretching thus from Lat. 60° S. to Lat. 80° N., and in every case the experiments were found entirely successful. Indeed the success of the plate in the northern voyages went beyond Mr. Barlow's anticipations, it having been found by lieutenant Foster (now captain Foster) that the plate not only corrected the compass, but likewise enabled it to traverse, when without the plate it had lost all its magnetic powers. This happened when the ship's head was to the southward: because in this, the iron drawing the needle southward, while the little remaining powers of the earth drew it northward. These forces destroying each other, the compass became useless; but when the plate was applied, as this neutralized the ship's attraction, that of the earth remained effective, and the compass took up its proper direction. In the account of sir Edward Parry's voyage in 1825, to Lancaster sound, he says, speaking of this property, "The plate thus placed was now to undergo a severe trial on the ship's arrival in Barrow's straits, and Prince Regent's inlet, where from the extraordinary increase of dip, and the conse quently augmented effect of the ship's iron upon the magnetic needle, the compass had before been rendered wholly useless on board ship. Never had an invention a more complete and satisfactory triumph; for, to the last moment of our operations at sea, did the compass indicate the true magnetic direction, requiring of course at times a considerable degree of tapping with the hand merely, to relieve the needle from friction. And even at Port Bowen, where the dip is 88°, and the magnetic intensity acting on a horizontal needle extremely weak in consequence, the azimuth compass on board actually gave the same variation as that observed on shore, within the fair and reasonable limits of error of observation under such circumstances. Such an invention as this, so sound in principle, so easy of application, and so universally beneficial in practice, needs no testimony of mine to establish its merits; but when I consider the many anxious days and sleepless nights which the uselessness of the compass in these seas had formerly occasioned me, I really should esteem it a kind of personal ingrati tude to Mr. Barlow, as well as great injustice to so memorable a discovery, not to have stated my opinion of its merits, under circumstances so well calculated to put them to a satisfactory trial." The principle upon which the action of the plate depends is as simple as its effects are efficacious. If we conceive all the action of the ship's iron to be concentrated in one general focus, we obtain the resultant of all these actions, as in all other cases of compound systems of forces; and it resulted from the experiments made by Mr. Barlow, that this centre would remain constant in all parts of the world, whatever changes might take place in the dip, intensity, and variation of the needle. And hence, if another mass of iron could be placed in a corresponding situation on the opposite side of the needle, another centre of force would be produced, which would counteract the former, and thus leave the needle as free as if no iron were in its vicinity. This, however, would have been impracticable, had it not been for the discovery to which we have alluded, namely, that the magnetic power of iron bodies is resident on the surface only. Availing himself of this property, Mr. Barlow found that by using an iron plate about fourteen inches diameter, he was able to obtain sufficient power to correct the local attraction of any ship in the British navy, although in some the disturbance of the needle from the iron is known to amount to more than a point of the compass, in opposite directions, making an extreme difference of two points or 22°. This takes place in England, where the dip of the needle is about 70°; but as the vessel proceeds to the north, the dip increases, and the deviation amounts, in many cases, to four points, as was the case in the voyage of Sir Edward Parry, above referred to. The description of the plate is thus given by Mr. Barlow: "It consists of two thin circular plates of iron screwed together, in such a manner as to combine any strong irregular power of one plate with a corresponding weak power of another, by which means a more uniform action is obtained. The iron commonly used is that weighing about three pounds to the square foot; and to prevent any accidental bending, and to give some thickness without much increasing the weight, a thin light piece of board is interposed between them. The method of screwing the two plates is as follows: a central hole is cut in both plates about an inch in diameter, and into this is inserted a brass socket, three inches long, having a broad flanch at one end, and an external screw at the other, with a brass nut; by which means the plates are compressed very strongly in the centre, and they are also held together at their circumference by six small brass nuts and screws. The annexed figure shows the plate attached to the tripod stand of the azimuth compass; but it is best, inom stead of the tripod stand, to have a fixed pedestal in any convenient place in the vessel, carrying the compass on its top, with the plate attached to its side. The plate being thus described, the method of fix ing it, and making the experiments, will be best understood from the following directions, supplied by Messrs. W. and T. Gilbert, Leadenhall street, with the plates made by them. Directions for fixing Barlow's Correcting Plate. The local attraction of a ship may be found in several different ways, viz. by observations on a distant object, or by reciprocal and simultaneous ob servations between an observer on board and an- not, therefore, to be less than six or seven miles. Let the vessel now be brought to any one point of the compass, and steadied there, while the bearing of the distant object is taken and registered. This being done, bring her head to another point, and repeat the observation; do the same at every point, or at least at the north, south, east, west, northeast, northwest, southeast, and southwest, which will be sufficient in most cases. It will now be found that the bearings thus observed will differ from each other according to the attracting powers of the vessel, from 6° or 8o to 26° or 28°: a difference which is caused by the iron of the ship attracting the needle out of its proper direction, generally the north end going to the eastward (in these latitudes) with the ship's head towards the east, and to the westward, with the ship's head to the west; but the contrary in southern latitudes, or where the dip is to the southward.* On examining these several bearings, there will be found two at opposite points of the compass (and commonly those made when the ship's head is north and south) that are very nearly the same, the mean of which must be accounted the true magnetic bearing of the object: and the difference between this bearing and the several bearings at the other points, will be the local attraction at each. In general the errors increase both ways, as the ship's head passes from the north and south towards the east and west, It should also be observed, that the local attrac- the actual variation; and this being found correctly by means of the plate at the fixed station, it is as easy The east, are less than >the The northwesterly are great- truth; Land southeasterly S The southwesterly er are great- than er are less >the truth; And in marking the local attractions, the four latter cases are denoted by (+), and the former by (-); because in the latter the apparent westerly variation is greater, and the former less, than the to allow for the variation and local attraction to- The west, To ascertain the Local Attraction by a distant object. The proper place for using the azimuth and correcting compass being selected by the captain, and a well defined distant object being chosen, the ship moreover being moored or lying with a short scope of cable, with anchors so arranged as to admit of her head being brought to each point of the compass, every thing is ready for observation. true variation. The following example will in some measure illustrate the above directions. Observations on the Bearing of a Distant Object in H. M. S. ISABELLA, with the view of ascertaining the amount of her Local Attraction. Local Attraction. • There are some instances, as in H. M. steam vessel Comet, where the north end of the needle is drawn aft instead of forward, from the effect of the iron chimney, and by being placed too near the iron work a bast. VOL. XVIII.-PART I. 2 A⭑ Direction of Ship's Head. -4. 49 24 2 54 54 so that if one be north 35° east, the other ought to be south 35° west; and when these two bearings do not so agree, the difference shows the local attraction at each point respectively. To ensure the two observations being made at the same time, when the ship's head is steady upon a point, the ensign, or any other flag proposed for the signal, is dipped or drooped: the observer on shore then keeps the station on board fixed on the line of his sights, and at the moment when the observer on board registers his observation, the flag is hoisted, which is the signal for the shore observer reading and registering his bearing,both also noting the time by two watches set to each other. This latter precaution is intended to prevent any mistake in comparing the simultaneous observations, which may otherwise happen, by the signal being misunderstood, or not seen.‡ The following is an example of observations made according to this method, on board H. M. S. Hecla, Captain Parry, May 8th, 1824. Local Attraction of H.M. S. HECLA. Bear'g of shore Bearing of ship Local Attraction. N.W. by N. +4 36 N.N.E. -1 34 -3 51 N.E. by N. +2 36 Here the two nearest opposite bearings are at N.N.E. and S.S.W., of which the mean is 49° 50'; this, therefore, is accounted the true bearing,-and the difference between 49° 50' and the bearing at each point, is the local attraction at each respectively, as registered in the 3d and 6th columns. Also the attraction being nothing, or nearly nothing, with the ship's head at N.N.E., the line of no attraction passes from the compass over the larboard bow, making an angle with the keel of two points, and in this line produced aft the correcting plate is ultimately to be fixed.* To find the Local Attraction by simultaneous Observations on board and on shore. In this case, the shore station may be taken as near to the vessel as may seem convenient; here a delicate azimuth compasst must be placed, and another at the station on board. The ship's head being then brought by warps, &c., as in the last case, steadily to each point, the observer on board registers the bearing of the station on shore, and the observer on shore the bearing of the station on board, at the same time, which, independently of local attraction, ought to be diametrically opposite, This case (which corresponds nearly with the observations in the Isabella, as given by Captain Sabine) is selected as an example of what sometimes, but very seldom happens, viz. that the line of no attraction lies oblique to the keel of the vessel. In all the ships that I have experimented upon, the line of no attraction is fore and aft.-P. Barlow. The only ship-compass sufficiently delicate for these observations, is the patent azimuth compass, by Messrs. W. and T. Gilbert, 148, Leadenhall street. It may sometimes happen, that in vessels with poops, or where the bulwarks are very lofty, the shore station cannot be seen in all positions of the vessel. In this case a second station may be taken on shore, when the first is thus hidden from the observer on board, or the bearing of the sun may be taken by both observers at the same instant; then the difference in these bearings will be the local attraction. In some cases the officers have preferred taking the bearings of the sun; observing also the time, while the ship is swinging to the tide, as her head is passing through N.E., E., S.E., &c. points, and afterwards computing the true azimuths, thus substituting calculations instead of the direct observation above described. This method of course requires no shore station. "Look for three corresponding or the nearest local attractions in any one line in the following table, filled up with written figures sent with the plate, and opposite to them, in the first two columns, stand the proper depth and distance that the plate is to have with respect to the compass, that is, the first column shows the depth, in inches, the centre of the plate is to be fixed below the pivot of the needle; and the second, the distance it is to be placed from a plumb-line falling from the centre of the needle,-observing always to place it in the line" of no attraction, which in the last example, and in the generality of cases, is fore and aft; but in the first example of the Isabella, it is in a line passing from the compass at an angle of two points with the keel of the vessel over the larboard bow. In this line of no attraction, and at the depth and distance as above described, the plate may be fixed either fore or aft of the compass; but the latter is best, particularly in northern voyages, because when thus situated, it gives considerable freedom to the needle, and causes it to traverse where it would otherwise be useless for want of directive power; and the action of the iron being neutralized by the plate, the bearing of the needle is always correct while the latter is in its place. When it is placed before the compass, the plate is only used occasionally, its attraction is the same as the ship's, but it is in the same direction; by applying it therefore at any time, the amount of the attraction may be ascertained, but it is not neutralized as in the former! case. It will of course be understood that the brass conical part sent with the plate is to be screwed upon the pedestal or compass-stand, to serve as a socket for the brass pin which carries the plate; a hole is to be drilled through the brass pin, to corand that when the place for the plate is determined, respond with the hole in the socket, into which a smaller pin is inserted to keep the plate to its place. We have inserted this table blank; but with any particular plate it is filled up with written figures, as observed from actual experiment on that plate. Attractions, determined experimentally, of Plate No. VARNISH. VARNISH, is the name of a fluid substance, consisting of a gum or resin dissolved in some fluid, and used by painters, &c. to give an artificial polish to their works. The most common varnishes are, amber varnish, made with amber, drying oil, and oil of turpentine; mastich varnish, made with spirit of turpentine and gum mastich; lac varnish, made of gum lac dissolved in spirit of wine; and gold coloured or lacquer varnish, which is made of gum lac 2 oz., yellow amber 2 oz., dragon's blood 40 grains, half a dram of saffron, and 40 oz. of good spirit of wine. One of the most useful varnishes is caoutchouc varnish, made of caoutchouc dissolved in kyapootee or cajeput oil, or, what is much cheaper, in naphtha prepared from coal tar. Its utility in rendering cloth, leather, &c. waterproof, is very great, and the manufacture of such cloths has been carried to great perfection by our able countryman, Mr. Charles Mackintosh, of Glasgow. The black var nish of the East, the nature of which was long unknown, has been found to be the juice of a large tree, now known by the name of Melanorrhea usitata, a new genus established by Dr. Walker. Some of these trees are 40 feet high, with stems 12 feet long, and 11 in girth four feet above the ground. Captain Grant speaks of some as having clear stems of 42 feet to the first branch. The following interesting account of it is given by Dr. Wallich:* "As long since as the year 1812, the late Mr. M. R. Smith, for nearly forty years an inhabitant of Sillet, and during the latter part of that long period a zealous contributor to the Honourable Company's botanic garden at Calcutta, furnished some very curious information concerning our tree to Mr. H. Colebrooke, then in charge of that institution. He must therefore be considered as the first person who brought this valuable tree into notice, although he failed in his endeavours to procure either dried specimens or fresh seeds of it. I shall here subjoin some of his remarks. I have discovered a sort of varnish, which I consider as the identical one made use of by the Chinese in their eastern and northeastern provinces. It is procurable, in great quantities, from Munipur, where it is used for paying river crafts, and for varnishing vessels destined to contain liquids, such as oil, ghee (clarified butter), milk, honey, or water. The drug is conveyed to Sillet for sale, by the merchants, who come down annually with horses and other objects of trade. The tree which yields it grows to an amazing size. I am informed that it attains one hundred cubits in height, and twenty in circumference, and even more. It forms extensive forests, which commence at a distance of three days' journey from the capital, and stretch in a northerly and easterly direction towards China for many miles.' 399 That the Kheu which Mr Smith describes is the same as that found by captain Grant, there cannot be any doubt; nor that it is identical with the Theet-tsee, or Varnish-tree of the Burmese. It follows, hence, that the tree has a very wide geographical range, extending from Munipur (in latitude 25° N. and longitude 94° E.) to Tavoy (in latitude 14°, longitude 97°). The valley of Kubbu, which has been ascertained by actual survey, made by lieutenant Pemberton, to be only five hundred feet above the plains of India, is distant two hundred miles from the nearest sea shore. The tree there attains its greatest size, and I believe it be comes smaller as it approaches the sea on the coast of Tenasserim, where it grows in comparatively low situations. Our tree belongs to the Deciduous class, shedding its leaves in November, and continuing naked until the month of May, during which period it produces its flowers and fruit. During the rainy season, which lasts for five months, from the middle of May until the end of October, it is in full foliage. Every part of it abounds in a thick and viscid greyish-brown fluid, which turns black soon after coming in contact with the external air. In Dr. Brewster's Journal of Science, vol. viii, page 96 and 100, there are two interesting articles, containing valuable information concerning the varnish produced by our tree, and its deleterious effects on the human frame. It is a curious fact, that, to my certain knowledge, the natives of the countries where the tree is indigenous, never experience any injurious consequences from handling its juices: it is strangers only that are sometimes affected by it, especially Europeans. Both Mr Swinton and myself have frequently exposed our hands to it without any serious injury. I have even ventured to taste it, both in its recent state and as it is exposed for sale at Rangoon, and have never been affected by it. It possesses very little pungency, and is entirely without smell. I know, however, of instances where it has produced extensive erysipelatous swellings, attended with pain and fever, but not of long duration. Of this description was the effect it had on the late Mr Carey, a son of the Rev. Dr. W. Carey, who resided several years in the Burma empire. Among the people who accompanied me to Ava, both Hindoos and Mahomedans, no accident happened, although they frequently touched the varnish, except in a slight degree to one of my assistants, whose hand swelled and continued painful during two days. Dr Brewster informs me that, after resisting its effects for a long time, it at length attacked him in the wrist with such violence that the pain was almost intolerable. It was more acute than that of a severe burn, and the Doctor was obliged to sleep several nights with his hand immersed in the coldest water. He considers it as a very dangerous drug to handle. One of his servants was twice nearly killed by it. In the neighbourhood of Prome a considerable quantity of varnish is extracted from the tree; but very little is obtained at Martaban, owing, as I am told, to the poverty of the soil, and partly also to the circumstance of there being none of the people in that part whose business it is to perform the process. This latter is very simple: short joints of a thin sort of bamboo, sharpened at one end like a writing-pen, and shut up at the other, are inserted in a slanting direction into wounds, made through the bark of a trunk and principal boughs, and left there for twenty-four and forty-eight hours, after which they are removed, and their contents, which rarely exceed a quarter of an ounce, emptied into a basket made of bamboo or rattan previously varnished over. As many as a hundred bamboos are sometimes seen sticking into a single trunk during the collecting season, which lasts as long as the tree is destitute of leaves, namely, from January until April; and they are renewed as long as the juice will flow. A good tree is reckoned to produce from one and a half to two, three, and even four viss annually, a viss being equal to about 3 lbs. avoirdupois. In its pure state it is sold at Prome at the rate of one tical, or 2s. 6d. the viss. At Martaban, where every thing was dear when I was there, the drug was retailed at two Madras ru * See Dr. Brewster's Journal of Science, No. III. p. 66, New Series, which contains the whole of this valuable paper. |