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lished, it occurred at once, that a thin iron plate of invention as this, so sound in principle, so easy of 5 lbs. or 6 lbs. weight, might be made to represent application, and so universally beneficial in practice, and to counteract the whole amount of the attrac- needs no testimony of mine to establish its merits; tion of the vessel, and thereby leave the needle per- but when I consider the many anxious days and fectly undisturbed; the action of the ship and that sleepless nights which the uselessness of the comof the plate, as regards their action on the needle, pass in these seas had formerly occasioned me, I neutralizing each other.
really should esteem it a kind of personal ingrati. Mr. Barlow having laid his proposition before tude to Mr. Barlow, as well as great injustice to the Admiralty, orders were given for experiments so memorable a discovery, not to have stated my being made in different parts of the globe, -it hav- opinion of its merits, under circumstances so well ing been expected by some, that the laws which had calculated to put them to a satisfactory trial.” been laid down were of a local kind, and that they The principle upon which the action of the plate would be found to change, like every thing else depends is as simple as its effects are efficacious. which had yet been observed of a magnetic needle If we conceive all the action of the ship’s iron to in different parts of the earth. Those, however, be concentrated in one general focus, we obtain the who could appreciate the investigations of the au- resultant of all these actions, as in all other cases thor, felt no doubt on this point.
of compound systems of forces; and it resulted from The correcting plate was accordingly placed un. the experiments made by Mr. Barlow, that this cender the superintendance of captain Bartholomew tre would remain constant in all parts of the world, and captain Baldy, on the east coast of Africa; un- whatever changes might take place in the dip, inder that of captain Basil Hall, in his long and in- tensity, and variation of the needle. And hence, if teresting voyage to the western parts of South another mass of iron could be placed in a corresAmerica; under captain Clavering, in his voyage ponding situation on the opposite side of the needle, to Spitzbergen; under captain Lyons, in Hudson's another centre of force would be produced, which bay; and under sir Edward Parry, in his voyage to would counteract the former, and thus leave the Lancaster sound; stretching thus from Lat. 60° S. needle as free as if no iron were in its vicinity. to Lat. 80° N., and in every case the experiments This, however, would have been impracticable, had were found entirely successful. Indeed the success it not been for the discovery to which we have alof the plate in the northern voyages went beyond luded, namely, that the magnetic power of iron boMr. Barlow's anticipations, it having been found dies is resident on the surface only. Availing himby lieutenant Foster (now captain Foster) that the self of this property, Mr. Barlow found that by plate not only corrected the compass, but likewise using an iron plate about fourteen inches diameter, enabled it to traverse, when without the plate it he was able to obtain sufficient power to correct the had lost all its magnetic powers. This happened local attraction of any ship in the British navy, al. when the ship’s head was to the southward: be- though in some the disturbance of the needle from cause in this, the iron drawing the needle south- the iron is known to amount to more than a point ward, while the little remaining powers of the of the compass, in opposite directions, making an earth drew it northward. These forces destroying extreme difference of two points or 22o. This takes each other, the compass became useless; but when place in England, where ihe dip of the needle is the plate was applied, as this neutralized the ship's about 70°; but as the vessel proceeds to the north, attraction, that of the earth remained effective, and the dip increases, and the deviation amounts, in the compass took up its proper direction.
many cases, to four points, as was the case in the In the account of sir Edward Parry's voyage in voyage of Sir Edward Parry, above referred to. 1825, to Lancaster sound, he says, speaking of this The description of the plate is thus given by Mr. property, “ The plate thus placed was now to un- Barlow: “It consists of two thin circular plates of dergo a severe trial on the ship's arrival in Bar- iron screwed together, in such a manner as to comrow's straits, and Prince Regent's inlet, where from bine any strong irregular power of one plate with the extraordinary increase of dip, and the conse a corresponding weak power of another, by which quently augmented effect of the ship's iron upon means a more uniform action is obtained. The iron the magnetic needle, the compass had before been commonly used is that weighing about three pounds rendered wholly useless on board ship. Never had to the square foot; and to prevent any accidental an invention a more complete and satisfactory tri- bending, and to give some thickness without much umph; for, to the last moment of our operations at increasing the weight, a thin light piece of board sea, did the compass indicate the true magnetic di. is interposed between them. The method of screwrection, requiring of course at times a considerable ing the two plates is as follows: a central hole is degree of tapping with the hand merely, to relieve cut in both plates about an inch in diameter, and the needle from friction. And even at Port Bowen, into this is inserted a' brass socket, three inches where the dip is 88°, and the magnetic intensity long, having a broad flanch at one end, and an exacting on a horizontal needle extremely weak in ternal screw at the other, with a brass nut; by which consequence, the azimuth compass on board actu. means the plates are compressed very strongly in ally gave the same variation as that observed on the centre, and they are also held together at their shore, within the fair and reasonable limits of error circumference by six small brass nuts and screws. of observation under such circumstances. Such an The annexed figure shows the plate attached to the
origen y tripod stand of the azimuth Let the vessel now be brought to any one point
ab agam compass; but it is best, in- of the compass, and steadied there, while the bearscron stead of the tripod stand, ing of the distant object is taken and registered.
to have a fixed pedestal in This being done, bring her head to another point, og 11 any convenient place in the and repeat the observation ; do the same at every
avessel, carrying the com- point, or at least at the north, south, east, west, 2pass on its top, with the northeast, northwest, southeast, and southwest,
plate attached to its side.fi which will be sufficient in most cases.
Il The plate being thus de It will now be found that the bearings thus obPubscribed, the method of fixa served will differ from each other according to the
ing it, and making the ex- attracting powers of the vessel, from 6° or go to
periments, will be best un- 26° or 28°: a difference which is caused by the iron derstood from the following directions, supplied by of the ship attracting the needle out of its proper Messrs. W. and T. Gilbert, Leadenhall street, with direction,-generally the north end going to the the plates made by them.
eastward (in these latitudes) with the ship's head
towards the east, and to the westward, with the Directions for fixing Barlow's Correcting Plate. ship’s head to the west; but the contrary in south
ern latitudes, or where the dip is to the southward.* The local attraction of a ship may be found in
On examining these several bearings, there will several different ways, viz. by observations on a dis- be found two at opposite points of the compass (and tant object, or by reciprocal and simultaneous ob- commonly those made when the ship’s head is north servations between an observer on board and an- and south) that are very nearly the same, the mean other on shore, or by one observer on board. In of which must be accounted the true magnetic bearthe second case, the distance of the object is of no ing of the object: and the difference between this importance; but in the former, it must be such that bearing and the several bearings at the other points, the length of the ship and cable shall produce a will be the local attraction at each. In general the parallax of only a few minutes of a degree; it ought errors increase both ways, as the ship's head passes not, therefore, to be less than six or seven miles. It should also be observed, that the local attrac- and commonly with the ship’s head towards
from the north and south towards the east and west, tion of a ship is different in every different point in her, and therefore the observations should be made
The southwesterly from some selected spot proper for taking all azi.
bearings and the are less
than muth and amplitude observations in, during the The east, northeasterly
the voyage; and in which spot a compass can remain
The northwesterly are great truth; for conning the vessel, and for correcting the course
i and southeasterly ) shown on the binnacle-compasses. In all cases it is necessary at sea, in working a traverse, to know And with the ship's head towards the actual variation; and this being found correctly
The southwesterly by means of the plate at the fixed station, it is as easy
bearings and the
than to allow for the variation and local attraction to the west, northeasterly
the gether on the binnacle-compass, as for variation
truth; alone: the errors, therefore, at the binnacle will be
are less corrected by the plate. For example, if after the variation has been determined, it be necessary that
And in marking the local attractions, the four the course should be east by compass, and that latter cases are denoted by (+), and the former by when the correcting compass shows the course to (-); because in the latter the apparent westerly be east, the binnacle compass shows east-by-north, variation is greater, and the former less, than the then east-by-north will be the course to be observed true variation. by the man at the wheel,—but the actual course of
The following example will in some measure ilthe vessel will be accounted east. This being pre- lustrate the above directions. mised, the following directions for ascertaining the local attraction of the vessel at the fixed station Observations on the Bearing of a Distant Object in should be attended to.
H. M. S. ISABELLA, with the view of ascertaining
the amount of her Local Attraction. To ascertain the Local Attraction by a distant object.
Bearing of Object.
Attraction. recting compass being selected by the captain, and a well defined distant object being chosen, the ship
North N. 51° 26' E. +1° 36' moreover being moored or lying with a short scope N. by E.
to 36 of cable, with anchors so arranged as to admit of
9 her head being brought to each point of the com N.E. by N.
-1 59 pass, every thing is ready for observation.
There are some instances, as in H. M. steam vessel Comet, where the north end of the needle is drawn ast 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.
so that if one be north 350 east, the other ought to
be south 35° west; and when these two bearings do Direction of a
not so agree, the difference shows the local attrac
tion at each point respectively. N.E. by E. 46
To ensure the two observations being made at
the same time, when the ship's head is steady upon E. by N. 45 56
a point, the ensign, or any other flag proposed for East 45 26
the signal, is dipped or drooped: the observer on E. by S. 44 26
shore then keeps the station on board fixed on the E.S.E.
line of his sights, and at the moment when the obS.E. by E.
server on board registers his observation, the flag
is hoisted, which is the signal for the shore obS.E. by S. 45 36
14 server reading and registering his bearing;-both S.S.E. 46 26
also noting the time by two watches set to each S. by E.
other. This latter precaution is intended to pre-
vent any mistake in comparing the simultaneous S. by W. 48
observations, which may otherwise happen, by the S.S.W.
signal being misunderstood, or not seen. S.W. by S. 50 26
The following is an example of observations made S.W. 51 11 +1 21
according to this method, on board H.M.S. Hecla, S.W. by W. 52 56
6 Captain Parry, May 8th, 1824. W.S.W. 53 56
6 W. by S. 54
Local Attraction of H.M.S. HECLA.
Direction of Bear'g of shore Bearing of ship Local W. by N. 55
Ship's Head. stat’n from ship. froin shore. Attraction. W.N.W. 55 46
+5 56 NW. by W.
S. 419 O'W.N.40°50'E. +0°10'
N. by E. 42 20 43 54 -1 34 N.W. by N. 54 26
45 51 -3 51
N.E. by N.
50 38 4 38 N.E. by E. 44 10
50 36 -6 26 E.N.E.
43 10 49 33 -6 23 Here the two nearest opposite bearings are at
E. by N. 40 50
47 29 N.N.E. and S.S.W., of which the mean is 49° 50';
-6 39 East 30 56
43 28 6 38 this, therefore, is accounted the true bearing, and
E. by S. 34 0
40 59 the difference between 49° 50' and the bearing at
-6 59 E.S.E. 30 20
3 each point, is the local attraction at each respec
S.E. by E. 28 O
33 39 -5 39 tively, as registered in the 3d and 6th columns.
S.E. 25 40
30 24 -4 44 Also the attraction being nothing, or nearly no
S.E. by S. 27 50
3 11 thing, with the ship's head at N.N.E., the line of
S.S.E. 29 40
32 0 no attraction passes from the compass over the
-2 20 $. by E. 30 0
31 30 larboard bow, making an angle with the keel of
- 1 30 South 37 0
36 58 two points, and in this line produced aft the cor
to 2 recting plate is ultimately to be fixed..
S. by W.
34 53 +3 32
S.W. by S. To find the Local Attraction by simultaneous Obser
S.W. 42 20
36 30 + 3 50 vations on board and on shore.
S.W. by W. 0
38 30 +5 30 In this case, the shore station may be taken as
W.S.W. 46 10 39 46 +6 24 near to the vessel as may seem convenient; here a W. by S. 47 20
+6 32 delicate azimuth compasst must be placed, and an.
West 47 0
+6 11 other at the station on board. The ship's head W. by N. 49
41 10 +7 50 being then brought by warps, &c., as in the last
W.N.W. 49 50
42 49 +6 51 case, steadily to each point, the observer on board N.W. by W. 49 40
42 58 +5 42 registers the bearing of the station on shore, and N.W.
43 52 +5 8 the observer on shore the bearing of the station on N.W. by N. 47.0 43 24 +3 38 board, at the same time, which, independently of
N.N.W. 45 30
+2 46 local attraction, ought to be diametrically opposite, N, by W. 43 10
41 36 +1 34 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 bidden 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 swing. ing 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.
Method of fixing the Plate.
case. It will of course, be understood that the brass The local attraction being determined by either
conical part - sent with the plate is to be screwed of the above methods, take the mean of the two
upon the pedestal or compass-stand, to serve as a deviations when the line of no attraction is N.E.
socket for the brass pin which carries the plate ; and N. W., the mean of the two at east and west, and that when the place for the plate is determined, and the mean of the two at S. E. and S.W. In the a hole is to be drilled through the brass pin, to cor
respond with the hole in the socket, into which a present case these would be
smaller pin is inserted to keep the plate to its place. Mean at N. E. and N. W.
We have inserted this table blank; but with any
40 53' Do. at E. and W.
particular plate it is filled up with written figures,
6 Do. at S.E. and S.W.
as observed from actual experiment on that plate. Look for three corresponding or the nearest local Attractions, determined experimentally, of Plate to
No. 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
10 of no attraction, which in the last example, and in
11 11 the generality of cases, is fore and aft; but in the
15 first example of the Isabella, it is in a line passing
15 from the compass at an angle of two points with
15 16 the keel of the vessel over the larboard bow.
1517 in In this line of no attraction, and at the depth and
1115 an distance as above described, the plate may be fixed
16 13 either fore or aft of the compass; but the latter is
12.12 best, particularly in northern voyages, because when
1616 thus situated, it gives considerable freedom to the needle, and causes it to traverse where it would
12 15 16 17 otherwise be useless for want of directive power;
13 01212 17 13 and the action of the iron being neutralized by the
60 13 13 1714 plate, the bearing of the needle is always correct
17 15 while the latter is in its place. When it is placed
to 13 | 15
13 16 before the compass, the plate is only used occasion
17 ally, its attraction is the same as the ship's, but it
| 14 | 13in2 | 1814 is in the same direction; by applying it therefore at any time, the amount of the attraction may be as
14 15 18 16 Job certained, but it is not neutralized as in the former
14 | 16 ans 18 | 17
VARNISH, is the name of a fluid substance, con 40 grains, half a dram of saffron, and 40 oz. of good sisting of a gum or resin dissolved in some fuid, : spirit of wine. and used by painters, &c. to give an artificial pol. One of the most useful varnishes is caoutchouc ish to their works." The most common varnishes', varnish, made of caoutchouc dissolved in kyapootee are, amber varnish, made with amber, drying oil, or cajeput oil, or, what is much cheaper, in naph." and oil of turpentine; , mastich varnish, made with tha prepared from coal tar. Its utility in rendering spirit of turpentine and gum mastich; lac varnish,' cloth, leather, &c. waterproof, is very great, and made of gum lac dissolved in spirit of wine; and the manufacture of such cloths has been carried gold coloured or lacquer varnish, which is made to great perfection by our able countryman, Mr. of gum lac 2 02., yellow amber 20z., dragon's blood Charles Mackintosh, of Glasgow. The black var
nish of the East, the nature of which was long un- after coming in contact with the external air. In known, has been found to be the juice of a large Dr. Brewster's Journal of Science, vol. viii, page 96 tree, now known by the name of Melanorrhea usi- and 100, there are two interesting articles, containtata, a new genus established by Dr. Walker. Some ing valuable information concerning the varnish of these trees are 40 feet high, with stems 12 feet produced by our tree, and its deleterious effects on long, and 11 in girth four feet above the ground. the human frame. It is a curious fact, that, to my Captain Grant speaks of some as having clear certain knowledge, the natives of the countrie's stems of 42 feet to the first branch.
where the tree is indigenous, never experience any The following interesting account of it is given injurious consequences from handling its juices: it by Dr. Wallich:*
is strangers only that are sometimes affected by it, " As long since as the year 1812, the late Mr. especially Europeans. Both Mr Swinton and 'myM. R. Smith, for nearly forty years an inhabitant self have frequently exposed our hands to it withof Sillet, and during the latter part of that long out any serious injury. I have even ventured to period a zealous contributor to the Honourable taste it, both in its recent state and as it is exposed Company's botanic garden at Calcutta, furnished for sale at Rangoon, and have never been affected some very curious information concerning our tree by it. It possesses very little pungency, and is ento Mr. H. Colebrooke, then in charge of that insti- tirely without smell. I know, however, of instances tution. He must therefore be considered as the first where it has produced extensive erysipelatous person who brought this valuable tree into notice, swellings, attended with pain and fever, but not of although he failed in his endeavours to procure long duration of this description was the effect either dried specimens or fresh seeds of it. I shall it had on the late Mr Carey, a son of the Rev. Dr. here subjoin some of his remarks.
W. Carey, who resided several years in the Burma I have discovered a sort of varnish, which I empire. Among the people who accompanied me consider as the identical one made use of by the to Ava, both Hindoos and Mahomedans, no acciChinese in their eastern and northeastern provinces. dent happened, although they frequently touched It is procurable, in great quantities, from Munipur, the varnish, except in a slight degree to one of my where it is used for paying river crafts, and for var- assistants, whose hand swelled and continued painnishing vessels destined to contain liquids, such as ful during two days. Dr Brewster informs me that, oil, ghee (clarified butter), milk, honey, or water. after resisting its effects for a long time, it at length The drug is conveyed to Sillet for sale, by the mer- attacked him in the wrist with such violence that chants, who come down annually with horses and the pain was almost intolerable. It was more acute other objects of trade.' The tree which yields it than that of a severe burn, and the Doctor was grows to an amazing size. I am informed that it obliged to sleep several nights with his hand imattains one hundred cubits in height, and twenty mersed in the coldest water. He considers it as a in circumference, and even more. It forms exten- very dangerous drug to handle. One of his sersive forests, which commence at a distance of three vants was twice nearly killed by it. days' journey from the capital, and stretch in a In the neighbourhood of Prome a considerable northerly and easterly direction towards China for quantity of varnish is extracted from the tree; but many miles.'
very little is obtained at Martaban, owing, as I am That the Khevi which Mr Smith describes is the told, to the poverty of the soil, and parlly also to same as that found by captain Grant, there cannot the circumstance of there being nonc of the people be any doubt; nor that it is identical with the in that part whose business it is to perform the Theet-tsee, or Varnish-tree of the Burmese. It fol- process. This latter is very simple: short joints lows, hence, that the tree has a very wide geo- of a thin sort of bamboo, sharpened at one end like graphical range, extending from Munipur (in lati a writing-pen, and shut up at the other, are inlude 25° N. and longitude 94o E.) to Tavoy (in serted in a slanting direction into wounds, made Jatitude 14°, longitude 97°). The valley of Kubbu, through the bark of a trunk and principal boughs, which has been ascertained by actual survey, made and left there for twenty-four and forty-eight hours, by lieutenant Pemberton, to be only five hundred after which they are removed, and their contents, feet above the plains of India, is distant two hun- which rarely exceed a quarter of an ounce, emptied dred miles from the nearest sea shore. The tree into a basket made of bamboo or rattan previously there attains its greatest size, and I believe it be- varnished over. As many as a hundred bamboos comes smaller as it approaches the sea on the coast are sometimes seen sticking into a single trunk of Tenasserim, where it grows in comparatively during the collecting season, which lasts as long as low situations.
the tree is destitute of leaves, namely, from January Our tree belongs to the Deciduous class, shed- until April; and they are renewed as long as the ding its leaves in November, and continuing naked juice will flow. A good tree is reckoned to prountil the month of May, during which period it duce from one and a half to two, three, and even produces its flowers and fruit. During ihe rainy four viss annually, a viss being equal to about 31 season, which lasts for five months, from the mid- lbs. avoirdupois. In its pure state it is sold at dle of May until the end of October, it is in full Prome at the rate of one tical, or 28. 6d. the viss. foliage. Every part of it abounds in a thick and At Martaban, where every thing was dear when I viscid greyish-brown fluid, which turns black soon 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.