On the Methods proposed for ascertaining the Longitudes and Latitudes of Places by Magnetical Instruments.

THE facility and readiness with which the longitudes and latitudes of places upon the surface of the earth might be ascertained, by means of magnetical instruments, if their performance could be depended upon, has at various times brought forward the proposals of instruments, and calculations made expressly for the purpose; but the actual experiments made on those plans have constantly shewn, that both the instruments and the calculations are insufficient to answer that object; and the principal reason of the failure is the uncertainty of the motion of the magnetic poles of the earth, upon which the variation of the compass principally depends.

That those poles (which may be considered to be the centres or focuses of all the magnetic bodies contained in the earth) do not remain fixed, but that they do actually move from place to place, is sufficiently evinced by the results of accurate observations; and no modern writer seems to entertain a doubt about it. The variation of the declination, or the change of variation, is principally attributed to the motion of those poles; but the difficulty consists in determining whether this motion is regular or irregular, viz. whether it may, or may not, be foretold by calculation, according to any rule what

soever.

That nature is regular in her works, and that every natural operation depends upon adequate causes, no person, who is at all acquainted with philosophy, can possibly deny. But when a certain phenomenon depends upon the combination of a variety of causes, some of which, and perhaps all of them, are out of the reach of our senses, and of calculation, we call it irregular or accidental; not for want of a natural dependance upon adequate causes, but because we are unable to discover and ascertain the laws of that dependance. Thus we know that the temperature of the atmosphere in London depends on the time of the year, on the point from which the wind blows, on the clearness of the air, &c. yet no one can foretel the precise degree of heat that will be indicated by the thermometer on a particular day of the next year; because, in the first place, we are not acquainted with all the concurring causes, and secondly, because, from the action of those causes upon each other, their ultimate effect upon the body of the earth becomes the result of an immense and incommensurable combination.

With respect then to magnetism, we must first endeavour to discover the causes which occasion the motion of the magnetic poles of the earth; and secondly, we must consider whether the effects of those causes may or may not be subjected to calculation.

The projectors of the methods for ascertaining the longitude and latitude by means of the variation compass, and of the dipping needle, generally frame hypotheses of regular movements, and establish upon them all their rules and calculations, and overlook the natural causes of irregularity or uncertainty. But hypotheses that are not founded upon a constant coincidence of effects, and especially when

they are insufficient to account for all the phenomena, cannot be considered as guides in the investigation of future events.-The magnetic poles of the earth have been supposed to be four in number; though they are at present generally, and more properly, thought to be only two. They have been supposed to reside on the surface of the earth, and to move upon it at a certain annual rate: They have been supposed to be fixed to a sort of nucleus within the earth, and to move along with it either from east to west, or from west to east: They have been supposed to be in the atmosphere; and in short, the conjectures have been very numerous; but let the hypothesis be what it will, we have no reason to believe that their motion is so regular as to be foretold, which will more evidently appear from the following

reasons.

In the investigation of natural properties, when they are out of the reach of actual observation or calculation, the imagination must start with, and be guided by the analogy of, established facts and laws; otherwise the probability of being right vanishes entirely.-Agreeably to this rule, if we attempt to form conjectures relative to the magnetic poles of the earth, we must in the first place say, that as the magnetic polarity has been found to be a property only of iron or ferruginous bodies, therefore it is likely that the magnetic poles of the earth reside not in the atmosphere, but in the ferruginous bodies contained in the earth. Secondly, we have no reason whatever to believe that the earth contains a moveable nucleus or kernel, but we know that in a magnet, whether natural or artificial, the poles frequently change their places, though the magnet has no nucleus; and therefore the magnetic poles of the earth may be susceptible of motion independent of a nucleus. Thirdly, it is natural to suppose that the same causes, which have been found to alter the situation of the poles of a magnet, act in the same manner upon the earth, and occasion the motion of its poles. Those causes, and the method of manifesting their effects, may be reduced to four; viz. the action of one magnet upon the other; the action of heat and cold; the chymical alteration or decomposition of the substance affected with magnetism; and, lastly, the mere mechanical derangement of parts.-That all those causes take place in the earth, no body can deny ; and of course it seems to be as evident, as the nature of the subject will admit of, that the motion of the magnetic poles is governed by the concurrence of all those causes.

So far we have considered only the motion of the magnetic poles of the earth, which undoubtedly govern the general variation of the magnetic needle; but if we also take into the account the local causes, which have been indisputably found to affect the needle in particular places, such as the vicinity of great tracts of land, promontories, volcanos, &c. we must acknowledge that the prospect is very discouraging, and the probability of our ever becoming able to ascertain the longitude and latitude by means of magnetical instruments, seems almost to vanish. In fact, when we examine the different projects hitherto published, and compare their results, we find an astonishing diversity as well among themselves, as between them and actual observations.

Of the Latitudes and Longitudes of the Principal Observatories in the World, from the latest and most accurate Observations.

Constantinople (St.Sophia) 41 1 27 N. 28 55

48 17 49 N.
45 28 2 N.

"

5 22 15 E.0 21 29 E.

1 52 26

E. 0

7 30 E.

22 43 27

E. 1

34 54 E.

1 20 45

E.0

5 23 E.

3 52 40

E. 0 15 31 E.

37 33

E.2 30 12 E.

43 5 7 N.
56 39 6 N.
44 0 55 N.
43 36 16 N.
155 45 45 N.
148 8 20 N.
40 50 15 N.
149 26 55 N.
51 45 39 N.
45 24 2 N.
38 6 44 N.
48 50 14 N. 2 20 15 E.0 9' 21 E.
39 54 13 N. 116 27 45
59 56 23 N. 30 18 45
43 43 11 N.
50 22 20 N.

E.0 57 3 E. 4 15 E.0 44 17 E. 1 15 22 W.0 5 1W. 11 51 32 E.0 47 26 E. 13 22 0 E.0 53 28 E.

On the Method of determining the Flatness of the Oblate Spheroid in France, by the comparison of an Arc of the Meridian with an Arc of a Parallel.

THE two lines of curvature on the earth's surface at any place, being well calculated to determine with precision the dimensions of the oblate spheroid in that place, geometers and astronomers have long wished that the great geodesic operations executed in Europe, like those upon which the new system of French measures is founded, should be immediately applied to the measurement of different arcs of meridians and parallels, in order to acquire, by this means, a more accurate knowledge of the real figure of the earth.

The trigonometrical labours undertaken by the French royal body of geographical engineers, under the immediate direction of the war department, and which are to serve as the skeleton of the new map of that country, already furnish very valuable results for the solution of the problem in question. For example: an arc of the parallel, measured trigonometrically and astronomically, in the latitude of 45 degrees, extends from the tower of Cordovan into Piedmont, and might even be continued to Fiume and farther. The observations on longitudes made last year, by Colonel Brousseaud and the astro nomer Nicollet, by means of signals of fire, give the amplitudes of four consecutive portions of this line, which embrace an extent of seven degrees. They are connected with those made by Messrs. Plana and Carlini, and other foreign philosophers a year before, upon the same line beyond the Alps, and thus form a geographical union between France and Italy.

Another arc of the parallel, not less important, comprised between Brest and Strasburg, the measure of which had been earnestly desired by the illustrious La Place, author of the Mecanique Celeste, is already known geodesically. It is upon this line that reverberating lamps have been constantly used for the purpose of ascertaining whether angular observations in places but little elevated, agree toge ther better by night than by day. It is also upon this line, that a trigonometrical survey has been effected by reciprocal and simultaneous observations from Paris to Brest; with the view either of deter mining exactly the absolute heights of the stations comprised between these extreme points, or of connecting by these stations all the secondary surveys intended to be afterwards undertaken for forming a hydrographic map of France.

It is proposed this year to make observations on the longitude of this second primordial line; but in order to avoid the trouble of establishing temporary observatories in an open country, and above all, the enormous expense which their construction would occasion, the difference of longitude between the extremities of this axis, will, with the assistance of chronometers placed at several intermediate points, be obtained, by the rapid transmission, by means of gunpowder signals, of the hours which will be counted, at the same instant of time, at Brest, and at Strasburg. This very simple proceeding is

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