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able to make in a very short time, and ascertain the source of the salt. Many of these lakes are of great extent, and in a great measure formed by an embankment of sand thrown up by a heavy sea, along a level shore. The water, that falls in torrents during the rainy season, is thus confined, and inundates a great part of the country; the sea, more or less, breaks over or percolates through the sand-banks, and thus the water is rendered brackish.-In the dry season the wind is very strong, and dry, and the air very hot; it was from 85° to 90° when I was there. The consequence is a very rapid evaporation of the water-the drying of the shallow lakesand the formation of salt. It is from these lakes chiefly that the island is supplied with salt. The revenue that this one article brings Government, amounts to about £10,000. annually. The Malagan-patton altogether is a singular country; its woods, and it is almost all wooded, are principally composed of euphorbia and mimosæ :-its few inhabitants are a sickly race, miasmata destroying their health, and the wild animals, with which the country abounds, as elephants, hogs, deer of different kinds, leopards, bears, &c. destroying the fruits of their labour. In the beginning of January I attended the Governor and Lady Brownrigg to Kandy, and had a good opportunity of becoming acquainted with the manners of the natives. The country in the interior, and particularly round Kandy, is magnificent; its grand features are high hills and mountains, and deep vallies, and perpetual wood, and perennial verdure; the wood is in faulty excess. The climate is fine; the air cool; generally at night below 75°, averaging all the year round the moderate temperature of 74°. From Kandy I made an excursion alone into Doombera, and explored a mountainous region, where a white man was never seen before. My object was to examine a cave that yields nitre. It is a magnificent one in the side of a mountain, in the depths of a forest surrounded by mountains of great height and noble forms. I shall send you a particular account of this, and of other nitre caves I have visited. The rock is a mixture of quartz, felspar, mica, and talc, impregnated near the surface with nitre, nitrat of lime, and sulphate of mag
nesia, and in one spot with alum, and in another incrusted with hydralite, similar to that round the Geyser, in Iceland. From the mountains of Dombera I looked down on the wooded plains of Birtanna, and saw the great lake of Birtanna, which no European I believe ever before visited; it is full of alligators. Returning to Kandy, after a short stay there I next came to this place, through a country almost entirely over-run with wood:---three days we travelled in a noble forest without seeing a single habitation. I wish you could see some of the noble ebony trees which flourish here, and without observing any traces of cultivation; but some fine remains of antiquity, especially about Candely lake, indicating that the country had once been in a very different state.
ART. V. Translation of a Letter from a learned Foreigner to a Friend in this Couutry, on the Figure of the Earth, and on the Length of the Seconds Pendulum in different Latitudes.
Ar the present moment, when the philosophers of France and England are conjointly engaged in attempts to determine, with the utmost precision, the length of the second's pendulum in different latitudes, and the measure of the earth, you have thought that it would be not uninteresting to point out in a simple and popular manner, the exact object of these operations; what means are made use of in their execution; and lastly, what useful results are likely to be derived from them to science and society. Such is the object of the letter which I have the honour to address to you.
Although these questions ought naturally to be discussed in the order in which they are proposed above, which is regulated by their mutual dependance, it may perhaps be advisable first to touch slightly upon the latter, and to indicate what of great and profitable, such researches may have for the human mind. Those who are specially engaged in the cultivation of the
sciences, know well that the importance of a discovery ought not to be estimated solely by the direct and immediate use that can be made of it. A multitude of examples, or rather constant experience, demonstrates to them that every acquisition of knowledge sooner or later brings forth its fruits; and those discoveries which have the good fortune to be useful from their birth, such as vaccination, the safety-lamp, and the electric pile, are in their eyes favourable anticipations, which give them a right to appeal to futurity, and to obtain credit from their contemporaries for abstract researches, of which the practical consequences, though not less certain, are more distant, and less immediately apparent. But whatever confidence in their researches such memorable examples naturally inspire, they ought never to neglect to justify it, even by remote indications, whenever it is in their power. We have this advantage in the class of researches which constitute the object of this letter, and I shall not fail hereafter to point out all their important applications.
I think it right, now however, to premise, that the knowledge of the figure of the earth, the determination of its mea■ure, and of the variation of its gravity upon different parts of its surface, are indispensable elements in the theory of universal gravitation; that they present so many confirmations of its truth of the first importance; that they serve to demonstrate the identity of terrestrial gravity, with that force which retains the moon in its orbit round the earth; that they only can discover to us if the density of the earth is equal throughout its mass, or whether it is different at different depths; and whether in every stratum at the same depth, this density is uniform, or variable at different points; and lastly, the precise determination of the dimensions of this globe which we inhabit, may enable us from its measure to form a fixed and universal standard of mensuration, whose subdivisions may be applied to the surveys of our fields, the measures of itinerary distances, and the construction of nautical and geographical charts, whilst its multiples may serve to express the vast extents of celestial space, which the astronomer has taught us to mea
sure. It is this unit of mensuration, deduced from the measure of the earth, which has been called meter by the French philsophers.
It may be easily conceived that these beautiful deductions of science have not been obtained without immense labour. There is reason to believe that the ancient astronomers of Chaldea and Egypt acquired notions sufficiently exact concerning the magnitude of the globe, but their first attempts are far indeed removed from the minute precision of modern astronomy. We are now certain of not making an error of 600 meters (about 300 toises,) in the calculation of the mean radius of the earth, which exceeds 6,000,000 meters. This may seem incredible to those who are not acquainted with the means made use of by astronomers, but nothing can appear more simple or certain when they are examined. Without entering into technical details, it is easy to explain the possibility of such precision. It will suffice for this purpose to remark, that the surface of the earth is not in fact so irregular as appears at first sight. That the mountains with which it is bristled, and the vallies with which it is furrowed, are, in proportion to its mass, no more than nearly imperceptible wrinkles, and that the little asperities upon the peel of an orange are comparatively much more considerable. If we also attend to the facts that the ter restrial continents are surrounded on every side by the sea, which penetrates them by numberless channels, that their coasts are no where much elevated above the waters which bathe them; that all the rivers by which these continents are divided, reach the ocean by very gentle declivities, since they are all commonly navigable; we shall perceive in this equilibrium the result of the general level of the terrestrial surface; we shall be able to conceive that its curvature must follow the regular curve of the ocean; and from thence we shall perceive that taking into the account, for greater exactness, the little undulations with which it is furrowed, the measure of such a curve may be accomplished with all the rigour of a mathematical operation.
Nothing more remains than to point out the processes by
which this measure is obtained. We have sometimes seen upon the sea-coast a vessel sailing from the shore; at first the whole is visible; but as it recedes it seems to sink into the horizon; the hull first disappears, then its lower sails, then the tops of its masts; and lastly, it totally vanishes. This arises from the convexity of the earth, which intervenes between the vessel and the spectator. At the same time, those who are on board observe a similar phenomenon. They first lose sight of the shore, then of the houses, then of the towers, then of the mountains, till at last they find themselves surrounded by the horizon of the sea. This progressive sinking, which they remark in receding from the shore, we may also perceive in the celestial constellations, in travelling upon the earth from north to south, or from south to north. The north pole and the stars which surround it sink towards the horizon as we advance to the south; on the contrary, it rises if we return. All the stars partake of these changes of elevation, which are caused only by our change of place. By measuring with care their meridian altititude above the horizon of each place, we ascertain the num ber of degrees this altitude has changed for the distance we have travelled on the same meridian; and as from the immense distance of the stars all the visual rays which proceed from them to every point of the earth at the same instant form no appreciable angle, it follows that the angle so observed, is really nothing but the mutual inclination of the horizons of the two places to which the observer has transported himself upon the same meridian, or in other words, it is the angle comprised between the vertical drawn in these two places perpendicularly to the surface of the earth. If then we measure also the distance of the two stations upon this same surface, by means of a determined standard of length, which may, for instance, be the toise, we shall know that in the part of the earth where the observation is made, such an angle between the two verticals answers to such a number of toises; so by simple proportion, we deduce the number of toises which correspond to an angle of one degree. This is what astronomers call measuring a degree of the meridian.