would have been in vain to aim at analysing it with strict accuracy. Yet I thought it worth while to endeavour to form as exact an estimation of its contents as I could, on account of its connection with the Dead Sea, into which, as was observed before, it pours its waters, and appears to remain in a stagnating state. This specimen was brought from a spot about three miles distant from that where the river enters the Dead Sea.

pure,

From the perfect pellucidity of this water, its softness, Apparentl and the absence of any obvious saline taste, I was led to suppose, that it was uncommonly pure, and could in no degree partake of the peculiar saline qualities of the Dead Sea. But I was soon induced to alter my opinion by the following results.

Dead Sea ex

1. The same chemical reagents, as were used to ascer- but analogous tain the general properties of the Dead Sea water, being to that of the applied to this, produced analogous effects. The same cept in three muriates and even the vestige of selenite were dis- strength. tinctly discovered; and this resemblance became more striking in proportion as the water was concentrated by evaporation.

2. 500 grains of this water being evaporated at about 200°, the dry residue weighed exactly 0.8 of a grain. This makes the solid ingredients amount only to 1.6 grain in 1000 grains of the water, a singular contrast with the Dead Sea, which contains nearly 300 times that portion of saline Apparently matter. As the water was concentrating, a few white parcontains only of its solid ticles were perceived on its surface, and a few others gradu- contents. ally subsided. When dried, the residue appeared in the form of a white incrustation, the upper edge of which exhibited great numbers of very minute crystals, which from their saline taste, and their cubic shape, discoverable by the aid of a microscope, were evidently common salt.

3. Distilled water being thrown on this residue, a minute portion of it remained undissolved, and on pouring an acid on this substance, a distinct effervescence was produced, showing the presence of carbonate of lime.

4. From the clear fluid a precipitate was obtained by oxalate of ammonia, which, dried but not calcined, weighed 0.12 of a grain.

5. From

The Dead Sea perhaps the

same water

5. From the remaining clear solution a magnesian precipitate was produced by ammonia and phosphoric acid, which, after driving off the ammonia by heat, weighed 0·18 of a grain.

6. The solution had suffered too many alterations to allow me to separate, with any degree of accuracy, the muriate of soda; but from a variety of circumstances, I thought it not unlikely, that it would have been found pretty nearly in the same proportions, with respect to the other salts, as it exists in the Dead Sea.

The inference I drew from this was, that the River Jor dan might possibly be the source of the saline ingredients of concentrated the Dead Sea, or at least that the same source of impregnaby evaporation. tion might be common to both, This inquiry, however, would require a much more correct knowledge both of the proportions of the salts, and of local circumstances, than I have been able to obtain.

VI.

An Account of the Measurement of an Arc on the Meridian of the Coast of Coromandel, and the Length of a Degree deduced therefrom in the Latitude of 12° 32'. By Brigade Major WILLIAM LAMBTON.

(Concluded from Vol. XIX, p. 317.)

Reductions of THE reductions from the hypothenuses to bring them to the hypothe the horizontal level were made by numbering the feet from

nuses.

the old chain as they were measured, viz. by calling 32 chains 3200 feet, which would be 3200 115 feet by the new chain; but this would produce no sensible errour in the versed sign of a very small angle, and on that account these decimals were not taken into the computation, which was thought less necessary, since the whole deduction did not amount to three inches. Neither was any notice taken of the different heights of the hypothenuses or levels one above another, as that difference was too trifling to affect a length of thirty or forty chains. The base has therefore been considered

sidered at the same distance from the centre of the earth, before it was reduced to the level of the sea, and the perpendicular height of the south extremity, which I have considered as nearly the general height, has been taken for that purpose. That perpendicular height was obtained by comparing the south with the north extremity, and the height of the latter was determined by observations made at the racestand and on the sea-beach, where allowance has been made for the terrestrial refraction. The following is the manner in which it has been determined:

On the top of the race-stand, the under part of the flag Determination of the height on the beach was observed to be depressed 9' 30"; and at above the sea, the beach, the top of the race-stand was elevated 7′15′′. When the instrument was on the platform of the race-stand, the axis of the telescope was on a level with the top of the railing, which was observed from the beach, But at the beach the axis of the telescope was four feet below the part of the flag which had been observed.

The horizontal distance from the station on the stand to that on the beach is 19208 feet. Then as 19208:4:: Rad: tan. 43", which must therefore be added to the observed depression of the flag. Hence 9' 30" + 43" 10′ 13′′ is the depression of the axis of the telescope on the beach, observed from the race-stand.

2

Now the station on the beach is nearly at right angles to the meridjan, therefore, by allowing 60957 fathoms to the degree, 19208 feet will give an arc of 3′ 9′′ very And the difference benearly, which is the contained arc. tween the depression and elevation being 2' 59", we have 3'9′′-2′ 58" 55 for the terrestrial refraction. Hence, since the observed elevation of the stand, plus half the contained arc, would give the angle subtended by the perpendicular height of the stand above the telescope at the beach, were there no refraction, we shall have 7 15" + 2 -5"58' 44" for the true angle subtended by the per pendicular height, which being taken as tangent to the ho rizontal distance and radius, we have R: tan. 8′ 44′′ :: 19208 48 797 feet the height required. But the axis of the telescope on the beach was determined, by levelling

down

down to the water, to be 21.166 feet above the sea. Which, added to the above, give 69.963 feet for the perpendicular height of the top of the stand above the level of the

sea.

Now the top of the race-stand was determined by levelling to be 31.25 feet above the north extremity of the base; which, taken from the other, leaves 38-713 for the north extremity of the base above the sea, which extremity being, by the table, 22.96 feet above the south extremity, we shall have 15-753 feet from the perpendicular height of the south extremity of the line above the level of the sea; and from this height the length of the base has been reduced.

The angles of elevation and depression were taken by the circular instrument, from a mean of several observations, and the errour of collimation was corrected by turning the transit over, and the horizontal plate half round. But the weather was rather dull during the whole of these operations.

Major Lambton then proceeds to give the particulars of the measurement of his base line, commencing in lat. 13° 00′ 29′59′′ N., and extending 40006-4418 feet south-westerly, making an angle of 10' 36" with the meridian.

Commencement of the operations from the base. The large theodolite.

After the completion of the base line, there remained nothing of importance to be done until I received the large instrument, which arrived in the beginning of September. I had however made an excursion down the sea coast, as far as Pondicherry, for the purpose of selecting the properest stations for determining the length of a meridional arc. This and the measurement of a degree at right-angles to the meridian I considered as the first object of this work: I accordingly lost no time in proceeding to accomplish these desiderata.

The instrument above alluded to was made by Mr. Cary, and is in most respects the same as that described by General Roy in the Philosophical Transactions for the year 1790, with the improvements made afterwards in the micro

copes,

scopes, and in an adjustment to the vertical axis, by which the circle can be moved up or let down by means of two capstan screws at the top of the axis. These are mentioned in the Philosophical Transactions for 1795, in the account of the trigonometrical survey. By sinking the circle on the axis, it is better adapted for travelling, and when the microscopes are once adjusted to minutes and seconds, on the limb of the instrument, the circle can always be brought back to the proper distance from them. Great attention however is necessary in bringing the axis down, so that the wires in each microscope being fixed at opposite dots on the limb, they may coincide with the same dots when the circle is turned half round, or made to move entirely round, and in a contrary direction to what it had been moved before; which latter method has been recommended by the maker. This circumstance respecting the axis should be most scrupulously attended to before the adjustment of the micrometers begins, so that when by arranging the lenses in such a manner that ten revolutions of the micrometer may answer to ten minutes on the limb, and therefore one division to one second, the circle can always be brought to its proper height, by trying the revolutions of the micrometer.

It has however been found from experience, that unless in cases of very long and troublesome marches, it is not necessary to sink the axis. The carriage being performed altogether by men, there is not that jolting which any other mode of conveyance is subject to, and as I found, that a considerable time was taken up in adjusting the axis before the revolutions of the micrometers could be brought to their intended limits, I therefore laid it aside, unless under the circumstances above mentioned.

The semicircle of the transit telescope is graduated to 10' Semicircle of of a degree in place of 30', which was the case with the the transit te lescope. semicircle described by General Roy, and the micrometer to the horizontal microscope applied to this semicircle, making one revolution in two minutes, and five revolutions for ten minutes on the limb; and the scale of the micrometer being divided into sixty parts, each part is therefore two seconds of the circle.

T

A number of experiments have been made for determin- Errour of the

ing

semicircle.