apply a correction on this account. For this purpose the principal or barometric scale mn, is made to slide upon another scale op, placed either below it or on one side of it, which is divided into degrees and parts, so as to represent the change of bulk in the gas produced by a change of temperature under the same pressure, and corresponding to the degrees of a common Thermometer attached to the instrument.

This scale is constructed in the same manner as the scale of a common thermometer, by changing the temperature of the bulb while the pressure is the same, and noting the range of the oil occasioned by it.

In using the instrument, observe the temperature by the thermometer, and set the index which is upon the sliding Sympiesometer scale, opposite to the degree of temperature upon the fixed scale; and then the height of the oil, as indicated on the sliding scale, will be the pressure of the air required.

When the height of one place above another is to be measured by the diminution of the pressure of the atmosphere, another correction is necessary to insure perfect accuracy in all instruments indicating this change, because the pressure of a column of air of a given altitude varies according to its humidity or moisture. I have therefore added to the Sympiesometer a new Hygrometer, which has already been described in a preceding article. See page 32.

In some of the Sympiesometers which I have made, the scale is divided into parts corresponding to the increase in bulk which takes place in the gas by the diminished pressure of the atmosphere on ascending a given height, the temperature being 32° of Fahrenheit. This scale is also formed by experiment, as follows: The instrument being placed in the glass-case, as before described, increase the density of the inclosed air until it support a column of quicksilver of 31 inches, the temperature being 32. Mark this point zero; then from the logarithm of 31 subtract .01C0, and find the corresponding number, which is 30.294; regulate the density of the air to support a column of quicksilver of this length; number this point on the scale 100, and divide the space into100 parts; cach part will equal the increase of bulk, or fall of the oil in the tube by ascending one fathom. In the above manner proceed, by subtracting 0100 from the logarith

last found, and marking the points corresponding to these densities, until the scale is complete.

By the above scale, the approximate height will be given without the aid of a table of logarithms, by subtracting the number of fathoms indicated by the Sympiesometer at the under station from that indicated at the upper station, the difference being the number of fathoms which the one station is above the other.

Previous to laying this instrument before the public, I wished to have it submitted to a fair trial, by comparing it with observations made in the same ship with the Marine Barometer. For this purpose Quintin Leitch, Esq. of Greenock, the proprietor of the ship Buckinghamshire, obligingly sent one of the first which I had made with this ship on her voyage from the Clyde to the East Indies, in the year 1816; and the following is the report given of the instrument by the late Captain Christian the commander, on his return.

"I am glad to say that I consider your Barometer a valuable instrument at sea, having given it a fair trial on the outward passage to India, by keeping a correct register of it, as well as of the common Marine Barometer, taken every third hour, night and day, during the passage; and I not only found that it was fully as sensible of the changes of the atmosphere as the other barometer, but that it had a great advantage over all barometers I have ever seen used at sea, namely, that of not being in the smallest degree affected by the motion of the ship, which will often make the quicksilver in the common tube plunge, or rise and fall, in such a degree as to make it very difficult to come within at least one or two tenths of an inch of the truth, even in the largest ships. On the passage home I also found it very correct in the indication of the winds and weather."

An opportunity of trying the Sympiesometer in a very different climate occurred last year, when the Expedition under Captain Ross sailed to the Arctic Regions. Lieutenant Robertson of the Isabella kindly undertook the charge of this instrument, and regular observations were made every four hours with the Sympiesometer and Marine Barometer, the results of which were highly satisfactory. The observations commenced on the 24th of April, in North latitude 51° 39′, and longitude 1° 7 E.; and were

continued to the latitude of 76° 50' N. and during the return of the Expedition to Deptford till the 13th of November. These observations, in the form of a graphical representation of the progress of the Sympiesometer and Marine Barometer, have been published in Captain Ross's Account of the Expedition, and will enable navigators to form a correct estimate of the re lative value of the two instruments *.

The following is Captain Ross's official report upon the Sym piesometer:

"This instrument acts as a marine barometer, and is certainly not inferior in its powers. It has also the advantages of not being affected by the ship's motion, and of taking up very little room in the cabin. I am of opinion that the instrument will supersede the Marine Barometer, when it is better known."

Lieutenant Robertson, in a letter to the Honourable Captain Napier of Merchistoun, has spoken of it in the following

manner:

In

"The Sympiesometer is a most excellent instrument, and shews the weather far better than the Marine Barometer. short, the barometer is of no use compared to it. If it has any fault, it is that of being too sensible of small changes, which might frighten a reef in when there was no occasion for it; but, take it altogether, in my opinion it surpasses the mercurial barometer as much as the barometer is superior to having none at all."

I have also had it in my power to make trial of the Sympiesometer on coasting voyages, through the favour of my friend Mr Stevenson, Engineer to the Scots Lighthouse Board, who placed one of them in the cabin of the Lighthouse Yacht beside a good marine barometer. Along with a register of both instru

• We have now before us a copy of these observations, both in a tabular and projected form. The superior delicacy of the Sympiesometer is very remark able. It frequently exhibited variations in the pressure of the atmosphere when the Marine Barometer was perfectly stationary, and thus predicted changes in the weather, when the other instrument gave no indications at all. We have likewise seen the observations made with the Sympiesometer by Captain Dalling, of his Majesty's ship Nimrod, who has expressed the highest opinion of the superiority of the Sympiesometer. On one occasion in particular, it enabled him to make successful preparations against a storm, which was not indicated by the Marine Barometer. ED.

ments, extracted from the ship's log-book, he has favoured me with a communication, which states, that, "after an experience of two years, the Sympicsometer affords the most delicate and correct indications of the weather;" and that "it is a great favourite on board, being commodious even for the smallest cabin, and at the same time easily read off."

"The master, mate, and steward of the Light-house Yacht, (Mr Stevenson adds), give such accounts of the utility and conveniency of the Sympiesometer, as are well calculated to recommend it to the attention of those sailing in vessels of the smallest burden. It is now in use in the service of the Commissioners of the Northern Lights, on board the Light-house Yacht, of 80 tons register, and the Pharos, or Bell Rock Tender, of 45 tons."

EDINBURGH, March 20. 1819.

ART. X.-Method of Weighing Anchors, used by the Natives of the Coast of Coromandel. Communicated by the Author.

WHEN his Majesty's ship Minden struck upon the Cole

roon shoal, on the coast of Coromandel, in September 1814, a bower anchor was laid out by the boats; but after the ship was hove off, these were found insufficient to raise the anchor off the ground; and the water being too shallow for the ship to approach, it became necessary to resort to some other means of effecting this object. While the officers were considering which of the ordinary expedients was best, the master-attendant of Porto Nuovo, who had come on board to give his assistance, suggested that the natives should be allowed to try the method in common use amongst them for weighing anchors. This proposal was immediately agreed to by the Admiral, the late Sir Samuel Hood, whose ardent and inquiring mind caught eagerly at every thing new, which promised to be of use in his profession.

The natives were supplied with a number of spars, such as topmasts, jib-booms, &c. These they lashed together, so as to form a raft in the form of a rude cylinder, between three and four feet in diameter. Round the middle of this, they wound the buoy-rope of the anchor, and made it fast. Thirty small

ropes, were now made fast to the spar, and passed round it several times, in an opposite way from that in which the buoy-rope had been wound; thus forming what are technically called "slewropes," (turning ropes). Sixty of the natives now mounted the spar, and, having taken hold of the ropes, hauled upon them so as to turn the spar round In a short time, the buoy-rope became tight, which prevented the further rotation of the spar. All the slew-ropes being now rendered equally tight, were held firmly in both hands by the natives, who stood erect, and in a line, along the top of the spar; and, upon a word of command being given, the whole party threw themselves suddenly backwards, so that they all fell flat on the surface of the water at the same moment. By this operation the spar was made to perform one quarter of a revolution; but this of course did not start the anchor, though it made the buoy-rope so tight, as to require a considerable force to prevent the spar from turning back again. The next turn was made by the alternate pairs of men remaining extended on the water, while the rest gradually climbed up to the top of the spar, by means of the slew-ropes. Having reached this point, and having drawn their ropes equally tight, they again threw themselves on their backs, while those who were already down, merely " took in the slack,” as it is termed, of their ropes, that is, kept them uniformly tense, while the spar was performing another quarter revo lution, by the effort of the number who had climbed up. The same thing was repeated, always by half of the party, till the anchor was fairly lifted off the ground. As soon as this was accomplished, the whole of the natives continued stretched on the water, while the boats towed the spar, together with the anchor, and all the apparatus, into deep water, where the ship lay; and the anchor was then hove up in the usual way.

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The anchor in question weighed above three tons, and was much heavier than any which the natives had ever before raised. They complained much of this circumstance; and indeed, it had nearly cost them very dear; for, when they had nearly reached the ship, some of them, either becoming tired or frightened, let go their ropes. This threw additional weight upon the rest, who in their turn, becoming alarmed, also quitted their hold. In an instant the anchor sunk to the bottom, and by unwinding the buoy-rope gave the spar so rapid a rotatory