ment, anxious as we naturally were, to ascertain the fate of a fleet from which we had separated eleven weeks before under such unpropitious circumstances. This suspense, however, was of short duration; our worthy Commodore, with five of his convoy, were soon discovered to be safe at anchor in the Bay, the remaining three ships were missing, and, sad to tell, have never since been heard of. Of those which were safe, four, including the seventy-four gun-ship, had been in more or less danger of foundering in the storm, whilst two escaped without injury; owing, as it appeared from a comparison of Journals, to their having escaped the brunt of the storm by being cons derably to windward of the others; thus corroborating, the theory with which I commenced, in my endeavours to prove that where the storm begins there will it soonest end; a great part of the third day, which was by far the most tempestuous with us, these two ships lay perfectly becalmed.

Such were the disastrous effects of this memorable hurricane, from a summary of which I think myself entitled to draw the following practical inference; namely, that had we instantly attended to the timely warning of the Barometer, by bringing the ship to the wind, and making preparations for the storm, instead of scudding before it, until we could scud no longer, we should have escaped with as little injury as the two ships I have just alluded to, and that had the three unfortunate ships which foundered in the storm pursued a similar course, which it may be fairly presumed they did not, a very different fate might have befallen them too.

But, lest the fatal catastrophe of this hurricane should not be deemed sufficiently conclusive, I shall mention the result of another, no less fatal in its consequences, which was encountered in the following season by another fleet of Indiamen, nearly in the same latitude and longitude, whilst under convoy of the late Lord Exmouth. On this occasion four of the finest ships of the fleet, crowded with passengers from Calcutta, were supposed to have foundered, as they were missing immediately after the storm, and were never heard of more. The last time they were seen was by Lord Exmouth himself, when they were observed to be scudding before the gale, whilst the rest of the fleet were lying-to.

Here, then, we have another melancholy instance in point, which, coupled with the preceding, ought to satisfy the mind of the most sceptical seaman, as to the infallibility of the Barometer in indicating the approach of hurricanes, within the tropics more particularly,

and consequently of the inestimable value of this instrument to every commander of a ship, and more especially to those whose destination is India.

I am, Sir, your obedient Servant.
MERCURIUS.

V. MEMORANDUM ON THE TIDES AT MADRAS, November 1821. By Lieut. Col. De Havilland of the Madras Engineers. The results of the observations made on the tides at Madras by the means of a Tide Gauge near the north east angle of the glacis of the Fort from the 31st May to the 10th October 1821 inclusive, were as follows. That high water on the full and change of the Moon, was at 8h 58 or 9h. That on those days the difference of elevation of the surface of the sea at high and low water, was 2 feet 7 inches; that on the 4th days of the Moon's age, and the corresponding days after the full, this difference was 3 feet 5 inches the maximum and that the 2d days after the 1st and 3d It was quarters, it was reduced to 1 foot 7 inches the minimum. also observed, that the daily decrease was greatest, on and immediately before the 1st and 3d quarters, and the increase least, immediately after. It was likewise remarked, that the average level of the sea at mid-tide, or between high and low water, varied with the Phases of the Moon. That it appeared highest on the 3d days of the Moon's age, and the corresponding days after the full, and lowest on the 2d days after the 1st and 3d quarters—and the difference was observed to be about 1 foot 4 inches; but it is probable that this being so great, was owing to some anomaly, in the observations made on the 3d days of the Moon's age, and those corresponding after the full, and that in point of fact, it was not more than 1 foot. The following table contains the particular of these results. The retardation of the tides from day to day was considerably less for the first four or five days after full, and change than at any other time; it was then about 30 per diem.

Results of the observations made on the Tides at Madras from the 31st May to the 10th October 1821 by means of a Tide Gauge fixed near the north east angle of the Fort.

Average mean level of the sea in the whole period 6

10

It is necessary to notice that until the 29th of July the observations were frequently interrupted, but that after that date they were made daily, at every tide, in every 24 hours; and as there appears some difference in the results obtained from the subsequent period, they are given in the following statement.

Similar statement as the above from 29th July to 10th October 1821 both inclusive.

Difference be

tween high and

low water mark.

Ins.

111 3

31

21

21

81

5

05

8/1/20 10

10%

039

45

81

Although this statement appears less anomalous than the last in some respects, it is not so in all, and as the other has the advantage of including the period of the long shore winds and strong southern currents, it is a better average for the whole season than the last. Much variation appears in all the daily observations, as the rise of the sea is evidently affected by the winds and currents which both tend to accumulate the waters.

The present Tide Gauge it is feared will not answer for the ensuing season as the N. E. winds have filled up the angle it is fixed in with sand, in a great degree. If it should be replaced, it would be best situated at one of the projecting angles of the Fort bulwark.

The Gauge consists of an iron pipe syphon, having one horizontal branch, sunk, in the sand below low water-spring-tides mark ; and one perpendicular branch with a float and rod in it; showing the rise and fall of the water. The length of the horizontal branch

is about 70 feet and the end of it under the surf is fixed to cross timbers to prevent its sinking deep into the sand.-The water in the upright arm of the syphon is drawn every day to ascertain that it flows freely. As the pipe is doubtless in a great measure filled with sand, it follows that the water neither flows nor ebbs in the syphon to its exact extremes, and therefore, that the rise and fall given in the Tables must be considered rather under the truth.-In many instances, the difference of high and low water is as much as 4 feet 3 inches and even 4 feet 6 inches. The beating of the surf affected the water in the pipe and the rod oscillated up and down at each stroke, but this oscillation did not exceed half or three quarters of an inch in the highest surfs.

The variation of mean level of the surface of the sea noticed above (5th column) accounts for the rise of the water in the river at Madras during the dry season. The sea on intering at the Ennore Bar, when the Chepauk Bar is shut up, comes down the canal from thence; after the full and change, the water is observed to rise for two or three days, in about as many days more, falls again; while however the daily operation of the Tides is not perceptible, from the distance of the Ennore Bar. This rise and fall has at the Wallajah bridge been frequently observed to be as much as 8 or 10 inches.

It is possible that the sea water filtering below the sands may aid this operation a little, but not much; for the ditch of the Fort which is kept full by means of this filtering underneath the sea face counterscarpe, is scarcely affected by this rise of the sea above alluded to no more than by the flowing and ebbing of the Tides daily.

VI.-Translation from a work by, M. M. Botté et Riffault, detailing the method pursued in France, of extracting Saltpetre from the soil, and of ascertaining the quality of Saltpetre by Assay. Communicated by Lieutenant Colonel Napier.

As natural mines of Saltpetre exist in France, it has long been an object of primary importance with the Government to improve, and increase the manufacture of that article, by the employment of men of science to establish manufactories upon the best and most economical principles, by which means the produce of Saltpetre was doubled in four years.

A prize of 4,000 Livres, was offered by the Academie of Science,