ANCHORAGE AT SANTA CRUZ, Tenerife.

SIR. The officers of several ships that have come here for the first time, have complained that the directions in Horsburgh, for the anchorage, are extremely unsatisfactory. I think these must have been formed when the buildings of the town did not extend so far north as they do now. I send you a copy of directions I have drawn up, of which, some officers to whom they have been submitted, have expressed their approval. The bearings and depth of water I have taken from a plan of the bay given me by Lieut. Arlett, when here in the Etna.

I have been much surprised to hear officers talk of this bay as a ticklish place to come to. A most erroneous notion. It is much superior as an anchorage to Madeira.

Very truly yours, Santa Cruz, August 12th, 1842.

RICHARD BARTLETT,

Directions. (Compass Bearings.)

H.B.M. Consul.

While running for the anchorage keep both leads going, and bring up to the northward of the Mole head; or, bring the clock-front of the square Church Tower on with a cupola, bearing W.S.W., and anchor with this mark on or to the northward of it.

Ships may anchor when in less than thirty fathoms. Give a large scope of chain cable. When the northernmost fort (Fort Paso Alto) bears N.N.W. the depth of water will be about twenty-five fathoms on the lines pointed out. The shore may be neared without risk, the water being deep, and no dangers that are not apparent. The anchorage to the south of the lines indicated, is reserved for vessels in quarantine. Variation 22° 41' west.

STRAITS OF BASILAN, SAMBOANGAN, ETC.

HAVING previously drawn the attention of your readers to the difference between Mr. Horsburgh's Directory and the charts, I will now point out another instance which has lately led to rather serious consequences.

On the 15th of last June, while we were lying at anchor in Samboangan Roads, the "Ann," of Greenock, from China, bound to London, struck on the reef to the south-westward of the St. Cruz Islands, Straits of Basilan. This ship was aground seventeen hours, and was ultimately got off with assistance from the ships in Samboangan, after receiving considerable damage,-false keel knocked off, all the pintles of the rudder broken, and making a foot of water an hour. Had this occurred twelve hours sooner she must have gone to pieces, there being then a strong breeze with a heavy swell.

The master of the Ann had no suspicion of any danger in this situation. He was trusting solely to Mr. Horsburgh's chart, in which it will be seen that nothing is inserted extending above a mile from the St. Cruz Islands. This reef is at least four miles from them. By referring to the Directory however, (a copy of which was on board,) we find a statement of H.M.S. La Sybille, having grounded on a coral bank to the south-westward of these islands: also advice given to give them a ENLARGED SERIES.-NO. 4.-VOL. FOR 1843. 2 F

berth in passing. I subjoin a few bearings taken on board the Anu, while on shore,-Point Caldera, N.W.; Sangboys Island, W.b.S.; Easternmost Peak of Basilan, S.E.b.E; Easternmost St. Cruz Island, E. N.; Western St. Cruz Island, N.E. E.; and the mastheads of the ships which were lying in Samboangan Roads were to be seen over the Western St. Cruz Island;-estimated distance from the latter island about four miles. This, I think, is the outer verge of the reef,* the least water we found on it was 13 feet, and this shoal patch is not above 300 or 400 yards in length by 100 in breadth, other patches extend from this in the direction of the Eastern Island, between the shoal and the west point of the islands there is plenty of water; but to the westward of this point there is another shoal rather extensive, on which an American ship struck, and remained some time, immediately after the above occurrence.

I have had no opportunity of examining around these islands, yet this notice may serve to caution ships, when in their vicinity, to keep the lead going, there being anchorage, I believe, on the edge of all these banks, with plenty of drift for keeping clear; and from my own observations, when a ship, near these islands, during the night shoals the water under seven fathoms it is time to let go.

As many ships pass through these Straits, a few remarks on the town and anchorage of Samboangan may be serviceable. The anchorage off the town is not good. The bottom being foul, and the bank steep, many anchors have been lost and broken, yet when the localities are known there will be little danger of these casualties. I have anchored here four times within the last eighteen months,-the first time we lost an anchor, the chain broke endeavouring to weigh it, and the last time our anchor brought up two others. Several other anchors have been lost and broken within the same period; and the "Warrens," of London, a short time prior to this, hove up two other anchors with her own as we did, one of which weighed 20 or 30 cwt. These facts will show the nature of the anchorage.

Immediately in front of the town, and at depths of from fourteen to eighteen fathoms, the bottom appears to be composed of large rocks, and here all the accidents have happened of which I know. Abreast of of either end of the town, the fort being at the eastern extreme and the church at the western, the ground is considered more even. Some think off the fort there is better anchorage, as you then let go in seventeen or eighteen fathoms; but as a general rule it is affirmed by the coasters here, that all along this part of Mindanao from Point Caldera, the bottom is foul when in deeper water than twelve fathoms. This depth, however, off Samboangan is very close in. With fifty fathoms of chain out a ship will swing into seven fathoms, and will not be more *The above description will serve as a useful caution to vessels navigating these Straits when in the neighbourhood of the Santa Cruz Islands. In the old chart of the Sooloo Archipelago, published by Dalrymple, in 1771, and still used by H.M. ships, there is a shoal of three fathoms mud due south of the westernmost Santa Cruz Island, and about midway between the position of the shoal above given by Captain Miller, which we have no doubt forms one of the patches extending to the eastern island to which he alludes, perhaps the same alluded to by the Sybille. If the chart in question had been on board of the Ann of Greenock, this would have been some, if not sufficient warning to her captain of the danger he was running on; and thus saved all the ills which followed. Horsburgh'a chart ought to have shewn the shoal laid down by Dalrymple.-ED,

than half a cable's length from the shoal water. Yet there is hardly any danger of driving in that direction, the tides being very strong, usually from 3 to 4 knots, and taking the direction of the land. It is, therefore, necessary to moor. Should a ship, in swinging, foul her anchor she will be sure to drift with it. From eighteen or twenty fathoms the water deepens very suddenly to thirty, then to thirty-five fathoms, and although this is deep water for ships to bring up in with chain cables, several have done so, and have all recovered their anchors, thus proving the bottom to be more even.

The refreshments to be obtained at Samboangan are bullocks, fowls, fruit, and yams; and in the shops many trifling articles of utility and consumption may be bought. All except the fruit are dear. A small bullock or calf, weighing about 2 or 3 cwt. costs six dollars; they are very lean, and the meat very dry. Fowls are not plentiful, but in general a few dozen may be gleaned up, the price to strangers (there being two prices for every thing at Samboangan) is from 2 to 3 dollars per dozen. Yams 2 and 3 dollars per pical. Fruit, consisting of pineapples and plantains always in plenty, and tolerably cheap, also cocoanuts. The water is excellent and easily procured. It runs behind the beach in a fine stream, but unless filled very early in the morning it is rather thick; the whole female part of the population making a point of washing in it every day, and stirring the mud up; yet it soon settles. We have always cut our wood on the St. Cruz Islands, which I believe is customary with most ships. There is, nevertheless, plenty of it on the large island, and I have been told some of rare qualities, of which, at present, no advantage is taken.

The town of Samboangan contains about 6 or 8,000 inhabitants, of whom, perhaps, 20 are Europeans, chiefly officials. The Governor is a Lieut.-Colonel in the Spanish army; and it is said they muster here about 200 soldiers, such as they are, mostly militia; the others are convicts from Manila, this being a penal settlement. On arrival they are made soldiers, and continue so, I believe, for life, unless they can make their escape, as many do. There is also a gun-boat station here for the suppression of the Sooloo and Mindanao pirates. About 200 men are in this service, commanded by a Lieutenant in the Spanish navy. The other naval and military officers, with the exception of a Commandant of Artillery, are the coloured natives of the town.

The little trade which is carried on scarcely deserves notice. A few Sooloo proas come across occasionally, bringing Beche de Mer, or, perhaps, a small quantity of gold dust, or, the edible birds'-nests, which are bartered for cotton goods supplied from Manila. I believe no shipments of indigenous produce of any kind are made, a result not proceeding from want of capabilities in the land, or, an ill chosen situation, but from lassitude and absence of all spirit and energy in the governing party influencing the whole population.

In connexion with Samboangan, it may be observed that, the Spaniards have a small fort at Caldera, which is about seven miles to the westward, and at which place I have been credibly informed there is a secure but small haven, having four fathoms least water, perfectly safe in all weathers; and having steep shores where a ship may heave down, or, undergo any repairs, which they are able to do without assistance from Samboangan, as at present nothing of this kind may be

looked for from there. In the case of the Ann, (which ship it was necessary to heave out,) all that could be obtained from the authorities was the offer to store the cargo in Samboangan, then to put the ship in this port of Caldera, and they would supply the master with a vessel. to go to Manila for workmen and materials to put the ship in order, although, there was attached to the gun-boat station, enough to have answered the purpose. But, being a stranger they objected to supply her from this source. Natives of the town were also hindered from entering on board to work the pumps while she made a passage to Ilo Ilo or Manila, both their own settlements, saying the men's lives were not safe on board, even should a small vessel be hired to convoy them. She ultimately proceeded to Sourabaya under convoy, but not that of the Spaniards.

ON RATING CHRONOMETERS.

Charlotte Town, Prince Edward Island, 14th January, 1843.

SIR. Although chronometers are now so generally used, I have never yet met with a simple, and at the same time accurate rule, for finding the correction to the difference of longitude measured by them, which becomes necessary when it is subsequently discovered that they have changed their rates. Yet, as chronometers on board a ship are seldom or never found to have the same rate at the end of a voyage that they had at its commencement, it is manifestly a matter of some importance that simple rules, easy of application in all cases, should be given; by which either the required correction to the result of a previous computation may be directly obtained, or new and more accurate data supplied for a repetition of the calculation for the chronometric difference of longitude.

In hydrographical operations, especially, the necessity for such a rule is very apparent; and, I have, therefore, been induced to offer one founded upon the hypothesis which an experience of many years in the use of chronometers has taught me to consider as most probable :excepting in those cases where the daily comparison of the chronometer with a number of others, a consideration of its previous rate, or its habitual performance show us that it is inadmissible.*

Let it be supposed that, in a ship about to proceed on a voyage the errors of her chronometers on Greenwich mean time, have been ascertained by the necessary astronomical observation, and at the same time, by comparison with a former observation, their mean rates for a certain period, or number of days, previous to the last observation. With these first rates the longitude of the places which she calls at in the course of her voyage are approximately determined; but at the last of those places she remains a sufficient time to obtain the rates of her chronometers for another period or number of days, and finds that these second rates are different from the first.

It is required to find the correction to the longitude that becomes

The hypothesis is not new, having been adopted by my respected friend and former commander, Capt. W. F. W. Owen, R.N., in his survey of Lake Ontario, in 1816; but I have applied it somewhat differently, and also endeavoured to develop its application more fully than he had occasion to do at that time.

necessary in consequence of such a change of rate, in the case of each watch separately, and under the supposition that the acceleration or retardation of its rate had been uniform.

Letv express the difference between the first and second rates; and t the number of days between the middle of the periods for which the two rates were obtained. Then will r, the ratio of acceleration or retardation per day.

v

Let n express the number of days for which the first rate was obtained; and p the period, or number of days for which the correction to the longitude is required :—then will " + p × r = x the cor

rection to the longitude in time.

n+p

Because "+ × p may be considered as expressing the sum of a series in arithmetical progression, the ratio of which is r, considered as a unit; the number of terms = p, and the first and last terms of which represent the space passed through by the watch (considering alone that part of its motion which is accelerated or retarded,) on the first and last days of p.

Or, as the acceleration or retardation is assumed to be uniform,* and to commence from the middle of the period (n,) for which the first-rate was obtained; and as, moreover, the space passed through by the chronometer in the first day is equal to half the velocity acquired at the end of that day; therefore

(} n + p2) — } n2 × 1 r = x.

for the whole amount of the space described by the uniformly accelerated or retarded motion of the chronometer in any time will be as the square of that time.

But (n + p2)- } n2 × } r = n + p xp × 2 r therefore

n' + p
2

xpr = x.

The rule may thus be expressed in words. To the number of days for which the first rate was obtained, add the number of days for which the correction is required; multiply half the sum by the last named number of days; and the product by the ratio of acceleration or retardation per day. The last product will be the correction to the longitude in time; which in west longitude, is minus with an accelerated gaining, or a retarded losing rate; and plus with a retarded gaining, or an accelerated losing rate; and in east longitude vice versá

EXAMPLE I.

Let us suppose the ship to sail from Plymouth, and that the rate of her chronometer had been previously ascertained, by observations seven days apart, (say on the 1st and 8th of June,) to be 3.5s. gaining on mean time per day. Moreover, by the last observations the error of the chronometer was found to be Oh. 1m. 10s. fast of Greenwich mean time.

⚫ It is, of course, not pretended that the acceleration or retardation of the rate, any more than the rate itself, is ever absolutely, or, in mathematical strictness, uniform, but only that it often approximates so nearly thereto in good chronometers as to justify the adoption of our hypothesis under certain limitations, the nature of which has been mentioned, and which must be left to the judgment of the observer.