hesitation the packet was admitted to free intercourse with that garrison. In fact, a portion of the passengers remained on shore there, and embarked on board another vessel for England. But on their arrival at Falmouth, they found their friends, who had come all the way in the government steamer (having also touched at Cadiz, and there had pratique,) performing quarantine, because the plague existed at Tripoli!

But of what use are our English regulations in effecting this care of the public health, which is contemplated by the quarantine laws? To carry such laws into actual effect seems to require the existence of an arbitrary government, or at all events, a garrison establishment such as Malta and Gibraltar; another name for despotic government. It would seem that the kind of liberty to which Englishmen are accustomed at home, precludes the possibility of enforcing quarantine laws. Here is another fact bearing on this part of the question that has fallen under my observation. On one occasion, being under quarantine at Standgate Creek, there was at that time a vessel there which had come from Trieste, having cotton on board. This cotton had come from Egypt. It does not, however, much matter where it had come from. Our quarantine regulations considered it capable of introducing the plague, and enforced its being put into an old hulk, stationed there for such purposes; and on board which the cotton was to be aired, and undergo the necessary probation. The merchant vessel was made fast along side the hulk; had her sails loosed, and got ready for a start; and as the last bale of cotton was put into the Lazaretto, she slipped her lashings, made sail, and in a few hours was in London, and the crew dispersed in all directions! Now this was so soon after they had been handling an enumerated article," as our laws have it, and one perhaps, of all others, most likely to be the means of conveying contagion! It has been my fortune to come up channel in a vessel with a foul bill of health; that is to say, coming from a place where the plague existed. The freest intercourse with boats, fish bought from them, and paid for, or more generally bartered for, &c,; and communication unrestricted has gone on between the parties, and even some letters have been sent on shore from her. But enough of this has passed under my observation, sufficient of this kind of breaking of quarantine laws, to convince me that they are repugnant to the free institutions of Britain; that they cannot be carried into execution, and had therefore better be done away with entirely. It only requires one (or two would be better) of the leading powers of Europe, to see this in its proper light; to take the thing up in earnest, and not to allow the minor powers to awe them into the continuation of what they decide on as useless. Seeing that these pettifogging and powerless nations do, in fact, make use of their quarantine laws, (with western Europe and unsuspected places,) more as measures of revenue and police, than with any regard to their professed object.

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Some twelve or fourteen years ago, this subject was much discussed; but nothing resulted except some alterations of who was to bear the expense, which was previously thrown on the ships, or rather those who consumed Levant produce. It was upon that occasion spread over the broad shoulders of honest John Bull," and made a publie charge, which was no doubt quite correct, if suffered to remain at all. Let us hope, Mr. Editor, that through the medium of your pages the subject may again come under discussion, and that the result may be the rescinding of the quarantine laws entirely.

AN ADVOCATE FOR THE FREE INTERCOURSE OF NATIONS. London, August, 1838.

[The reader will find at page 778 of our volume for 1836, an interesting and valuable article on this important subject, from the pen of an American writer, whose opinions as being those of a medical man are entitled to additional respect. With regard to the plague itself, against which an effectual quarantine is most necessary, some useful discussion has taken place at the meetings of the British Association, which may form matter for another number of the Nautical.—ED.]

DESCRIPTION OF ALACRAN AND CAY ARENAS, IN THE Gulf of MEXICO.

FROM the survey of Don Ciriaco De Cavallos, in 1802, we supposed the port of Alacran capable of admitting small craft only. It is true, a vessel drawing eleven feet may pass over the rocky heads between Perez and the South Spit, which make the entrance intricate; still, with the wind to the south of east, which it generally is in the morning or rather before noon, a vessel drawing eighteen feet may sail in. It is one of those harbours that can only be taken by eye; and from aloft every shoal may be plainly seen. A vessel entering should pass close to the South Spit, and run N. & W., until within a cable's length of Perez Spit; then haul close round it, keeping in the blue water, and anchor with the Huts W. by N. one-fourth of a mile. The white water on Perez Spit can be plainly traced from the island, coming in from the westward, with the Huts bearing N. and N.W. by N. There are heads with sixteen feet over them, a quarter of a mile off from the reef; therefore the eastern extreme of Perez Spit should not be brought to bear to the eastward of N.E. by E.

Alacran affords a very secure harbour; the dry reefs protecting it as effectually as would the land. The outside anchorage, two cables north of the South Spit, in six fathoms and a half, coral sand, is very safe, with all but westerly winds, which are rare, and give timely warning.

The tide, if it can be so called, is very remarkable. It was new moon on the 25th of March, about three in the afternoon. On the following day the tide fell from 6 A.M. to 6 P.M. 74 inches, and the

DESCRIPTION OF ALACRAN AND CAY ARENAS.

next day 13 inches, the wind being from E.N.E. to E.S.E. moderate. Again, on the 26th April, two days after the new inoon, wind in the same direction, rather less, it fell twenty-six inches from 7 A.M. to 6 P.M. This was a very unusual tide, and several reefs were quite dry that were not seen before above water. We may suppose this rising of the waters in the morning to have been occasioned by the land and sea wind, (the few inches are scarcely worth On the plan therefore, noticing,) the mean height being about noon. there may be, sometimes, a foot more or less than there shown. The fishermen we found, on our second visit, are supplied with water from the main-land; a strong proof that it cannot be procured at Alacran. The dampness of the atmosphere was remarkable; the sails wet with salt water, exposed to the burning sun for two or three days, would not dry, and the fog in the morning was regular and very uncomfortable.

its

The main reef forms a regular segment, convexing to the N.E., base or extreme length tending N.W. by N. fourteen miles. The lead does not give sufficient warning of approach, the abrupt descent from twenty fathoms to dry rocks is very remarkable, as are the shelves on the west side from seven to twenty-four fathoms in a ship's length. The nature of the deep water soundings is very fine sand; that called grey white and yellow is much of the same kind; a hornprotractor laid on white paper will give the exact tint. This will be found the predominant colour on the ground of Campeche Bank. To the eastward of the Alacranes there is no appearance of soundings, with a constant current to the N.W. from one to one-half knots per hour. The dry sand-bores are conveniently situated for our work. Three of the same stars as were observed at Pajaros make the high north bore in lat. 22° 32′ 15′′ N.. These sand-bores soon get covered with grass, samphire, and various kinds of herbs, when ahove water. The first formation of all is branches of dead coral. These we found by digging to the level of high water mark, and is probably the reason the Alacran does not contain fresh water. All the cays swarm with boobies and man-of-war birds, now with their young. The only eatable kind are plovers and sand-pipers. Fish of all kind are very abundant, particularly groupers and rock-cod. The fishermen dry them for the Campeche market; they had nets for taking them and the hawk's-bill. Turtle are also plentiful.

T. S.

[Much has been said of the current on the south side of Cuba. This is the third time I have found it settling to the eastward when the moon is increasing, or in her two first quarters. It continues from Cape Antonio to Cape Maize. Mr. Shepherd, a pilot, residing at Port Antonio, informed me it ran to the eastward a fortnight, and then to the westward about the same time. Coasters from the Caymans also give this as a reason for making their passages to Jamaica at this time.

[Arenas. In our last number the meridian distance of Arenas from Alacran was given as 6' 51",5, it should have been 6m. 51s. 5.-ED. N.M.]

ON THE ADAPTATION OF DIFferent Modes OF ILLUMINATING LIGHTHOUSES DEPENDING ON SITUATION, AND THE OBJECT INTENded in THEIR ERECTION. By W. H. Barlow, Esq.

(From the Philosophical Transactions.)

Constantinople, March 14th, 1837. HAVING made several experiments with the Drummond light, and other means of illuminating light-houses, undertaken at the request of the Turkish government, with a view to placing lights at the entrance of the Bosphorous from the Black Sea, I have been led to observe some facts regarding the illuminating powers of the lights themselves, and the increase obtained by the use of reflectors and lenses, which, I trust, may not be found uninteresting.

On the increase of illuminating power obtained by lenses and reflectors.

Let L in the annexed figure represent a lamp; mm, two reflectors, which may be so adjusted as to throw the reflected images either in parallel lines on a screen at P' and P", or at such an inclination as to unite with that of the light itself at the centre point P. Let also ss represent a screen of such imperfect transparency as to absorb the same quantity of light in transmission as the mirrors m m absorb by reflection; then in the first case the three images P' P P'' will have equal surfaces and intensities, and the illuminating power will be three times that of the central lamp; and when by a different adjustment of the mirrors the three images are blended in one, then the surface will be equal to that of the central image, but the intensity is three times greater, so that in either case the illuminating power will be proportional to the number of mirrors, or to the surface of those mirrors. If, therefore, we conceive the whole space between m m to be filled with mirrors, to reflect the light in parallel lines on

* We understand that Mr. Barlow was presented on this occasion by the Sultan with the diamond order of "Nishaun é iftihar," or "Badge of Honest Pride,” accompanied with a firman expressive of the Sultan's satisfaction with his services and zeal.-ED. N.M.

We reject here the difference in the length of the trajectory of the direct and reflected light.

the screen P' P P", it is clear that the illuminating power of the lens will be expressed by the number of times the surface of the central image is contained in the whole surface of the screen P', P"; and this is true, whether we consider the several images to be thrown in parallel lines, or condensed in a focus, or dispersed over a larger surface; for as the illuminated surface is contracted, the intensity is increased, and as it is extended, the intensity is diminished in the same proportion; so that under all circumstances the product of surface and intensity will be a constant quantity. Hence the illuminating power (abstracting from absorption) will be increased by the reflector in the ratio of the surface of the lights to the surface of the end or section of the reflector. Or, in other words, the area of the end of the reflector divided by the area of the light, will be a numerical measure of the illuminating power.

The result is obtained by supposing the reflector to be composed of a number of small plane reflectors, each throwing the light in parallel lines, and each image therefore as having the same intensity as the direct light (screened as above) when viewed from the same distance; but with a continuous curve surface, such as a parabolic reflector, we must consider the divergency of the emanating ray at the point where it falls on the reflector, which will vary inversely as the square of the distance of that point from the centre of the light, or directly as the square of the sine of half the angle which the light subtends from that point, and therefore as the versed sine of half the same angle; and the sum of all these must be compared with the area of the reflector, that is, of its section or end, which varies also as the versed sine of half the angle which its extreme edge subtends at the light.

In order, therefore, to compute the increase of illuminating power due to a parabolic reflector, according to this principle, we must find a mean focal distance, that is, a distance (from which to estimate the constant angle subtended by the light) that shall be equivalent to the several variable distances.