ON

THERMOMETRICAL NAVIGATION.

HAVING for the last twenty-seven years been in the practice of using the thermometer, with a view to ascertain its practical advantages, I am enabled to lay before the reader a statement of facts derived from my experience, which I have full confidence will be useful to them, particularly such of them as frequently sail on the coast of North America.

By the use of this instrument, the navigator is enabled to determine whether he is or is not in the gulf stream, on the American coast; for its water being several degrees warmer than that of the adjoining ocean, the rise of the mercury in the thermometer, when placed in the water, will indicate the presence of the gulf stream-and its fall, afterwards, that you have passed out of it. The advantages arising from this knowledge will be obvious, when it is considered that vessels may shorten their passage from North America by keeping in this stream, and in returning from Europe, by avoiding it; vessels after passing through the gulf passage, between the Bahamas and the Florida shore, bound to the United States, particularly when the wind is a head, may also shorten their passage by keeping in the stream. The general indications of this instrument gives also notice of their approach to soundings. This, however, is subject to considerable variation, which is greater or less on different parts of the coast, from the effects of particular winds, and other circumstances, which will be hereafter noticed.

It is well known that the gulf stream is occasioned by the eastwardly trade winds driving the water into the gulf of Mexico, where, becoming above its natural level, it endeavours to seek it, by running between the Bahamas and the Florida shore; it then takes a direction along the coast of America to and over the southern extreme of the Banks of Newfoundlandthence through and among the Western Islands-from thence it takes a south-eastwardly direction, becomes very much spread, runs among the Canary Islands, and to the southward of them strikes the Barbary shore, as far south as cape Blanco, where it arrives at its first starting point; and is again under the influence of the trade winds.

It is extremely probable that the loss of the Brig Commerce, Captain Riley, was occasioned by this stream, and that it is this which drove her on the coast of Barbary.

It is by some thought that the gulf stream, or what is called the Florida stream, is occasioned by water flowing from the Mississippi. But this opinion is not upheld by sufficient plausi. bility to obtain it general assent. The most that can be allowed is, that this river may contribute in a very small degree towards forming that stream. If it is even a principal cause, how is the greater warmth of the water at this place than the adjoining ocean water accounted for? It is not possible that it can be materially affected, by water issuing from a river, at a distance of S. E. of 1200 miles.

What ought to put this question at rest, is the fact, that from a survey made across the isthmus of Darien, it is found that the water on the Atlantic side is always the highest, that its height varies in proportion to the strength and constancy of the trade winds, and that at the time of this survey, it was fourteen feet higher on the Atlantic than on the Pacific side.

In 1801, when in New Providence, I made many particular inquiries, concerning some interesting facts, connected with this subject, of several masters of those small vessels which were in the habit of fishing and wrecking among the Florida keys. By them I was informed, that when there is a long continuance of westwardly winds within the limits of the eastwardly trade winds, the water among those keys becomes much lower than when the strong eastwardly trade winds prevail; and that at such times they have seen the summits of rocks above water, which they have, during the trades, sailed over in small vessels. Those who rely on these facts cannot surely attribute the origin of the gulf stream to the flux of the water of the Mississippi.

The water of the gulf stream is at all times warmer than that of the adjoining ocean; and it is from the indication by the thermometer of the change of the temperature of the water, that your arrival in, or departure from the gulf is known-the mercury suddenly rising when you enter it, and as suddenly falling when you pass out of it, between the Capes of Virginia and the Banks of Newfoundland.

The water on the coast, in spring, is colder than in autumn, althought the sun's declination is the same in both cases; for the cold it acquires during the winter renders a long succession of warmer air necessary to change it materially; and after having been heated in summer, a proportional time is necessary to render it cool by the presence of colder air.

Having crossed the gulf at all seasons of the year, I will here subjoin the following notice of the variations in its temperature to which I have found it subject. It is not, however, offered as a standard, but as a collateral notice, as I have found a reference to my former journals of little service to me, on any subsequent voyage. This may, however, be of some advantage

the observations of others; who, by proper precautions, will, twithstanding, find the thermometer a very useful instrument. In summer, in lat. 37° and 38°, I found it to vary in tempeture from 72° to 80°-in winter, same latitude, from 70° to 3o.

In lat. 30°, in summer, from 75° to 89°-in winter, from 74° 78°.

In the latitude of Cape Hatteras-in summer, from 78° to 4°-in winter, 70°, 72°, 74°, 75°, 76°, 77o, &c.

It has been already observed, that the temperature in the gulf tream, varies, in proportion as it is affected by particular cirumstances. The greater its volocity, the warmer the waer will be found; because, passing more quickly from a warmr to a colder climate, it retains a greater degree of warmth. The water of this stream, as will be hereafter more fully noced, being driven towards the shore by an eastwardly wind, and riven from it by a contrary wind, I conjectured that the wind hus affected it only to an inconsiderable depth, perhaps only a ew fathoms, and that the common water of the ocean would e found beneath it of the same temperature, as that of the ame latitude in other parts of the ocean. I therefore deternined to try an experiment which I thought might inform me on this subject. Count Rumford having made the upper surFace of a vessel of water boil and evaporate, while a cake of ice remained frozen at the bottom; and knowing, that without agiation, water in the upper part of the vessel, which is warmer than that beneath it, would not descend and intermix; I, therefore, took the first opportunity of endeavouring to ascertain whether the gulf water did or did not only flow over the common water of the ocean on soundings, and not communicate its heat below, or entirely displace the ocean water. I was then in the schooner Nancy of Philadelphia, bound to Havanna. To effect this purpose, I caused a keg to be made, such as is described by Dr. Franklin. It was made with a valve in the upper head, opening on the outside, and another on the lower head, opening inside the keg. This being let into the sea, and sunk by a weight, the resistance of the water kept the valves open in descending so as to admit the water of any depth to which it might be sunk, and which, being drawn up, with a taught line, kept the valves closed, so as not to admit any other water into it. By this means, the water of any depth might be obtained. I let this keg down to the depth of 100 fathoms, and found the water it contained, 4, 5, 6 and 7 degrees colder than that on the surface. Returning home, on the same voyage, I tried this experiment again, off Cape Lookout. I ran in towards the land from the gulf stream-wind S. S. E. but from the vessels drift

ing, could not get more than 60 or 70 fathoms perpendicular line: at that depth I found the water to be only two degrees colder than that at the surface; which difference I concluded might be found in any other part of the ocean. I therefore abandoned all further experiments on that subject, reflecting also, that the gulf stream communicates more latterly than perpendicularly.

For those who are curious, I will also add another expedient, by which this experiment may be made. If an empty corked bottle be attached to the lead, and let down to the depth of 35 fathoms or more, the weight of the water will force the cork into the bottle, which immediately filling with water, and being swiftly drawn up, will furnish water of the depth at which it was received into the bottle.

Mr. Jonathan Williams observes, in his treatise on Maritime observations, page 80, that the water on soundings, on the American coast, is colder than the air. But experience has taught me, as well as many others, that this allegation is very erroneous. I have found the relative temperature of the air and water in such places extremely variable, and on some occasions, the former 12° colder than the latter. This may be easily accounted for. When so large a body of water has become heated by avery warm summer, it requires the presence of cold air for a considerable time, in order radically to affect the water. When therefore there is a sudden change of wind from south to north, while the water has not yet undergone this change, the air will be found colder than the water, as it would not immediately be materially affected thus suddenly by the air.

Mr. Williams, however, himself, furnishes a refutation of this fact, which he has above stated. In a journal kept by him, on his passage from Virginia to England, he asserts, that the whole time they were on soundings on the coast of America, the air varied from 5° to 8° colder than the water.

He also says, p. 14, that water over banks, is always much colder than that of the main ocean. This is also incorrect.-It is indeed colder over some banks, but it is warmer over others, which will be seen by referring to my observations in this article, on Cape Lagullas, bank off the Cape of Good Hope, the Bahama bank, the waters of tropical lattitudes, and those of the coast of America, from Cape Carnival to Cape Hatteras; and some distance to the northward of the latter Cape,-which latter places will be found to disagree with his account, when the wind blows towards the shore on the coast of America.

The velocity of the gulf stream varies under different circumstances. When strong westwardly winds prevail within the limit of the trade winds, it in a partial degree overcomes the force with which the water is driven by the trade winds into the gulf of Mexico; and in this case, there being at that place less water

accumulated than under different circumstances, it takes its course from thence with less velocity, which affects it throughout its whole course; and as the water is the warmer the nearer it is to the line, it also, from this circumstance, taking a longer time to reach the North American coast, is found cooler, from its longer exposure to the air of a more northern latitude. It is also found, that the precise course of the gulf stream is not always the same: but that it also varied with the action of the winds upon it, during its course along the coast of America: for a south-eastwardly wind drives it nearer to the coast, and a north-westwardly wind drives it further out in the ocean. Another effect is also observed from this action of the winds. It is contracted into a narrower channel by the south-east wind, and its velocity thereby increased, and becomes more spread by a north-west wind, and its velocity thereby diminished.

I have had repeated opportunities of observing the variations in the width of this stream, from lunar observations and good chronometers; and I have uniformly found its edges to differ in longitude, at different times in the same latitude, in proportion as the winds tended from the east or from the west; and I will here recite one instance of the gross and dangerous mistakes to which persons are liable, who are unacquainted with its variations.

In 1804, returning from Bordeaux to Philadelphia in the ship Orion, I spoke a schooner on the outward edge of the stream, bound to Rhode Island. Her captain inquired, and I told him my longitude, from a lunar observation taken that morning. But he insisted that I was at least fifty miles nearer the shore, as he had entered the gulf stream; and it was in vain that I endeavoured to convince him of what was actually the case, that a north-west wind, which was then blowing, had set the stream that distance to the eastward of its regular course as laid down in the charts; and thus he left me. Now, if the wind had been in a contrary direction, and, instead of driving the stream fifty miles eastward, had driven it fifty miles to the westward of its usual course; and if he had remained thus obstinate, and taken a fresh departure, he would at least have run his vessel into danger, and perhaps quite ashore; and such instances, I doubt not, have been frequent from such mistakes; and that the disasters of many vessels which have been run on shore between Long Island and the Highlands may be attributed to them.

Though a sudden fall of the mercury generally indicates the time when a ship comes into the water between the gulf stream and the shore, and by a further fall declares when she is arrived upon soundings, from latitude 36 degrees north quite to the banks of Newfoundland, yet it is sometimes found otherwise; cases of which I will here recite.

In September, 1800, on my passage from Amsterdam to