seas, as the currents from the southern seas running into the northern are found warmer."

Notwithstanding, however, the authority of so great a man, experience has taught me that this theory is liable to much objection. I have often found my vessel set to the northward, and at the same time the ocean water becoming colder. As currents are known frequently to shift to contrary points of the compass, in the open sea, as well as near the shore, that which I have just remarked may with plausibility be attributed to my having entered the current at the commencement of the change of its direction from south to north; and that I have been carried northward by the same body of water that had just run to the south. From a similar cause, the commencement of a southwardly wind in winter, where a north wind has just ceased to blow, is found nearly as cold as the north wind.

In cases of sudden change in the temperature of the sea-water, I have generally found my ship to be more or less affected by a current, which I discovered by the aid of my chronometer.

But nothing in the temperature sufficiently indicates the direction of the current. Sometimes, however, on a change of the water's temperature, I have not been sensible of any current whatever. Perhaps this has been when the current was on the turn. Yet, notwithstanding this uncertainty, the thermometer's indication may serve to put navigators on their guard, as thus to avoid danger.

Description of the Thermometer.

The thermometer is an instrument to measure different degrees of heat. It is a small glass tube, with a bulb at the bottom. Having the bulb and part of the tube filled with mercury or spirits of wine, the tube is closed at the top, and the part not occupied by the fluid is a vacuum. Against the tube there is a scale to measure the expansion of the fluid, under different temperatures. The fluid expands by heat, and contracts by cold. An increase of heat will therefore make the fluid rise, and a decrease of heat will make it fall.

The thermometer now in use is that which was constructed by Fahrenheit. On this scale the fluid stands at 32 degrees, when it just begins to freeze; and at 212 degrees when put into boiling water. At temperate it stands at 55 degrees; summer heat at 76 degrees; blood heat at 98 degrees. If the scale be continued to 600 degrees, it gives the heat of boiling mercury; and if it be continued downward to 90 degrees below zero, it gives a degree of cold which will freeze mercury.

Cautions in using the Thermometer.-Remedy of its Defect.

This instrument should at times be turned upside down, in order to discover if any air has been admitted into the tube. When thus reversed, if the mercury run freely from one end of the tube to the other, there is a perfect vacuum in the upper part of the tube, and its indication of degrees of heat will be therefore correct. But if any air is contained in it, the mercury in this case will be suspended; and the upper end of the instrument should therefore be examined. If there should be found in this place the least opening to admit air, the instrument should be put into hot water (but not hot enough to burst the tube) when the mercury, if the heat be sufficient, will rise so as entirely to fill the tube, and thus force out the air; at which time the aperture should be immediately closed with sealing wax, which will cure the defect.

To seal a thermometer hermetically, is to heat its neck or end till it becomes soft, and then, with a pair of pincers, to close it, by twisting that part together.

16

ON THE

LAND AND MARINE BAROMETER.

THE Marine Barometer is a useful instrument, in high latitudes, in assisting navigators to anticipate approaching storms. Previously to a hard gale of wind, there is a great fall of the mercury, and even near the tropics, its fall before a storm or a hurricane is usually considerable.

Within nine or ten degrees of the equator, there seldom or never is a hurricane, or a storm of long duration, but whirlwinds and hard squalls of short duration are sometimes experienced within these parallels of latitude, without any fall of the mercu ry in the barometer.

The barometer is of little use in predicting storms which may happen within the tropics, except before a severe hurricane; then there is a considerable fall of the mercury, and that is when the latitude is not less than 14 or 15 degrees north or south.

In high latitudes the motion of the mercury in the barometer is uncertain; but previously to a storm or a gale of wind, there is a considerable or great fall of the mercury; and the mercury often rises before the conclusion of the gale, as the equilibrium of the atmosphere begins to be restored.

The mercury falls considerably before a heavy fall of rain; and when the mercury stands low in the barometer, the air is light, and deprived of its expansibility or elasticity. At such a period, rain generally falls. The mercury also sinks on the approach of thunder and lightning, or when the atmosphere is highly charged with electrical matter. In serene, settled weather, the mercury generally stands high, and also in clear frosty weather. In the northern hemisphere, in the open sea, the mercury rises with northwardly, and falls with southwardly winds; because the former, coming from the frozen points near the pole,

are more dense than the latter, which blows from the equatorial regions. In the southern hemisphere the contrary takes place; for there the mercury rises with cold southwardly winds, and falls with northwardly winds. These effects are more particularly observed in high latitudes, in the open sea, for obstructions and irregularities will always happen near the land, because there the rarification and expansibility of the atmosphere is not so equal as over the open ocean.

By a little practice with the marine barometer, and proper attention, the navigator may in some seas be enabled by its indications to take in and make sail.

I have, in several instances, found the marine barometer of material service; once in the ship William Savery, from Savannah, bound to Liverpool, in latitude 47° north, to the eastward of the banks of Newfoundland, after a long continuation of hard gales and squalls from the westward, and gusts, with thunder and lightning; and after clearing away and moderating a little, I was on the point of making more sail; but on observing the mercury to continue very low, I thought it best to defer it a little while; and in twenty minutes afterwards, it blew harder than before, during those several days of heavy gales and sudden gusts, and almost constant lightning; the atmosphere was so completely charged with electrical matter, that a person could not touch our lightning chain without receiving an electric shock in some degree, and the compass card frequently fluttered very much; but when the gale began to cease in reality, the mercury began to rise at the same instant with the first appearance of the gale's breaking; a great fall of the mercury in the barometer is a sure indication of the approach of a hurricane in latitude not less then 14° or 15° within the tropics, north or south. In the year 1806, while I was at the Isle of France, in the ship Orion, of Philadelphia, there was a very heavy hurricane in February, and another in March, which did much damage; and two vessels were lost at the island of Bourbon. The fall of the murcury at this place, is so certain an indication of an approaching hurricane, that the lieutenant of the port went on board every vessel then lying in port, to give instructions to have the yards and topmasts down, the vessels well ballasted and well secured with cables and anchors. Cables and anchors can always be hired at that place; and although I had the Orion secured with two anchors of 7000 lbs. each, they were brought home, and the ship went on shore, as well as every other vessel in that port, though all were got off without damage, except one ship, called the Topaz of New York, which, by drifting on to an anchor fluke, which went through her bottom, and, by that means, nearly filled with water. She was afterwards got up and repaired. The lieutenant of the port gave these instructions one

day previous to the hurricane, so that the barometer is certainly useful to those sailing in the hurricane months near the island of Roderigo, the Isles of France and Bourbon, and the bay of Bengal. Vessels and lives might be saved, by preparing for hurricanes in time.

RULES

For predicting the weather by the Barometer, by Drs. Halley and Hutton, Messrs. Pascal, Patrick, Rowing, Changeux, De Luc, Clark, Hattan, and many others, from whose writings the following rules are collected.

When the mercury rises it is a sign of fair weather, attended with heat, if in summer, but frost in winter. When the mercury falls it denotes wind and rain, or perhaps both. If the mercury rises suddenly during the time of rain, the ensuing fine weather will not continue long, but if the rise be gradual and continues for several days, the continuance of fair weather may be expected. If the mercury falls suddenly several divisions, it is a sign that the succeeding rain will not be of long duration; but if the mercury continues to fall regularly for several days, rain or wind, or perhaps both, will be of considerable duration. The mercury falling considerably in autumn, winter or spring, indicates gales of wind, commonly attended with rain, snow, or sleet; but in summer it denotes rain, and probably thunder and lightning. The mercury stands low with high winds, and still lower if accompanied with rain. If the mercury falls quickly in very warm weather, thunder showers may be expected soon after. If the mercury be in an unsettled and fluctuating state, the weather has the appearance of being very changeable.

If the mercury has been stationary during several days, its surface must be carefully observed, in order to ascertain whether it is rising or falling; for this purpose let the exact figure at the surface of the mercury be observed; then shake the tube a little, and observe if the mercury is more or less convex or concave. If it is more convex, it is a sign the mercury is rising, if the same as before, it is stationary.

If the mercury was concave before the tube was shaken, and more concave afterwards, the mercury is falling; if of the same concavity, or nearly so, it is stationary, but if less concave, it is rising.

Between the tropics there is very little variation in the heights of the mercury in the barometer, except in the before-mentioned cases; and the more distant any place is from the equa