tor, the greater is the change of the mercury in the barometer; thus at St. Helena the extreme variation is very little; at Jamaica it is only about three-tenths of an inch; at Naples it seldom exceeds one inch. In England the extreme range amounts to about two and an half inches; and at St. Petersburgh, three and an half inches nearly.

RULES

For predicting the weather by the mercury in the barometer, much improved by Mr. Patrick, and on the same principles with those of Dr. Halley.

1st, The mercury's rising in the tube, presages, in general, fair weather; and its falling, foul weather, as rain, snow, hail, sleet, high winds, and heavy gales.

2d, In very hot weather the fall of the mercury indicates thunder.

3d, In winter its rising presages frost; and in frosty weather, if the mercury falls three or four divisions, there will certainly follow a thaw; but in continued frost, if the mercury rises, it will certainly snow.

4th, When foul weather happens soon after the falling of the mercury, expect but little of it; and on the contrary, expect but little fair weather, when it proves fair shortly after the mercury has risen.

5th, In foul weather when the mercury rises much, and continues so for two or three days before the foul weather is quite over, then expect a continuance of fair weather to follow.

6th, In fair weather, when the mercury falls much and low, and thus continues for two or three days before the rain comes on, then expect a great deal of wet, and probably high winds.

7th, The unsettled motion of the mercury denotes uncertain and changeable weather.

8th, You are not so strictly to observe the words engraved on the plate, (though it sometimes agrees with them) as the mercury's rising and falling, for if it stands at much rain, and then rises to changeable, it presages fair weather, although not to continue so long as it would have done if the mercury were higher, so on the contrary, if the mercury stood at fair, and falls to changeable, it presages foul weather, though not so much of it as if it had sunk down lower.

From these observations, says Mr. Rowenge (Natural Philosophy), it is not so much the height of the mercury in the tube, that indicates the weather, as the motion of it up and down;

wherefore, to pass a right judgment of what weather is to be expected, we ought to know whether the mercury is exactly rising or falling, for which the following rules are given.

First, If the surface of the mercury is convex, standing higher in the middle of the tube than at the sides, it is generally a sign that the mercury is rising.

Second, If the surface of the mercury is concave or hollow in the middle of the tube, it is sinking.

Third, If it is level, or rather if it is a little convex, the mercury is stationary; for mercury put into a tube, especially a small one, will naturally have the surface a little convex, because the particles of the mercury attract each other more forcibly than they are attracted by the glass.

Fourth, If the glass be small, shake the tube, and if the air be growing heavier, the mercury will rise about half the tenth of an inch higher than it stood before; and if it is growing light, it will sink also that much. This proceeds from the mercury's sticking to the sides of the tube, which prevents the free motion of it until it be disengaged by the shock. Therefore, when an observation is to be made by such a tube, it ought always to be shaken first; for sometimes the mercury will not vary of its own accord until the weather it ought to indicate, be present.

In approaching the coast of Europe, the greatest heights of the mercury are found upon eastwardly winds, and it may often rain or snow, the wind being in those points of the compass; and the barometer may sink little or none at all, or it may even be in a rising state, the effects of these winds counteracting. But the mercury sinks, for winds as well as rain, in all other points of the compass; but rises when the wind shifts to the north or east, or between these points: but if the barometer should sink with the wind in that quarter, it must be expected to shift soon from that; or if the fall of the mercury should be considerable, a heavy rain is likely to ensue, as it sometimes happens.

The mercury rises with the winds from the east and north, and between these points, on approaching the coast of Europe; but in other places, where the winds are under different circumstances, the effects are different. When approaching the coast of the United States, the barometer rises with west and north winds, and between these points, and falls when it changes from these points. The atmosphere in westwardly winds, on the coast of North America is more dense, for the same reason that eastwardly winds are the more dense on the coast of Europe.

It has been mentioned that you are not so strictly to observe the words engraved on the plates though sometimes they agree. In Philadelphia, November, 1818, it blew nearly a hurricane from the south-east. Having a marine barometer in my house at the time, I remarked, that during the height of the gale, which was

from midnight till 5 A. M. the mercury in the barometer stood at the words much rain on the plate, and as the gale abated, the wind continued hauling until it came out from the north-west, when the atmosphere became clear, and the mercury rose to the word on the plate changeable.

Caution is necessary, not to unscrew the box at the bottom of the frame of the barometer, otherwise the mercury will run down out of the tube. This box contains a bag or reservoir open at the upper part. This bag is of flexible leather, and the air pressing upon it, keeps the mercury suspended at its proper height. Through the bottom of the box passes a screw, having upon its end a round plate, which screw presses upon the leather bag, and forces the mercury to the top of the tube, so that it is prevented from breaking the tube by dashing against the top of it, when the instrument is removed from one place to another. When this barometer is used, the screw at the bottom of the frame or box is to be turned, that the mercury may fall to its proper height, and indicate the corresponding changes in the weight of air. The screw is not only let down, that the mercury may subside to its proper height, but also give admission to the action of the external air upon the mercury contained in the bag. When the screw is let down the external air will then pass in through the same hole where the screw passes.

The tube in the Marine Barometer is very small, so as to prevent the mercury from dashing up and down by the motion of the ship, and for which reason, after a marine barometer has been removed from one place to another, and the screw is let down, a longer time is required for the mercury in it to subside than in a common house barometer, which has the tube much larger.

If the box at the bottom of the frame should be unscrewed by mistake or otherwise, and by that means the tube loses the mercury out of it; the mercury can be restored, if there should be any quicksilver on board; but before the mercury will run into the tube, the air must be expelled, which must be done by heating it, either by holding it in a hot gun barrel, or over a chafing dish of hot coals. The air thus being expelled by a paper funnel, that is, paper wrapped tight round the end of the tube like a funnel. Pour the tube quite full of quicksilver, so as not to allow any air to enter it. Do not allow any air bubbles to remain in or adhere to the sides of the tube.

If there should be any small bubbles of air in the tube, by moving them backwards and forwards in the tube, it will help to clear the mercury; and if the mercury be clear or pure, it will appear like a solid rod of steel in the tube.

Being filled after this manner, and being then closely pressed with the finger to prevent the air from entering, then invert the

tube; immerse the end of the tube in the reservoir of mercury, keep the finger to the end of the tube until immersed, not allowing the tube to touch the bottom of the reservoir or bag; then take away the finger and fix it in the frame as before, and the mercury will fall in the tube to its proper height, according to the weight or pressure of the atmosphere; leaving a vacuum in the tube above the mercury, which will allow the free motion of the mercury to rise and fall in the tube, according to the weight or pressure of the atmosphere on the surface of the mercury in the bag. The upper end of the tube is hermetically sealed.

The barometer may be applied to various other purposes than that of predicting the weather. In purchasing bulky goods by weight, such as wool, flax, or silks, the higher the barometer the greater the quantity of weight there will be.

No person ought to go down a mine, subject to fire damp, when the mercury in the barometer is low.

The barometer is also applied to the mensuration of mountains. In accurate observations for measuring mountains, &c. it requires attention in the construction and use of the barometer, and the temperature of the air, for, unless this remains the same, the dimensions of a given quantity of mercury will be variable, and the altitude of the mercury will be an uncertain measure of the weight of the atmosphere; because it is dilated by heat, and contracted by cold, when the weight and pressure are unchanged. Mr. De Luc attended particularly to this circumstance, and contrived to estimate the effects of heat on the quicksilver in the barometer, when it is used for the purposes above-mentioned, by means of a thermometer, the scale of which is divided in such a manner as to indicate with little labour of calculation, the correction to be made on account of heat. As an increase of heat that is sufficient to raise the mercury in the thermometer from the point of melting ice, to that of boiling water, will lengthen a column of mercury in the barometer six lines, which is half a French inch, or rather more than an English half inch. The scale of the thermometer marks the interval between the freezing and boiling points, answering to six lines of the barometer, which is divided into ninety-six equal parts, each of which will correspond to the 16th of a line in the motion of the mercury in the barometer, dilated by heat; which must be added or subtracted from the height of the mercury in the barometer for every degree of variation of the thermometer, so graduated. A scale of this kind continued above boiling or below freezing water, is annexed to his portable barometer and

thermometer.

Mr. De Luc prepared two barometers with their respective

*This is not noticed in predicting the weather by the barometer.

thermometers, graduated in the manner explained; he placed one pair in the cellar of one house, and another in the upper room of another house in the lower station, so as to be exactly on a level with the cellar; he found that the thermometer in the room rose nine degrees, and the barometer nine sixteenths of a line higher than those in the cellar, whence he shows that without allowing for the effect of heat, the difference in the heights of these two barometers would have indicated a difference of about forty-five feet in the heights of these two places, though they were both on the same level.

The aforesaid calculation is only necessary in accurate observations, as has been before mentioned; in predicting the weather it is not necessary.

The barometer is so formed that a column of quicksilver is supported within it, to such a height as to counterbalance the weight of a column of air, of an equal diameter, extending from the barometer to the top of the atmosphere. At the surface of the earth, the height of this column of quicksilver is, at an average, almost thirty inches. As a cubic foot of quicksilver weighs 13,600 ounces avoirdupois, and as the height in the barometer is 2 feet, or reduced into decimals, thus, 2.5 feet; therefore 13,600 multiplied by 2.5 is equal to 34,000, which is equal to 2,125 pounds on a square foot; and 2,125 being multiplied by 14, which is the number of feet on the surface of a middle sized man's body, is equal to 29,750 pounds pressure of the atmosphere on a middle sized man's body; that is, when the air is of the above gravity, a weight which would be insupportable and even fatal to us, were it not equal in every part, and counterbalanced by the spring of air within us, which is diffused through the whole body, and re-acts with an equal force against the outward pressure. More of this hereafter will be explained when treating of the atmosphere, &c.

The number of feet in height of the atmosphere, corresponding with one-tenth of an inch on the barometer is variable, depending on the temperature and density of the atmosphere. The variation depending on the temperature is shewn in the following table, calculated for every five degrees from thirty-two to eighty, Farenheit's thermometer; from whence it may be easily calculated for the intermediate degrees, allowing twenty-one hundredths of a foot for each degree.

17