shew the weight of the atmosphere with perfect accuracy; because as the mercury sinks in the tube at top, it rises in the vessel at bottom, and as it rises in the tube it sinks in the vessel. Hence, a fixed scale cannot give the heights accurately; and for this reason various contrivances have been fallen upon by which to remove the objection. One is a light float moving upon the surface of the mercury in the basin, but as this, however light, must always give some pressure, it is not perfectly accurate.

The only change which an alteration in the height of the mercury points out, is a change in the weight of the whole mass of the air over the surface of the mercury in the basin. It does not point out the cause by which that change is produced; and of course it cannot point out, or foretel, with certainty, any consequence that is to result from it The cause may be in any part of the atmosphere; and though it may be assumed, generally, that any motion or current of the air must tend to diminish its weight, upon the same principle that a body may be made to pass rapidly over another, without doing much injury, that would crush it if left to act upon it for any considerable time, and that this wind should be indicated by a fall of the barometer, and tranquil weather by a rise; but as the motion may be of an elevated portion of the air, or may be for the counteraction of a disturbing force, it does not follow, as a matter of absolute certainty, that the effect shall follow at the earth's surface. The barometer is usually styled a weather glass; but it is not an infallible one in the case of wind, and it is far from being so in that of rain; yet still, as the wind is nothing but a disturbance of the atmospheric strata, and as the tendency of these disturbances is to produce rain, the barometer is of some use in marking the changes of the weather, and the credulity of mankind leads them to place more confidence in it than it deserves.

In order that the barometer may show, accurately, the variations of the weight of the atmosphere, the tube must be very smooth, and so wide, as that the friction or rubbing against its sides may not affect the motion of the mercury; and it must be kept at as uniform a temperature as possible, because the mercury expands and becomes lighter when exposed to the heat, and contracts, or becomes heavier, when exposed to cold.

The total range of the barometer is much smaller in the regions of the equator, than in the more distant parts of the hemispheres; because there the motions of the atmosphere are more dependent upon the sun, and less upon those other causes which produce variable winds and currents; but the daily fluctuations, which depend on the presence and shining of the sun, are greater as well as more regular. Whatever those disturbing forces may be, the sun seems to moderate their action; for, in the six months, from the middle of March to the middle of September, the variations are not much more, in our Latitude, than half of what they are for the remainder of the year.

Sympiesometer. This instrument answers the purpose of a barometer, is more portable, and not so apt to be broken by the weight of liquid in it. The bulb, D, at the top, with a portion of the tube, C B, is filled with hydrogen gas, a fluid compressible like air, but much lighter; and the vessel, A, with the lower part of the tube, is filled with almond oil, coloured so as to be seen through the glass. The pressure of the air upon the surface of the oil in the vessel, A, forces the oil into the tube, and compresses the hydrogen in the upper part; and when any part of the pressure in the lower vessel is taken off, the elasticity of the hydrogen causes the oil to descend in the tube. The range of this instrument must be found by comparing it with a good barometer, at different heights, and both in marking the divisions, and in using it when made, it requires to be carefully corrected for temperature, by a sliding scale, H; for which purpose a thermometer, K I, is attached to the instrument. The sympięsometer is an ingenious instrument; but it is too delicate for common purposes; and, indeed, most of the attempts that have been made to render the barometer more sensible to the changes of the atmosphere, have failed in the essential requisite of simplicity.

JUDGING OF THE WEATHER.

The following rules are usually given for this purpose; but they are not to be implicitly relied on:

1. The rising of the mercury presages, in general, fair weather; and its falling, foul weather, as rain, snow, high winds, and storms. When the surface of the mercury is convex, or stands higher in the middle than at the sides, it is a sign that the mercury is then in a rising state; but if the surface be concave or hollow in the middle, it is then sinking.

2. In very hot weather, the falling of the mercury indicates thunder. 3. In winter, the rising presages frost; and in frosty weather, if the mercury falls three or four divisions, there will be a thaw; but, in a continued frost, if the mercury rises, it will certainly snow.

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

5. In foul weather, when the mercury rises much and high, and so continues for two or three days before the bad weather is entirely over, then a continuance of fair weather may be expected.

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

7. The unsettled motion of the mercury, denotes unsettled weather.

8. The words engraved on the scale are not so much to be attended to as the rising and falling of the mercury; for if it stand at much rain, and then rises to changeable, it denotes fair weather, though not to continue so long as if the mercury had risen higher. If the mercury stands at fair and falls to changeable, bad weather may be expected.

9. In winter, spring, and autumn, the sudden falling of the mercury, and that for a large space, denotes high winds and storms; but, in summer, it presages heavy showers, and often thunder. It always sinks lowest of all for great winds, though not accompanied with rain; but it falls more for wind and rain together, than for either of them alone.

10. If after rain, the wind change into any part of the north, with a clear and dry sky, and the mercury rise, it is a certain sign of fair weather.

11. After very great storms of wind, when the mercury has been low, it commonly rises again very fast. In settled fair weather, except the barometer sink much, expect but little rain. In a wet season, the smallest depressions must be attended to, for when the air is much inclined to showers, a little sinking in the barometer denotes more rain. And in such a season, if it rise suddenly, fast, and high, fair weather cannot be expected to last more than a day or two.

12. The greatest heights of the mercury are found upon easterly and north-easterly winds; and it may often rain or snow the wind being in these points, while the barometer is in a rising state, the effects of the wind counteracting. But the mercury sinks for wind as well as rain, in all other points of the compass.

MEASURING OF HEIGHTS.

From the rapidity with which the density of the atmosphere diminishes, the barometer forms an easy, and when properly used, an accurate means of measuring the height of mountains; but in this, also, allowance must be made for the changes of temperature.

The following are the common rules for estimating the different elevations of places, as indicated by different heights of the barometer.

1. Observe the height of the barometer at the bottom of any height or depth proposed to be measured, together with the temperature of the mercury, by means of the thermometer attached to the barometer, and also the temperature of the air in the shade, by another thermometer which is detached from the barometer.

2. Let the same thing be done also at the top of the said height or depth, and as near to the same time with the former as may be. And let these altitudes of mercury be reduced to the same temperature, if it be

thought necessary for correcting either the one or the other, viz. augmenting the height of the mercury in the colder temperature, or diminishing that in the warmer, by its 9600th part, for every degree of difference between the two.

3. Take out the common logarithms of the two heights of mercury so corrected, and subtract the less from the greater, cutting off, from the right hand side of the remainder, three places for decimals; so shall those on the left be fathoms, in whole numbers, the tables of logarithms being understood to be such as have seven places of decimals,

4. Correct the number last found, for the difference of the temperature of the air, as follows: viz. take half the sum of the two temperatures of the air for the mean one; and, for every degree which this differs from the standard temperature of 31°, take so many times the 435th part of the fathoms above found, and add them, if the mean temperature be more than 31°, but subtract them if it be below 31°; so shall the sum or difference be the true altitude in fathoms; or, being multiplied by, six, it will give the true altitude in English feet.

The greatest inconvenience in the use of the barometer for measuring heights, is the want of portability in the instrument, and the danger of its being broken by the great weight of mercury in the tube; for, to give perfect freedom to the motion, a proper barometer for the measuring of heights, should have a tube rather wider than those used for common purposes. Attempts have been made to remedy this defect by the substitution of a metallic tube for the whole height, below that to which the mercury is estimated to sink, and the best metal that can be employed for this purpose, is iron, because mercury, which combines naturally with many other of the metals, does not naturally combine with iron. But the metallic tube has one disadvantage: it is much more susceptible of changes of temperature than the glass; and this, of course, occasions greater change in the whole volume of the mercury. These changes may, no doubt, be in so far counteracted by a careful attention to the attached and detached thermometers, but still, even with the best construction, the instrument requires to be managed with great skill,-skill which can only be acquired by long practical experience.

THERMOMETER.

These instruments, though, like barometers, they may vary a little in form, all depend on the same principle,—namely, the tendency which substances have to expand with heat and contract with cold.

Simple thermometers, that is, thermometers which merely point out the temperature of the air to which they are exposed, or of the substance into which they are plunged, are named from the substance whose expansions and contractions point out the changes of temperature. An air thermometer consists of a long glass tube with a bulb at the end. The air is partially expelled, and the end of the tube plunged into water. If a warm

the

WATER

210

BOLL

200

BLOOD
HEAT

90

substance be applied to the bulb, the air expands and forces the water down in the tube; and if a cold substance be applied, the air contracts and the water rises. In consequence of the great sensibility of air to vary in bulk by changes in temperature, the air thermometer is very delicate, but for general purposes it is not so convenient as one filled with a liquid. The liquids used are spirit of wine and mercury. The bulb is filled with one or other of these substances; the air wholly expelled by heat; and then the end of the tube is closed hermetically, that is, the sides of it are melted together in such a way as that the air is excluded. After this the scale is formed thus: the bulb is plunged into snow just melting, or water just freezing, and the point at which the liquid stands is marked upon the glass, as the freezing point; then the bulb is plunged into boiling water, and the point to which the liquid rises is marked as the boiling point. The absolute distance between these depends upon size of the bulb as compared to the wideness of the tube, being always longer as the bulb is larger and the tube smaller; but the degree of heat at each of these points, is found to be in all cases the same. The number of degrees or divisions between those points, and also above boiling and below freezing, depends upon the kind of scale, which is different in different countries. According to Fahrenheit's scale, the one used in this country, freezing is marked 32 and boiling 212; the intermediate space is divided into 180 equal parts, and the divisions are carried upwards and downwards, as far as is necessary for the purpose intended, or as the tube or stem of the instrument will admit. Thermometers with small bulbs are most sensible to rapid changes of temperature; but those with large bulbs admit of greater divisions in the scale. The cut in the margin shows a common thermometer.

50

TEMP
ERATE

30

FREEZ
ING

40

Though Fahrenheit's scale be the one mostly in use in this country, it is far from the most convenient, as the beginning of it is not any fixed degree of heat in a substance easily obtained A better scale is that which has 0 or zero at the freezing point of water, and 100° at the boiling point. This is called the centesimal scale; 5 of its degrees are equal to 9 of Farenheit's; and, therefore, to change centesimal degrees to Fahrenheit's, multiply by 5 and divide by 9; and, to change the other way, multiply by 9 and divide by 5.