THE

QUARTERLY

JOURNAL OF AGRICULTURE.

ON METEOROLOGICAL OBSERVATIONS. By Mr JOHN ADIE, Edinburgh.

THE changing phenomena exhibited in the atmosphere have, from the earliest ages, excited the attention of mankind. But to none, perhaps, does a knowledge of the state of the weather become more useful and interesting than to the agriculturist; for it must often be to him a matter of much importance in his field operations, to be able to form some idea of the probable changes that may take place, or in what state the weather may for some time be expected to remain. The various instruments constructed for this purpose, afford the only correct means by which such knowledge can be obtained. What is here proposed, therefore, is to describe, in a manner which may be understood by every one, the various properties and uses of this

class of instruments.

The most prominent feature in the changes that take place

atmosphere, is the variation in its pressure. This change is indicated by the rise and fall of the mercurial column of the barometer. The invention of this instrument, by means of which those changes were first shown to exist, was achieved by Torricelli in 1643; and it has from that time undergone various alterations and improvements, till it has arrived at its present state of perfection. The instrument of this kind, which in all respects may be considered the best, is the perpendicular

VOL. III. NO. XIII.

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column of mercury, where the variations in its length are observed by a scale of inches and parts. The mean height of the barometer at the mean temperature of the air, reduced to the level of the sea, from nine years' observations, was found to be 29.948 inches, and its greatest range 28 to 31 inches.

In observing the barometer, the rising or falling of the mercury is to be taken as indicating changes in the weather, without regard being paid to the actual height of the column; for, whatever the situation of the instrument may be, whether at a great altitude, or at the level of the sea, differences in the height of the situation only cause a constant difference in the mean height; while similar changes in any situation indicate the same result. This will be more particularly explained hereafter.

When the barometer is high and stationary, a continuance of steady and good weather may be expected, corresponding to the season of the year; and in general it may be remarked, that the rise is slow and regular when such weather follows. A sudden fall is commonly prognostic of a gale of wind, whereas when the fall is slow and regular, rain generally ensues. Steady weather is never to be expected when sudden changes in the height of the mercury occur; good days may indeed intervene, but we cannot depend upon their continuance. In this climate, the east and north-east winds produce a very marked change in barometric indication, and often lead persons to suppose that the instrument is not to be depended upon. These winds cause the barometer to stand higher under the same circumstances, than winds from the other points of the compass, so that a change of wind from east or north-east to west, south-west or south-east, without any change in the barometer, is to be considered as equivalent to a rise, and the reverse of this, as equivalent to a fall. This quantity may be estimated at about of an inch.

In making use of the barometer as an indicator of the changes to be expected in the weather, by variations in its height, it is of some importance that the mean height or changeable point at the place of observation be found. If the altitude of the place above the level of the sea be known, the mean height of the barometer may be found from computation, or simply by sub

tracting from the mean height at the level of the sea of an inch for every 88 feet of altitude.

But where the height of the place of observation is not known, which will be most generally the case, a register of the height of the barometer should be kept, noting it morning and evening at 10 o'clock, for one year or more. And from these observations the mean height will be obtained, by dividing their sum by the number of the observations: For example, the sum of one year's observations, made at 10 A. M. and 10 P. M. in 1827, was 21615-410, and this number divided by the number of observations, 730, or twice the number of days in that year, gives 29-61 inches as the mean height or changeable point of the barometer. It is from the mercury's being above or below this point, then, that you are to conclude what weather may be expected from its indications or changes; from the want of this knowledge, many are led into a mistake, by supposing that the words Fair, Change, and Rain, engraved on the scales of barometers, indicate such weather in all places; whereas they can only do so at one particular altitude.

Having now explained the principal causes of failure in using the common barometer for general purposes, I shall next give a description of the best construction of that instrument, to be used where the greatest accuracy is required for the determination of minute changes, or the height of grounds above the sea.

The instrument to be used for such purposes should have a tube of large diameter, in which the mercury is boiled, to expel from it all air or moisture. In the cistern containing the mercury there is another tube, of the same internal diameter as that of the barometer, with a metal ring embracing it; and an adjustment is made at each observation, by forcing the merfrom the cistern by means of a screw, till its surface cuts off the light seen through the glass-tube below the ring. An instrument of this construction is said to have an adjustable surface; that is, the surface of the mercury in the cistern is always brought to a fixed point; consequently the length of the mercurial column is at all times observed above this point.

cury

For the measurement of heights, the scale of this instrument is made to read off to one thousandth-part of an inch. There has been lately constructed a cast-iron cistern, which renders the in

strument much less liable to injury from carriage, and gives great stability to the mercury during observation. The method of using the barometer for the purpose in question, is to observe its height at the lower and upper stations; and the temperature of the mercury in the barometer is to be ascertained by means of a thermometer attached to it, and also that of the air, by one freely exposed. All the instruments must be well defended from the sunshine; and, when time will permit, it is of great importance to allow them to remain, till both the attached and detached thermometers indicate the same temperature. When two barometers are used, the one at the lower station should be observed at the same moment as that at the upper, by signals, or at times previously fixed upon by the observers. When one instrument only is used, any change of the barometer that may take place during the time required in passing from one station to another, gives an error in the deduced height. Steady weather, therefore, should always be selected for making these observations; and though great distances intervene between the situations, the results may be equally accurate, from the simultaneous changes that take place in the barometer over a great extent of country. In a series of observations in which I was engaged for determining the height of Benlomond, where four observers were engaged, one at the top of the mountain, a second at the base, and the other two at Edinburgh, a distance of about sixty miles, after comparing the heights deduced from the three lower of these observations, the agreement was found to be very close.

We shall here give a table showing the elevation corresponding to the depression of every tenth of an inch on the barometer when at its mean state, by inspection of which an idea of the altitude may be formed. Thus, if the changeable point of the barometer, in any situation, be 29-4, and that at the level of the sea 29.9, subtracting the one from the other, leaves 5-10ths; the height corresponding to this in the table is 439 feet. This will be nearly the altitude of the situation*.

For the tables and formulæ from which altitudes may be computed, we refer the reader to Galbraith's Tables, and Baily's Astronomical Tables.

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Table showing the Number of Feet of Altitude corresponding to Depressions of the Barometer.

This may be useful to the agriculturist, as the temperature diminishes the higher we ascend, causing a consequent diminution in the productiveness of the soil.

The Sympiesometer, invented by Mr Adie of Edinburgh, is another instrument by which the changes of weight in the atmosphere are measured. Its indications are the same as those shown by the barometer, with the advantage of having a longer scale. For the measurement of heights, this instrument is very convenient, from its small size admitting of its being carried in the pocket, and not subjecting it to the same chances of accident as the portable barometer. The height is given in fathoms by the instrument; requiring only one correction, which is performed by a small table engraved on its case.

When the barometer is used within doors, the best situation for it is any room where the temperature is equal, and not exposed to sunshine.

The thermometer, by which the temperature of our atmosphere is determined, was invented by Sanctario in 1590. The instrument in its first construction was very imperfect, having no fixed scale, and air being the medium of expansion. It was soon shown, from the discovery of the barometer, that this instrument was acted upon by pressure as well as temperature. To separate these effects, alcohol was employed as the best fluid from its great expansion by heat, but was afterwards found to expand unequally. Römer first proposed the use of mercury as the expansive medium for the thermometer. This liquid metal great advantages over every other medium; it has the power of indicating a great range of temperature, and expands very equally. After its introduction, the melting point of ice

has