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214

EXPLANATION OF THE DELINEATION PREFIXED.

comparing his valuable observations with those of Greenwich, and derived much information therefrom.

It is necessary to say that the black curvilinear line represents the condition of the barometer at Orkney, while the red curved line represents that of Greenwich ; the red colour being supposed to be the symbol of the southern or warmer station, and the red colour throughout that of relative warmth; hence, the red arrows are the type of the southern or warmer winds.

The black line represents the northern or colder station, and the black arrows the colder or northern winds. Hence the colours are suggestive symbols of the relative states of temperature. The cardinal points are like those of maps in general, the north at the top, the east on the right, so that the direction of the arrows denote the direction of the winds, and the colour the relative quality as to heat.

The scale is large, to give freedom and ease, at the same time more correctness than a little drivelling miniature. The dots of each day represent the height of the barometer at 9 A.M. on the left hand, and 9 P.M. the right hand dot.

The number of arrows is intended to convey some idea of the force of the wind, considering a calm to be represented by the absence of any arrow, and a hurricane to be represented by six arrows, as deduced from the pressure in lbs. on the square foot. So that the square of any number of arrows will very nearly give the force or pressure of the wind in lbs. on the square foot. “Greenwich Observations,” foot note page 5, January, 1845.

THE BAROMETER AT GREENWICH AND ORKNEY. 215

The barometer is more intimately connected with wind than rain. It falls very little in the tropics for great and heavy rains, but it almost invariably indicates the gale or coming hurricane, whose power and impetus probably depend on the proximity of some adjacent high pressure ; like a waterfall or cataract, instead of a gradual fall of the bed of the river.

The delineation I have prefixed, although confined to one month, is a type and emblem of the whole, and may afford materials for much thought, investigation, and research. We cannot fail almost immediately to perceive that whichever coloured line is highest or uppermost, corresponds with, or indicates, the highest barometer at the place which that colour represents.

And it almost invariably follows that the wind arrow is of the same colour as the top or upper line of the barometer of that day, and the moment the coloured lines approach, or intersect, or alter their relative position, the wind immediately alters its power, force, or its direction.

When they approximate to an equilibrium the air is calm ; as they recede from each other, and their weights or pressure differ, the wind strengthens and increases in force and power as the barometric lines separate from each other.

It appears from the most casual inspection that the red line almost uniformly overtops or surmounts the black line, and we perceive that the red arrows, or warm winds from the south, generally accompany the red line barometer. The red being the south or Greenwich

216 CONNECTION OF THE BAROMETER WITH THE WIND.

square foot.

barometer, the red arrows are south winds. The barometer produces the winds.

On January 13th, there are five arrows, the square of which is 25. In the “Greenwich Observations,” about noon of the 13th, the pressure is given at 25 lbs. on the

foot. And in the like manner throughout three arrows would denote 9 lbs. pressure. And one arrow would denote light air from 1 to 3 lbs. pressure on the square foot, according to Osler's anemometer. Of course, the pressure of the wind is not constant throughout the day: the maximum effect only is given, in accordance with the highest and lowest barometer.

The barometer and the wind are natural allies, and associated, as cause and effect, so intimately and closely as scarcely to be dissevered.

The difference of the weight between any two columns of air is the immediate and direct cause of wind, the greater weight, force, or power rushing down upon or into the column or area of smaller weight, the strength, pressure, and rapidity of the wind being in direct proportion to the difference of weight or barometric pressure between the two columns or areas, large or small. The intervening distance between the columns or areas of different weights acts according to the general laws of gravitation. A difference of one inch between the barometers of Greenwich and Orkney, or 500 miles apart, would produce a breeze of perhaps 9 or 10 lbs. pressure, but if a difference of two barometers at 250 miles apart amounted to one inch, the pressure or force of the wind would be 16 or 20) lbs.

GREAT EXCEPTION AND APPARENT FAULT.

217

on the square foot. These are the fundamental laws as true and permanent on the atmosphere as in the celestial and planetary motions. The winds, therefore, most strictly conform to the great and general principle of gravity, and, as Job so correctly expresses, “ He made a weight for the winds.”

There appears to be one very great exception to the general rule upon which I have endeavoured to explain the phenomena of the winds. On 13th January, it will be seen, the barometer was unusually low, and the wind unusually strong, and that, contrary to the law, the black line being above the red one, by a tenth of an inch or more, at the usual hours of observation ; the depression of the barometer at Greenwich at one P.M., being only given for that hour, while the Orkney barometer at noon is unrecorded; therefore we do not know the exact difference at noon between them. But throughout the day the Orkney barometer was higher than the Greenwich barometer, and yet the wind did not blow from Orkney as it should have done ; but a very strong gale blew directly contrary, or from the S.W., and this would appear to be a fatal objection to all we have previously said, but the evidence and the appearances are only partial. There was no gale at Orkney; all was tranquil, and a light air from the northwest. See, what a very slight difference in the pressure stopped this mighty impetuous gale rushing onwards with such force and power. The scale was turned, as it were, by a feather, and a limit set to the storm by a gentle force, unseen, unknown, and unrecognised,

218

IMPORTANCE OF RELATIVE CONDITIONS.

except by the barometer, which is the balance of the atmosphere, and shows its weight to the fraction of a grain, and no other means existing of perceiving why the wind was hushed, lulled, and checked, and died away into a calm.

The barometer began to rise in the north first, and gradually progressed towards the south, and came up only as a gentle breath against roaring wind, and stayed it. But the delineation to be perfect, and the evidence to be complete, should show where the pressure or the force that urged on the gale was. It was undoubtedly to the S.W., in or adjacent to the Madeira

area;

there was the higher pressure, I am as fully convinced as if I possessed the tables to demonstrate the fact. My limited access to information precludes the possibility of my exhibiting the deficient testimony, but it assuredly existed; for the Madeira tables of the one year which we possess demonstrate the fact of high pressure at Madeira producing S. W. gales with us. As the barometer began to rise in the north, so it began to fall to the southward of us, as towards Madeira ; thus the propelling force was diminished at the very same time that the opposing power was increased.

Exactly resembling in every point the arms of a balance, as the one rises the other falls in exact proportion, and the smallest fraction turns it. Nothing can exceed the mathematical demonstration of the balance; the barometer is the most exquisite balance, and to it we appeal with unfeigned confidence. Its testimony is ever true. Does a balance ever err except it is out of order?

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