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wing in fair summer weather, and which increase with the ephemeral temperature, and cloud the brows of day. The colours likewise of the Haze and of the Clouds depend greatly on the direction of the Wind, the most brilliant certainly attend East Winds, and form some of the glowing tints so beautifully imitated by Claude Lorraine, and other painters of natural scenery. It is with those Winds that Aurora rises blushing at her golden tresses. And it is usually a Southern Wind that makes Greyhooded Even like a sad Votarist in Palmer's weeds.

CHAPTER IX.

SOME MISCELLANEOUS

OBSERVATIONS ON AT. MOSPHERIC TEMPERATURE AND PRESSURE.

AFTER the invention of the Barometer and Thermometer, many important discoveries, about the Pressure and Temperature of the atmosphere were made by philosophers. The knowledge, however, of their variations, and the relations of these variations with other phaenomena, remains still very obscure, and leaves a wide field for future investigation.

When Galilaeo and Torricellus had discovered that the Pressure of a column of atmosphere was equal to that of a column of mercury of equal base, and of about thirty inches of height, and to a column of water of the same base, and about thirtyfive feet in height: but that the height of the mercury or water, which balanced the column of air, varied a little at different times, philosophers began to measure the atmospheric pressure by such means; and finding that its variations were very irregular and uncertain, and were not referable to any known

laws, they began to investigate their causes. Without detailing the particulars of the observations made by philosophers, from time to time, it will be sufficient to observe, that the variations of pressure, as far, at least, as we can discover, may be caused, first, by a variation in the volume of atmosphere, the density remaining the same; or, secondly, by a variation of density, the volume remaining the same: or, lastly, by a variation in both density and volume. But though these circumstances may be conceived capable of effecting barometrical variations, yet it is probable there may be many other causes yet unknown. Many hypotheses have been assumed to account for variations in the density and volume: but after all, these do not seem capable of accounting for all the phaenomena which attend alterations in the atmospheric pressure. As it is not my intention to detail former experiments and hypotheses, I shall conclude this chapter with a few simple observations on facts, which I have made myself, many whereof do not appear to have been noticed before.

It is a common observation, that the falling of the mercury, when gradual, is followed by long continued Rain; when it suddenly sinks, or

sinks and rises alternately, by Showers; and when greater or more sudden depressions take place, Storms are generally the consequence. These observations are, generally speaking, true; though, perhaps, with occasional and rare exceptions; for sometimes the Barometer shall sink, and even the clouds present all the appearances of Rain, and yet the rainy symptoms shall subside, and clearness return without any fall. But there appear to me to be some other remarkable connexions between the state of the Barometer, and other phaenomena, which do not appear to have been noticed.

Sudden changes in the barometrical pressure of the air produce certain corresponding effects on the nervous system: thus a rapid rise in the Barometer will generally cause a peculiar sensation in the ears of many persons amounting to a degree of temporary deafness; and what renders it more strikingly apparent, that the increase of atmospherical pressure is the cause of the sensation is, that a sudden descent from high mountains has often the same effect, as I experienced myself after coming down from Cader Idris, Aug. 14, 1814, and again after descending from Skiddaw, May 19, 1816. I perceived it also in a less degree during the

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gradual descent from the Jura into Switzerland, on Monday, July 29, 1822. Aëronauts have frequently become deaf for a time after rapidly descending in Air Balloons, and this effect seems, according to accounts, to be in some measure proportionate in a degree to the height to which they have ascended.

Some writers have ascribed the above phaenomenon to the relaxation of the tympanum, but to me it seems clearly referable to determination of blood to the internal parts of the ear, and to be a similar impulse of blood to those which we find often to follow nervous irritation in any part or organ.

Rain, as is known, sometimes falls with a rising Barometer; and, when this happens, it is usually followed by fine healthy weather. Some philosophers have called it Rain of the recomposition of the air.* And I have noticed Rain, with a rising of the mercury, to be attended with circumstances which seem to indicate a strong positive electricity.†

* Van Mons. Nicholson's Jour. Sept. 1809.

+ The strong and refreshing smell, which sometimes results when Showers first fall, after a long drought, is not an invariable attendant on them, even under these circumstances. The highly electrified water of summer's Thundershowers

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