turning to my journal for 1799, I was agreeably surprised to find, on the 12th of November a "N. B. Several large meteors and much lightning in the morning from five to seven o'clock." The day's account was, "Showery; violent wind P. M.," and the barometer was near the low extreme.

A similar exhibition of meteors was seen over Great Britain, and part of the Continent as far as Geneva, on the morning of the 12th of November, 1832. And what is certainly very remarkable, though probably fortuitous, a like exhibition seems to have taken place in North America, on the morning of the 12th of November, 1833.

These meteors, it would seem, are of the same nature as the smaller ones, called falling stars, and the larger ones or fire balls, and probably belong to the same region of the atmosphere, or that region which lies between the clouds and the aurora borealis.

NOTE [C] SEE page 109.

FORTY years have elapsed since this Essay on the variation of the Barometer was written; and I have met with no arguments since, that appear to me more cogent than those which I have used in it to explain the phenomena. Another fact respecting the variation of the barometer indeed appears to be well established, by the attentive and careful observations of that instrument. I mean the diurnal variation first observed in the torrid zone, and

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since then traced through the temperate zone, though it is there blended with other and more powerful variations, from other causes.

Generally speaking, the fact is this, that early in the morning, about sun-rise, or soon after, the barometer is higher, all other circumstances the same, than it is afterwards; that it droops a little, as the heat of the day advances, and is lowest nearly in the warmest part of the day; after which, it rises as the air cools, and in the evening nearly recovers what it had lost since morning.

The sun's power being greatest in the torrid zone, this effect of it, (for it is evidently an effect of temperature,) is there a maximum ; and on this account it is more conspicuous there, as well as on account of the other variations being of less magnitude than in the temperate and frigid zones. The effect diminishes, in leaving the equator, in some proportion as the latitude, the seasons, and other circumstances.

The sun is constantly heating the earth and air, successively from east to west: the air being heated expands in various directions, to restore an equili brium of pressure; if this expansion was only in a perpendicular direction, it would not disturb the barometer; but as the air will go in any direction where the pressure is least, it has a lateral motion as well as a perpendicular one; and hence the column pressing on the mercury is less in quantity during the high temperature; but when the excess of temperature is withdrawn, the air falls back into its former position.

It will be perceived that the principle we have adopted in the Essay on the variation of the Barometer is, that an equality of elasticity in two vertical columns of air, will, in great part, counteract an inequality in their weight. To illustrate this position: suppose a cylindric vessel of indefinite length were filled with hydrogen gas, and placed perpendicular to the ground plane, having no communication with the atmosphere; suppose then that a small hole were made in the side of said vessel, at a point where the elasticities of the two gases were equal. A communication being now opened, an intercourse would immediately commence; but this would not be occasioned by the specifically heavy gas rushing into the light gas, nor the light gas into the heavy gas, exclusively. The two gases, having equal elastic forces, would, by virtue of those forces, be diffused through each other slowly and gradually, according to the law which I have pointed out in another Essay-that of one gas being as a vacuum to another in regard to their mutual diffusion.

The application of this principle, in accounting for a temporary existence of a warm vapoury volume of atmosphere, supporting itself against heavier but colder volumes of atmosphere on the right and left of it, is too obvious to be insisted upon.

Of all the phenomena of meteorology, there is none more difficult to explain, than those relating to the fluctuations of the pressure of the atmo

sphere indicated by the barometer. After forty years' additional observations and occasional reflections on the subject, I am persuaded that the explanation already before the public in this third Essay, is sufficient, and will be generally considered so, when properly understood. The fact probably is, that the Essay has fallen into the hands of few who have paid much attention to meteorology; or who have had their attention directed more particularly to this part of the subject. Indeed the explanation itself involves certain physical facts by anticipation, which were not known at the time the Essay was first published. I allude to the mutual action of different elastic fluids. It has been long known, that if a tall jar, of two or more inches in diameter, be filled with a heavy elastic fluid, and held with its mouth downward, it will be emptied in a moment or two, and at the same time filled with common air; but if placed with its mouth upward, it will remain for a considerable time in the jar, and be gradually diluted with common air, till at length it becomes filled with the same; and vice versâ, if filled with a light elastic fluid. But it was not known, I believe, till I published my Chemistry in 1810, that if a vessel of any shape be filled with any either light or heavy gas, and it be made to communicate with the atmosphere by a tube of one tenth of an inch diameter, it matters little whether the tube be held up or down, as regards the time of the exit of the gas. It will be slow and gradual, in both positions..

From these observations it is not difficult to conceive, that a vertical column of warm vapoury air may be projected into a heavier column of cold dry air, such that their elasticities may be nearly equal for a time, but that the adjustment of their weights may require a slow and gradual operation, sufficient to account for the interval of time observed between the extreme and mean state of the barometer.

NOTE [D] SEE page 125.

In 1817, the celebrated traveller HUMBOLDT published his Essay on Isothermal Lines on the Globe, in the third volume of the Memoirs of the Society of Arcueil. The author's object was to ascertain facts, rather than to account for them; he has furnished a more copious collection than that given by KIRWAN. The effect of elevation in reducing the mean temperature of any place, is now well known to be about 1° for every hundred yards above the level of the sea; and this is occasioned by the temperature of the atmosphere diminishing, in ascending, at the same rate. It is pretty obvious too, that distance from the equator must cause a diminution of temperature, though it may not be easy to discover the law of that diminution; but it is not so obvious why the same parallel of latitude should have such great variations of temperature in its different parts. In most parallels of latitude, between the tropics and the polar circles, there