60° Fahrenheit, 100 cubic inches of air weigh 30 grains troy; and a perpendicular column of the whole atmosphere weighs about 14 pounds. This is just balanced by a mercurial column of about 30 inches in height; and it is on this principle that the mercurial barometer is constructed.

The weight or density of atmospheric air is not uniform throughout its whole extent; a gradual diminution taking place as we ascend above the level of the sea. The rate of

decreasing density is such, that at the height of seven miles, the atmosphere would have a density equal only to onefourth of that at the surface of the sea: and at the height of fourteen miles, it would have only one-sixteenth, and so These heights have never been attained by man, but the ratio is the same at smaller elevations, and the consequence is, that on ascending elevated ground, the atmosphere exerts less pressure on any substance on which it rests.

on.

The average pressure of the atmosphere (though subject to temporary alterations from meteorological causes), is found to be the same, or very nearly so, at any one place from year to year; and over the whole globe its lines of equal density, or pressure, are parallel to the sea level. If, therefore, we know the mean density at any one station, for instance, 2,000 feet above the sea, we may conclude that any other place, having the same density, has a similar elevation above the sea. This decreasing density is indicated by the barometer, the mercury in the tube falling at greater elevations. Thus, at the level of the sea, near the foot of Chimborazo, M. de Humboldt found that the barometer stood at exactly thirty inches; whilst at the elevation of 19,332 feet, to which he ascended on that mountain, it was no higher than fourteen inches and eight lines. And in December, 1831, when M. Boussingault, accompanied by Colonel Hall, ascended the same mountain, to the elevation of 19,699 feet (the greatest terrestrial height yet accomplished), the barometer fell to thirteen inches, eight lines. At all other places having the same elevation above the sea (allowing for some variation of temperature), the indication

of the barometer would be similar; and thus, by the use of that instrument, we are enabled to obtain a very near approximation to the heights of all places which are accessible to the foot of man.

The mean height of the barometer in London is twentynine inches and nine lines, nearly. The height of the mercurial column varies in this climate, from about twentyeight to thirty-one inches.

Another mode of measuring heights, dependent also on the density or pressure of the atmosphere, is by the boiling point of water. In London, and at all other places having a similar elevation, water boils at 212° Fahrenheit; and we are so accustomed to associate the notion of its ebullition or boiling up, with that particular temperature, that perhaps at first we may find some difficulty in reconciling the idea of "boiling water" with any other degree of heat. This ebullition, or boiling up of water is, however, dependent on the pressure of the air above it; and the boiling point of water decreases in a ratio nearly equivalent to the decrease of atmospheric density. And accordingly, at considerable elevations above the sea level, water, when boiled, does not acquire so high a temperature as 212°, and, under ordinary circumstances, cannot be raised to that temperature. At great elevations it is, therefore, not so available for some culinary purposes; and we much doubt (allowing all other conditions to be similar) whether so good a cup of tea could be made at the Hospice de St. Bernard, which is 8,600 feet above the level of the sea, as in the metropolis of England, at the former place the temperature of boiling water being only 203°: and still less could this be accomplished at the Cerro de Pasco, in Bolivia (celebrated for its silver mines), the height of which is 13,673 feet above the sea, and where the boiling point does not exceed 189°. The boiling point of water forms, however, a highly useful standard for measuring the altitude of any accessible locality.

Although a certain proportion of water or aqueous vapour is always present in the atmosphere, the quantity is liable

to great variations, this being in great measure dependent on the temperature of the air. If water be exposed to the air, it gradually disappears, finding its way into the atmosphere by the process called evaporation. Warm air, however, is capable of taking up, and holding in suspension in the state of invisible vapour, more water than cold air. Thus, a cubic foot of air, at the temperature of 32o, the freezing point, has not capacity for holding more than two grains and about a third; whilst, at the temperature of 60° it will hold about five grains and three-quarters; and at that of 70°, nearly eight grains. This, it will readily be perceived, accounts for the greater rapidity with which a lake, or pond, or any other surface of water is dried up, or carried off in the form of invisible vapour, in warm than in cold weather. This, also, in great measure, accounts for the phenomena attending the formation of dew.

One of the conditions essential to the deposition of dew, is, that the atmosphere must contain an excess of moisture; that is, more than it has capacity to hold. When the air becomes heated during the day, and raised to a high temperature, its capacity for holding moisture is increased, and, under ordinary circumstances, by the means of evaporation, it will become charged with moisture. If, then, it hold in suspension aqueous vapour proportioned to its capacity, it will, when the temperature decreases towards night, contain an excess of moisture, or more than it has capacity for holding suspended at that lower temperature; and the superabundant moisture will accordingly, as the temperature of the air gradually diminishes, be gradually and gently deposited in the form of dew. The deposition will be greatest when a clear cool evening succeeds a sultry day. In our climate, the difference of temperature between day and night is usually greatest in spring and autumn; and consequently the dews are generally most abundant at those seasons.

In calm weather, and under ordinary circumstances, the moisture evaporated from a given tract of land might be supposed to be deposited on the same tract at night. But

even with a gentle breeze, this portion of air may be removed to a considerable distance before night-fall. If, however, the wind should pass over a similarly-constituted district, it may carry with it a similar supply of moisture; but should it cross an arid, sandy tract, the air will not be replete with aqueous vapour, and little or no deposition of dew will take place. If, on the other hand, the wind should pass over water, or a swampy surface, the deposition of dew may be increased. The difference in the quantity of dew deposited, according to local variations of the surface, is displayed to a certain extent in our own island. It may be familiar to our readers that dews are usually less copious during the prevalence of easterly, than of westerly winds; a circumstance attributable to the different nature of the surface over which these winds pass; the easterly crossing the continent of Europe, whilst the westerly sweep across the wide Atlantic Ocean. This phenomenon is, however, more strikingly exhibited in hot climates, where winds which pass over an expanse of water, usually become loaded with moisture, and a very copious deposition of dew takes place. Thus, in Egypt, the north wind, which crosses the Mediterranean, deposits dew in such abundance, that it is sufficient to soak through the garments of the inhabitants: whilst, during the prevalence of the south wind, which traverses extensive tracts of sandy deserts, no dew is deposited.

A serene and cloudless sky is very favourable to the deposition of dew, little or no dew being formed if the sky be veiled in clouds; a circumstance which has probably not escaped the notice of our readers, and which is, in some measure, attributable to the comparatively higher temperature, which usually prevails at nights, when the sky is clouded, than when it is clear; and it not unfrequently happens, that, on nights when the sky is alternately clear and enveloped in clouds, dew will appear and disappear with the changing atmosphere.

It is also essential for the copious deposition of dew, that

the ground, or any surface on which it forms, should possess a temperature considerably below that of the atmosphere. The air in contact with the cold body becomes chilled, and thus losing its capacity or power of holding so much invisible vapour, the latter is condensed, and deposited in the form of dew. This effect may be observed when a glass of cold water is taken into a warm room in summer. The invisible vapour contained in the air of the apartment is condensed, and fixed on the exterior of the glass. It is also exemplified by the steam, as it is commonly called, or dew, frequently observable on the glass windows of an apartment. When the external air is considerably colder than that of the room, the glass acquires a corresponding low temperature, and the invisible vapour in the air of the apartment becomes condensed on the panes. The deposition will, of course, be greater or less, according to the heat and moisture of the apartment, and the cold of the external air; and if our apartments are kept warm, it is therefore generally more abundant in winter than in summer; and very copious deposits will be observed on the lights of hothouses and conservatories.

For the rapid formation of dew, it is essential that the surface of a body should be ten or fifteen degrees cooler than the air. All substances are not, however, equally subject to be covered with dew; and the consequences to which this leads in the natural world, are of the most interesting character. In clear weather, a thermometer laid on grass, has been known to mark twelve degrees lower than one laid on garden-mould, and sixteen and a-half degrees lower than one laid on a gravel-walk. It will be evident, that a much more abundant deposition of dew will take place on the grass, than on the other portions of the surface : and thus we find, that the dew is most copiously deposited on the herbs of the field, which need this nourishment; whilst land, uncovered by vegetation, and more especially stony ground, which does not require and would not benefit by it, receives a comparatively small supply. Such is