In treating of rivers, we remarked that the transparency of large bodies of water is very different, though the water be to all appearances equally pure in the different cases. For the method of determining the transparency, see OPE

RATIONS.

With regard to the temperature of the sea, it has become an object of very considerable importance since the discovery that, in the vicinity of, and immediately over shoals, the temperature is much lower than in the deep places; it is therefore extremely interesting to verify this fact by taking the temperature of the water wherever there is an opportunity of doing so. What is the effect of the waves in changing the temperature of the surface. The temperature must, if possible, be taken at various depths, and in all cases compared with that of the atmosphere. See OPERATIONS.

It has also been observed that the saltness of the sea is not exactly alike in different places, and that in some cases it is more bitter at the surface than lower down; precise observations on this subject are, therefore, very desirable. For the method of obtaining the degree of saltness, and also for bringing up water from any depth in order to examine it, see OPERATIONS.

The specific gravity of sea water, depending upon the quantity of saline ingredients, will also be different in different parts, besides which it is modified by the temperature. See OPERATIONS.

Observe the hygrometric state of the air over the sea, the indications of the thermometer and barometer, the magnetic variations. &c. See METEOrology.

The luminous appearance, or phosphorescence of the sea, is a striking phenomenon and well worthy the attention of the philosophic traveller. It is generally attributed to molluscous animals and zoophytes, the spawn of fish, and decaying animal and vegetable substances; most probably all these causes conspire to produce the effect, sometimes together and sometimes separately; it is, therefore, recommended to examine the phenomenon closely, both in itself and with reference to place, temperature of the water, and atmospheric temperature and pressure. Does the appearance precede or follow a storm; is it observed to have any reference to the electrical state of the atmosphere at the time or immediately before or after; what is the precise appearance, for this is often very different?

Besides the circumstances we have mentioned, there are

various others which he who is borne along upon the trackless deep will do well to notice; such as storms, calms, water-spouts, &c.; as also the molluscæ, the fish, and the birds that are met with in different parts. In a word, the traveller cannot be too often reminded, that the knowledge of a single well-authenticated fact, however insignificant in appearance, may serve to clear up some doubt, and, like the key-stone of an arch, consolidate reasonings otherwise well founded, but which, without this, would fall to the ground.

CONCLUSION.

When the traveller shall have obtained all possible information on the several subjects connected with the waters of a country, and shall have entered them neatly and distinctly in the way most convenient to himself in his note book, he will do well to put some of the principal data into a tabular form in a manner similar to No. 1. See the TABLES and their description.

Such a table has the advantage of showing the extent of inland navigation, and the water-surface generally, both running and stagnant; an object of great interest when compared with the other grand divisions of the country into forests, cultivated lands, &c. Moreover, the water-surface being known, this datum, combined with other circumstances, will serve to explain those meteorological phenomena of the country which depend upon evaporation, and to determine, in a great degree, the nature of its climate.

Few travellers, it is true, remain long enough in a country to be able to fill up such a table in all its details from their own observation, but the deficiencies may, in many cases, be filled up from authentic documents, and these should always be had recourse to if possible.

The other objects which may have been observed in examining the waters of a country, such as the commerce carried on by inland navigation, the fisheries, &c. must each be transported to its particular article in the traveller's notes; the traffic to the article COMMERCE, the fisheries to the ANIMAL PRODUCTIONS, or to INDUSTRY, and so of the rest.

E

SECTION III.

METEOROLOGY, AND THE CLIMATE OF A COUNTRY.

By the Climate of a country is understood the degree of heat and cold, humidity or dryness which prevails. Climates are very different; they are caused by the combined influence of latitude, relative position of sea and land, and height above the level of the ocean; and are also modified by the prevalence or absence of mountains, sterile sands, extensive lakes, marshes, forests, &c. The effect of climate on a country is to produce a greater or less fertility, and on its inhabitants to modify their physical energies and moral character. Hence climate is an object deserving the attentive consideration of the traveller, and the conclusion he arrives at regarding it, will result from a combination of all the observations he may have made of its various elements. Of the geographical and relative position, particular aspect and hydrography, we have already treated; we come now to speak of meteorology, or the phenomena of the atmosphere, whose habitual state in any country constitutes the climate of that country.

The various phenomena which take place in the atmosphere are termed meteors; they may be divided as follows:

Aerial Meteors; or the winds, the dry fogs, and the exhalations which emanate from bodies on the earth's surface, and remain suspended in the lower strata of the atmosphere.

Aqueous Meteors; as clouds, fogs, rain, dew, hoar-frost, snow, and hail.

Igneous Meteors; or the ignes fatui, falling stars, bolides and aerolites, together with the Aurora Borealis, lightning, &c. the two last are, properly speaking, electrical phenomena. Luminous Meteors; rainbows, haloes, parhelia, paraselenæ and zodiacal light.

But first, of the atmosphere itself.

Of the Atmosphere.-A learned French philosopher says, in speaking of the atmosphere, "Every one would like to foresee its vicissitudes and know their causes. The husbandman owes every thing to the atmosphere; on it he de

METEOROLOGY, AND THE CLIMATE OF A COUNTRY. 75

pends for his success or failure; he experiences its salutary influence, or dreads its cruel effects; his harvest is not the sole object which interests him; his health in most cases depends upon the state of the atmosphere. Prudent from habit and necessity, he commits no excesses, but the slightest alteration in the aërial fluid deranges the physical economy of his body. The air which he breathes may become a poison, and while in the fields he may be asking of the earth the recompense of his labours, the sustenance of himself and his family, he may, on his return home, bring with him the germ of long and acute disease. How important is it then that man should know the atmosphere."

The atmosphere, besides being essential to animal and vegetable life, is the grand laboratory in which are prepared and developed the various phenomena we have already enumerated. But we shall begin with speaking of the air itself.

The air is fluid, elastic, expansible and heavy; on this last property is founded all the indications of the Barometer and the various uses of this instrument. The expansibility of the air is developed by means of caloric, whose effect is measured by the Thermometer; its elasticity and fluidity, set in motion by temperature and luni-solar attraction, are the cause of winds; for, like all other fluids, the air seeks its equilibrium, and, in flowing, as it were, from a part where it is dense to one where it is rarefied, or which is momentarily deprived of it by some accidental cause, it produces those currents which we call wind. The air is composed of oxygen and nitrogen or azote, in the proportion of 21 parts of the former to 79 parts of the latter in bulk, or 23,25 and 76,75 in weight; a trace of carbonic acid gas, about 0.001 according to Dalton, is also discoverable. Its pressure is about 2160 lbs. on every square foot of the earth's surface. The height of the atmosphere, deduced from the duration of the twilight, is estimated at from 45 to 50 miles. The weight of a column of air of this height is equal to that of a column of water of equal base, and 35 feet high, or of a column of mercury 30 inches high. A man's body, taken at an average surface of 15 square feet, sustains a weight of air exceeding 32,000 lbs.

Besides the immediate elements of air, the atmosphere always contains a greater or less quantity of water in the state of vapour, and various gaseous exhalations.

The quantity of oxygen gas contained in the atmosphere

is found by means of the Eudiometer, and its degree of humidity is ascertained by the Hygrometer. The temperature varies not only with the latitudes of places, but according as they are more or less elevated above the surface of the sea. Thus an elevation of thirty fathoms causes the same variation of temperature as a removal of one degree from the equator towards the poles. At the height of 15,740 feet above the level of the sea, at the equator, we attain the region of perpetual snow, and arrive, as it were, at the 71st parallel of latitude. In the ascent from the plain we experience the influence of every variety of climate, and pass through every degree of vegetation.

The air is more or less mixed with exhalations according to the height, a circumstance which has great influence on health. According to the observation of De Saussure it would appear, that a height of 3,600 feet is not the most favourable situation for the human species. If, says he, the air of the plains is loaded with heavy exhalations, on the other hand, the air of regions elevated to 600 toises is vitiated by exhalations of another kind, which, though lighter than air, do nevertheless diminish its salubrity; so that the common air most favourable to health, is that of the plains and great valleys of Switzerland placed at about 1200 or 1800 feet, (French measure).

The salubrity of the air in different places, depends in great measure upon the state of the surface of the earth in these places; but, independent of the cause, the habitual state of the atmosphere in any place should be exactly observed by the scientific traveller, the more particularly as its influence is paramount on the animal and vegetable economy.

If I shall here dwell a little upon the effects of different states of the air, it is in order to guide the traveller in his determinations, whenever his object is the choice of spots for new settlements, which, after they are established, frequently prove unhealthy from want of a due consideration, a priori, of the habitual state of the air or weather.

An old writer, who has paid particular attention to the effects of the atmosphere on organized beings says, "The weight and elasticity of the atmosphere exercise a less immediate influence on the animal and vegetable economy than its temperature, its humidity or dryness, and its electricity. These four circumstances are the cause of all the changes, of all the states of health or sickness through which animated