9. The effect on the pulse and respiration of mountain travelling is to accelerate them. According to M. de Saussiere the air of low plains is less salubrious, because it is loaded with heavy exhalations, sustained by its density; on the other hand, that of mountains more than 500 or 600 toises (3196.85 or 3836-22 feet) above sea level is vitiated by other exhalations, which, possibly lighter than common air, do not the less impair its salubrity. CHAPTER V. 1. THE NEIGHBOURHOOD OF GREAT SEAS AND THEIR RELATIVE neighbouring sea. THE neighbourhood of the sea has a great influence in moderating the temperature of a country or town. It follows with Influence of great slowness alteration in the temperature of the air, and in consequence moderates any excess of temperature that is likely to be occasioned; thus, the nearer a town is to the sea, the less will the extremes of heat and cold be found: the further inland a country be, cæteres paribus, the more the extremes are observed. The sea-board towns of intertropical countries are always Temperature. cooler than the interior of the continent. Thus, during the Ashantee expedition of 1863-64, the encampment at the banks of the River Prah, about 180 miles from the sea coast, was at an average ten degrees hotter than Cape Coast; this is due to the cooling influence of the sea water over the wind as it passes over its surface. The atmosphere is generally more invigorating and healthier than in the interior, through the development of a larger quantity of ozone in the atmosphere. By the evaporation that is constantly going on along the Electricity. sea shore, places thus situated are plentifully supplied with atmospheric electricity generated through that action (Pouillet). In tropical countries the banks of rivers are generally very Banks of rivers. Surface of river water. Results of ebbing tide on river banks. Lakes and lagoons. unhealthy; the emanation from the surface of the river does not consist of aqueous vapour only, but also of deleterious gases, such as sulphuretted and carburetted hydrogen, &c., besides which the muddy swamp that is occasioned gives off very poisonous vapours. The mean daily temperature of the water of the river sometimes exceeds the minimum temperature of the country or town around, in which case the water exhales mephitic atmospheric gases, the result of putrefaction of the vegetable and animal matter which it receives in its course, and which is held in solution by them; and this water, when drunk by individuals predisposed to bowel complaints, at once produces dysentery and diarrhoea. These diseases continue in amount and severity in direct ratio with the increase of temperature, and decrease as the heavy rain increases the bulk of the water, and diminishes its temperature. Through the evaporations and exhalations that are constantly going on from the surface of water, especially in tropical climates, the banks and surface of the river and neighbouring country are always covered with mists, fogs, or haze, which is principally the case when there exists a great difference between the temperature of the atmosphere and that of the water. This haze or mist is observed generally in the morning before the heat of the sun dispels it. Thus, during the harmattan months, in the upper portion of the River Gambia, when the temperature of the water is very low, ranging from 50° to 56° in early morning, the surface of the river and the adjacent lands are generally covered with thick mist. The ebbing of tidal rivers by the ocean tide, such as those met with in every part of Western Africa, leaving muddy banks with dead vegetable and animal putrescent matter exposed to the direct rays of solar heat and the influence of the temperature, twice during the twenty-four hours, are productive of deadly emanations which might produce fever, diarrhoea, and dysentery. When lakes and lagoons are well supplied with water, and their banks are high and steep, they give out no pestilential vapour, but when their banks are low, and the water is subject to be diminished, their banks become exposed to the direct rays of the sun, and the vegetable and animal remains become decomposed, and form the source of malarious fevers. The lagoons in the Bight of Biafra are the source of the most deadly emanations: the banks are open, flat, and covered with rank vegetation, and here and there pools of water. During the dry season there remains an extensive tract of slimy mud, which sends forth pestilential vapours. But lagoons, lakes, and swamps, when surrounded by large umbrageous trees, by forest trees and green vegetation, are not unhealthy, nature having provided the leaves of these plants with the property of decomposing and destroying the malarious poison by the oxygen which they give out, forming ozone by combination with the oxygen of the air, and of absorbing certain noxious emanations; but when marshy land, covered with forest and underwood, is deprived of solar light and heat, of a free circulation of the air, and the woods are close and impenetrable to atmospheric influence, it becomes very unhealthy. Countries that are well supplied with forests or woods are Forests and very much cooler than those which are open and cultivated; the reasons are that they shelter the surface of the earth from the direct rays of the sun, and consequently prevent radiation from it (the process by which the atmosphere is heated)—that the leaves give out a large quantity of watery vapour-that they absorb solar heat, and increase the amount of cooling superficial or radiating surface. During the day, whilst there is sunshine, plants absorb and decompose carbonic acid from the atmosphere, assimilating its carbon and giving out its oxygen to the air: during the night, or in the shade, they absorb oxygen and give out carbonic acid. But malaria is most active and deadly from sunset to sunrise, at the very time when plants absorb oxygen and give off carbonic acid. Marsh miasma "is arrested, and probably neutralised, decomposed, absorbed or respired, by large thick G woods. Valleys. Mists and fogs-origin. leafy trees, by dense groves and forests, by masses of foliage and by evergreen shrubs ;" and Pliny was correct in his opinion that "groves and trees absorb and destroy mephitic vapours.' According to Haviland, a forest, intervening between a pestilential marsh and a city, often affords a protection to the inhabitants, and perchance the pestiferous air gets decomposed into innocuous gases during its enlargement among the trees. Sullivan, in his "Visit to Ceylon," corroborates this fact. He remarked that the vicinity of tanks and lagoons of the most foetid and agueish character was perfectly healthy. They were covered in abundance with various kinds of aquatic plants, including the lotus, of almost Victoria Regia magnificence, which, by a kind Providence, are made "to serve not only as filterers and purifiers of the water itself, but even as consumers of a considerable portion of the noxious exhalations that would otherwise poison the neighbourhood." The atmosphere of valleys is generally charged with moisture. It contains very little ozone; it is chilly in the morning before the rise of the sun. "During the forenoon, mid-day, and afternoon, the temperature has attained its maximum, and is succeeded by the cold mist and fogs of evening. Mountain chains influence the temperature of the valleys to a very considerable extent. Valleys and plains on the north side of lofty mountains, being deprived of the warming and cheering rays of the sun, are cold and gloomy, and their vegetation is retarded in a proportionate degree." The morning mists are generally dispelled by the morning These are caused by radiation of heat in calm nights and the cooling of the earth's surface or surface air below the air above; their origin is from the vapour which arises from the earth's surface and from water. Fogs have the same origin, their immediate causation being the admixture of airs of different temperatures. The temperature of the water at night is hotter than that of the superincumbent air, which, "becoming warmed by the water, rises surcharged with moisture, which sun. *Op. cit. vol. i. p. 167. |