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The most obvious circumstances which affect the climate of a region or coun-try, are temperature, humidüy, purity of the atmosphere, wind, atmosphericpressure, intensity oflight, and atmospheric equability or vicissitudes,

1. Temperature. —In considering the temperature of a place, we must regard, not merelyits annual mean.but its extremes, Inland tracts of country experience greater extremes thanthe coasts. This arises from land being more rapidly heated and cooled than water. Henceit attains a higher temperature in summer,—and a lower one in the winter. It also deservesnotice that the western coasts of the extra-tropical continents have a much higher mean tem-perature than the eastern coasts. This is explained by the heat evolved in the condensationof vapour, swept from the surfiice of the ocean by the eastern winds. (Daniell's Metereologi-cal Essays, p. 105, 2d ed. Lond. 1827.) The eft'ects of heat and cold on the human bodyhave been already considered. (See pp. 46 and 57.) Warm elimates are adapted for pulmo-nary invalids (especially consumptive palients,) the rheumatie, the scrofulous, and the para-lytic. Cold, or rather moderately cool, elimates are bracing, and are fitted for relaxed consti-tutione.

2. Humidity. Hygrometric State of the .4lmos^We.-»-Evaporalion from the culaneous andpulmonary surfaces is augmented by a dry state of the atmosphere, and checked by a dampor moist state. But the transudation which depends on vital action is augmented by a warmmoist atmosphere. (Edwards, De l'Influence des Agens Physiques, p. 338. Paris, 1824.)" Of all the physical qualities of the air," observes Sir James Clark, (The Sanative hifluenceof Climate, 3d ed. Lond. 1841.) " humidity is the most injurious to human life." A moist,or rather a soft, climate promotes vital transudation, and, therefbre, is adapted for chronicbronchitis of a dry irritable kind, frequently denominated dry catarrh, and for some othermaladies attended with a harsh, dry, parched skin. A dry climate, on the ether hand, ehecksvital transudation, and, therefbre, is better fitted for relaxed, languid constitutions, with pro-fuse secretion and exhalalion; as^humid asthma, and those forms of chronic eatarrh aecom-panied with copious expectoration.

3. Purity of the Atmosphere. —A pure conditioii of the atmosphere is an essential dementof all healthy cümates. The greater mortality of cities than of the country is principally ie-ferrible to the respiration of air vitiated by the congregalion of a large number of persona ina comparalively limited Space. Emanations from the soil, and from decomposing organicmatter, also contribute to the contanünation of atmosphere. 1 The injurious effeet of fogs onpulmonary invalids is well known to every one. Curiously enough, however, some patientsafFected with spasmodic asthma breathe better in a smoky atmosphere (as that of London)than in pure air.

4. Wind. —Wind greatly modifies the effeet of temperature on the body. Thus two suc-cessive days, whose temperature, as indicated by the thermometer, niay be the samc, shallproduce in us—the one a Sensation of warmth, in consequence of the calm, still, condition ofthe air,— while the other creates a feeling of cold, from the presence of a violent wind. Sothat, as Sir James Clark (Op. supra cit. p. 156.) has justly observcd, " the inflnenee of tempe-rature on the living body is indicated mucli more accurately by our Sensation than by thethermometer." JVIoreover, the humidity and the purity of the atmosphere are greatly modi-fied by the motion or calmness of the air. The preclse effeets produced on elimates by wind,must of course depend on its direction, violence, &c.

5. Atmospheric Pressure. —Diminished atmospheric pressure promotes evaporation. Ele-vated regions, therefbre, are colder, drier, more bracing, and, cceteris paribus, better adaptedfor relaxed individuals, with profuse secretion and exhalation, than the opposite localities :but, on the other hand, they are injurious in bronchial or tracheal Irritation, with diminishedsecretion.

In extra-tropical elimates, a fall in the barometer, without a change or lise of wind, isusually followed by rain. Now a humid condition of the atmosphere ehecks evaporation,while the reduced barometrical pressure augments it. Hence, we have two opposing influ-ences in Operation. This condition of the air induces a feeling df languor and fatigue, andgives rise to sweating on the slightest exertion.

6. Intensity op Light. —The influence of light has been already considered. (See p. 44.)

" The prodnetion of Agae, by the exhalaüons from stagnant water and marshy soils. is well known toevery one. My Criend, Professor Daniell (Lond. Edinb. and Bubi. rhu. Mag. Jaly, 1841.) has shown that thewaters upon the Western coast of Africa, to an extent of 40,000 square miles, are impregnated with snl-pbarelted hydrogen, to an amoant, in some places, exceeding that of some of the most celebrated sulphurSprings of the World; and he suggests that the existence of this deleterious gas in the atmosphere, whichmust necessarily aecompany its Solution in the waters, may be connected with the awfnl tniasma, whichhas hitherlo proved fatal to the explorere and seltlers of the deadly shores of Africa; as well as of other

The origin of sulpburetted hydrogen in sea, and some other waters, has been ascribed, by Dr. Marcet (Phil.Trans. ]HI», p. 195,) Mr. Malcomsoll (Trans, of the. Gcologir.al Society, SM Ser. vol. v. p 504, Lond. J810,) Dr. A.Pontan (■#»». de Chem. et de Phys. July, 1840.) and Professor Daniell (op. supra cit. ,) to the decomposition ofsulphates of the waters, by patrefying vegetable matter.

Vol. I. —14