UM 



METEOROLOGY. 



[crrBOTB or DRAINAGE. 



with moisture ; the constituent particles of the latter 

 radiating heat rapidly, whilst the former does so but very 

 slightly. Generally speaking, good radiators greatly pro- 

 mote rapid evaporation, just as we find in the case of the 

 ordinary porous wine-coolers, from the surface of which 

 moisture is quickly carried off. In precisely a similar 

 manner, the extensive surface of sodden earth, in swampy 

 and marshy lands, absorbs beat during the daytime, and 

 as readily radiates it at night. Evaporation is quickly 

 carried on ; but being attended with a cooling effect, the 

 moisture conveyed into the air becomes partly con- 

 densed, and hence the mists and fogs, so common in the 

 damp and fenny portions of Essex, Cambridgeshire, 

 Lincolnshire, and other parts of our own country. For 

 similar reasons, and arising from the same causes, we 

 may often, in summer time, notice a deep haze coating a 

 valley beneath us, as soon as the sun sets, if we are 

 standing on some moderate elevation. The depth of the 

 haze or mist will depend on the amount of moisture 

 which has been evaporated during the day, and tho fall of 

 temperature at night presuming, of course, that the air 

 is perfectly still, and that various circumstances, to which 

 we need not here allude, are favourable to radiation.* 



Such, therefore, being the causes of the superabundant 

 moisture, haze, fog, &c., of damp grounds, we may 

 proceed to notice tbeir general effect on animal and vege- 

 table life ; and here we may observe, that the natural 

 constitution of the subject will materially influence the 

 effect, prejudicially or beneficially. Many plants can 

 only vegetate in an exceedingly hot and moist air ; and 

 their healthy existence, in any place, is highly indicative 

 of the surrounding climate. Even human beings can be 

 so inured to a particular region, as to live, without serious 

 inconvenience, in places where others would find an 

 untimely grave. Many parts of the coasts of Africa, 

 North and South America, <tc., illustrate this ; and even 

 our own fenny districts, in which ague is so prevalent, 

 are inhabited by individuals whose families have long 

 resided there, and being acclimatised, are comparatively 

 free from diseases which a casual or temporary visitor 

 is sore to suffer from. It is not only, however, in the 

 districts themselves, in which such meteorological pheno- 

 mena are common, that danger to health may arise ; the 

 surrounding country is equally liable to be affected : and 

 hence every method should be adopted which may 

 possibly remedy the evil. The usual effect of too moist a 

 climate, is that of a general depression of the nervous 

 system, and, consequently, a liability to impression of 

 diseases, especially those of the low febrile class, as the 

 typhoid, &c. Most endemics partake of this character; 

 but they are frequently accompanied with others, in 

 which diarrlnca is usually a characteristic. In some 

 cases, a peculiar fever becomes endemical : as an instance 

 of which we may cite New Orleans, where yellow-fever 

 is a regular and periodical visitant It is stated, how- 

 ever, that the rate of mortality is not above the average, 

 for those born and brought up in tho city ; and that the 

 fever is chiefly fatal to visitors. This most likely arises 

 from want of precaution on the part of the latter, who 

 are likely to expose themselves to the dangerous influence 

 of the climate, which the regular inhabitants prudently 

 avoid. 



In most marshy places, but especially in warm or 

 hot climates, vegetable growth and decay proceed rapidly, 

 being stimulated by the heat and moisture together. 

 This is remarkably the case on the west coast of Africa, 

 and in many parts of Italy, of which the Pontine 

 Marshes are a well-known example. In such districts 

 pestilence is always to be found, arising from the abun- 

 dant miasmatic vapours which are generated. The 

 Niger is too well known as a most dangerous locality for 

 Europeans ; and its pestilential effects are readily traced 

 to tho presence, in immense quantities, of poisonous 

 gases which are given off from the rich tropical vegeta- 

 tion as it undergoes decomposition, caused, and rapidly 

 promoted, by tho hot and moist air. 



***, p. 1141. 



Of late years those facts have received tho earnest 

 attention of all classes, and the principles of meteorology 

 have been studied in reference to the best niotlnxl of 

 detecting, avoiding, or ameliorating such conditions.' 

 Tho hygrometer has been of great use for tho examina- 

 tion of the atmosphere in suspected localities; bei 

 as tho dew-point is formed at a temperature which 

 differs from that of the air, just in proportion to tho 

 dryness of the latter,! it follows, that the lower tho 

 dew-point average is in any place, the drier, and therefore 

 tho more salubrious, is the country for a permanent 

 residence. One of the objects which Mr. ' .rried 



out in his balloon ascents, was that of acquiring informa- 

 tion on this matter, which might subsequently ho of 

 service in directing the choice of stations for our troops 

 in India and other tropical countries. 



In attempting to improve the climate of marshy places, 

 of course drainage must bo had recourse to ; and as the 

 practical part of this belongs more especially to engi- 

 neering science, it will not fall within our scope to discuss 

 it in its mechanical details. Not only docs the, with- 

 drawal of the surface-water of swamps, marshes, A-c. , 

 lessen, and, if complete, expel standing fogs and mists, 

 but it also diminishes tho rain-fall, on the average, 

 throughout tho year. The reason of this is twofold : 

 first, there occurs a diminution of superincumbent 

 moisture, which, in part, causes the rain ; and, secondly, 

 through that, the addition to rain falling from hi-li 

 regions (due to the presence of moisture at a lower level), 

 is avoided ; for, as we have already seen, more rain falls, 

 in any spot, as we approach the surface of tho earth, 

 because of the drops gathering additional moisture from 

 tho air they pass through. J Tho application of a good 

 drainage system has, therefore, the effect of removing 

 the moisture itself, and, simultaneously also, tho cause 

 of its continued production. Occasionally, even the 

 geological character of a country may bo judged of by its 

 meteorological characteristics. Thus we generally find 

 that clayey soils, which will not allow the water <m their 

 surface to drain away, are covered with mists ; and the 

 temperature is usually lower than that of a surrounding 

 but better drained country. Such soils are generally 

 termed "cold" by the farmer, who, apart from any 

 scientific theories, has discovered, by long experience, 

 that their temperature, superabundant moisture, iVc., 

 unfit them for producing many kinds of crops. A few 

 years ago, such places were left in their natural stale, 

 but that enterprise which commenced in our manu- 

 facturing districts, has infused itself gradually amongst 

 the agricultural portion of tho community ; and many 

 thousand acres, considered by our forefathers as irre- 

 claimable wastes, now teem with excellent crop-. ; 

 and also have become places fit for residence, where 

 formerly they were little better than sources of 

 pestilence. 



It has been discovered, that where land is in course of 

 being well drained, and cleared of timber and brush- 

 wood, the annual fall of rain soon diminishes in amount. 

 Of all other countries, America has furnished the most 

 striking examples of this fact. There nature has done 

 everything on the large scale ; immense forests spread 

 over surfaces exceeding in area some of tho larger states 

 of tho old world : but as civilisation lias gradually 

 spread, the necessities of the colonists have encroached 

 on the domains of forest-land ; trees have been felled 

 over extensive areas ; and it has been found that, 

 gradually, the amount of precipitated moisture is 

 lessened. This result proceeds from the removal of 

 those objects which radiated the heat of the earth, Arc., 

 and tended to produce rapid evaporation and subsequent 

 condensation ; for the branches and twigs of trees have 

 an astonishing effect iu precipitating rain, and of holdin<.; 

 passing mists. We have often watched such flying over 

 MI extent of plain country, yet parting with but little, if 

 any, of their moisture. Tho moment, however, they 

 came in contact with a plantation of trees, they were 

 stopped in their course, and the previously suspended 

 moisture fell in copious showers, owing to the rodiati::.; 

 and cooling action of the trees amongst which the uiL>ts 



