THE ATMOSPHERE— PRESSURE. 



95 



springs ; the decomposition of the slight ad- 

 mixture of carburetted hydrogen contained in 

 the atmosphere, by the electrical discharges of 

 the clouds, so frequent in intertropical coun- 

 tries. 



Besides the substances which have just been 

 mentioned, and which may be held proper to 

 the atmosphere under all circumstances and in 

 all situations, there are other accidental mat- 

 ters associated with it, which occur especially 

 near the ground, and of which several, desig- 

 nated miasms and contagions, affect the animal 

 system prejudicially. The chemical nature of 

 these substances has not yet been made known 

 by any immediate analysis ; but, considering the 

 putrefactive processes which proceed inces- 

 santly on the surface of our planet, covered as 

 it is with animal and vegetable matters, and led 

 as well by combinations and analogies derived 

 from the domain of pathology, we may fairly 

 conclude on the existence of such injurious lo- 

 cal admixtures. Ammoniacal and other azo- 

 tized vapours, sulphuretted hydrogen, combi- 

 nations, indeed, resembUng the multibasic, (ter- 

 nary and quarternary), compounds of the vege- 

 table kingdom(3^''), may form miasmata, which, 

 in a variety of shapes, and by no means only 

 on naked swampy bottoms, or on sea-coasts 

 strewed with putrifying molluscs, or covered 

 with under-growths of mangrove (Rhizophora), 

 and Avicenniae, may produce fevers of aguish 

 or typhoid types. Fogs which diffuse a pecu- 

 liar smell, remind us at certain seasons of the 

 year of such accidental contaminations of the 

 lower strata of the atmosphere. Winds and 

 ascending currents of air occasioned by the 

 heating of the surface, raise even solid, though 

 of course finely pulverized substances, to con- 

 siderable heights. The dust, which makes the 

 air misty over a great area, and falls about the 

 Cape de Verd Islands, to which Darwin has so 

 properly directed attention, is found from Eh- 

 renberg's observations to contain an infinity of 

 silicious shelled infusory animalcules. 



As principal features in a general physical 

 picture of the atmosphere, we may distinguish, 

 1st. In the variations of the air's pressure : the 

 regular, and between the tropics, so readily ap- 

 preciable hourly oscillations, a kind of ebb and 

 flow of the atmosphere, which cannot be as- 

 cribed to the attraction of the mass of the 

 moon("^), and which is so different according 

 to the latitude, the season of the year, and the 

 height of the place of observation above the 

 level of the sea. 2d. In the climatic distribu- 

 tion of heat : the influence of the relative posi- 

 tion of the transparent and opaque masses — 

 the fluid and solid superficial areas, as well as 

 of the hypsometrical or perpendicular configu- 

 ration of continents, relations which determine 

 the geographical position and curvature of the 

 isothermal lines* in the horizontal or vertical 

 direction, in the ground-plane, or in the aerial 

 strata lying one above another. 3d. In the dis- 

 tribution of the moisture of the atmosphere : 

 the consideration of the quantitative relations 

 according to diversity in the solid and oceanic 

 surfaces, distance from the equator, and height 

 above the level of the sea ; the forms in which 

 precipitation of the watery vapour takes place, 



* Lines of equal mean temperatur*. 



and the connection of this precipitation with 

 the changes of temperature, and the direction 

 as well as the succession of the winds. 4th. 

 In the relations of the aerial electricity, whose 

 primary source, when the air is serene, is still 

 much disputed : the relation of ascending va- 

 pours to the electrical charge and the fashion 

 of clouds according to the time of the day and 

 the season of the year, the colder or hotter 

 zones of the earth, the lower or higher-lying 

 plains ; the frequency and rarity of storms ; 

 their periodicity and occurrence in summer and 

 winter; the casual connection of electricity 

 with the extremely rare occurrence of hail- 

 showers by night, as also with water-spouts 

 and sand-spouts, which have been so ably in- 

 vestigated by Peltier. 



The horary variations of the barometer, in 

 which within the tropics the instrument is twice 

 in the course of the day at its highest, viz., at 

 9 or 9i A. M. and 10 or 10| p. m., and twice at its 

 lowest, viz., at 4 or 4i p. m., and 4 a. m., nearly 

 the hottest and coldest hours in the round of 

 the twenty-four, consequently, long formed the 

 subject of my most careful daily and nightly ob- 

 servations(3^=^). The regularity of these is so 

 great, that the time, especially in the day, may 

 be ascertained by the height of the column of 

 mercury, without an error on the average of 

 more than from fifteen to seventeen minutes. 

 In the torrid zone of the New Continent, on the 

 coasts as well as on heights of more than 12,000 

 feet above the level of the sea, where the mean 

 temperature falls to 7=^ C (43° 8 F.), I have not 

 found the regularity of this ebb and flow of the 

 atmosphere to be disturbed either by tempests 

 of thunder or of wind, by rain or by earthquakes. 

 The amount of the daily fluctuation diminishes 

 from the equator on to 70° N. latitude (a par- 

 allel under which we possess very accurate ob- 

 servations made by Bravais at Bosekop) (3"), 

 from 1-32 line, to 18 hne. That, much near- 

 er the pole, the mean height of the barometer 

 is actually less at 10 a. m. than at 4 p. m., so 

 that the times of the maxima and minima are 

 severally interchanged, is by no means to be 

 concluded from Parry's observations at Bowen 

 Harbour (73° 14' N. latitude). 



The mean height of the barometer, by reason 

 of the ascending current of air, is somewhat 

 less under the equator, and especially under the 

 tropics, than in the temperate zone(^^*) ; it ap- 

 pears to attain its maximum, in the West of 

 Europe, in the parallels of 40° and 45°. If, 

 with Kaemtz, we connect those places which 

 present the same mean differences in their 

 monthly barometrical extremes by isobaromet- 

 rical lines, curves are engendered, the geograph- 

 ical position and direction of which yield us im- 

 portant conclusions in regard to the influence of 

 the configuration of continents, and the expanse 

 of seas upon the oscillations of the atmosphere. 

 Hindostan, with its lofty mountain ranges and 

 triangular-shaped peninsula, the East coasts of 

 the New Continent, at the point where the 

 warm gulf-stream turns eastward by New- 

 foundland, show greater isobarometrical fluctu- 

 ations than the West India Islands, and the 

 Western parts of Europe. Prevailing winds 

 exert the most especial influence on the dimi- 

 nution of the atmospheric pressure, and with 

 this, according to Daussy, as we have already 



