90 



THE ATMOSPHERE— CLIMATE. 



observed, the mean height of the sea is increas- 



ed(3"). 



As the whole of the most important varia- 

 tions in the weight or pressure of the atmo- 

 sphere — whether they occur regularly at certain 

 hours and seasons, or are accidental and ex- 

 cessive, when they are often accompanied with 

 danger("*) — like all the rest of what are called 

 weather phenomena, have their principal cause 

 in the heating power of the sun's rays ; so the 

 directions of the wind (partly on Lambert's 

 proposition) were at an early period compared 

 with the state of the barometer, with variations 

 in temperature, and with differences in the hy- 

 grometric state of the atmosphere. Tables of 

 the pressure of the atmosphere along with par- 

 ticular winds, designated by the title of baro- 

 metric-al wind-cards, have given a deep insight 

 into the connection of meteorological phenom- 

 ena(3^^). With wonderful acumen. Dove per- 

 ceived, in the laws of the rotation of the winds 

 of both hemispheres, which he discovered, the 

 cause of many grand variations (processes) in 

 the atmospheric ocean(^^^). The thermal dif- 

 ference between countries lying near the equa- 

 tor and those situated near the pole, engenders 

 two opposite currents in the upper regions of 

 the atmosphere and on the surface of the 

 earth. In consequence of the diversity of the 

 rotatory velocity in the parts lying nearer the 

 pole, or nearer the equator, the air which is 

 streaming from the pole acquires an eastern, 

 that which is pouring along from the equator a 

 western direction. From the struggle between 

 these two currents, the place of descent of the 

 higher, the alternating displacements of the 

 one by the other, depend the most important 

 phenomena of atmospheric pressure, of the 

 heating and cooling of the aerial strata, of the 

 precipitation of moisture, and, indeed, as Dove 

 has correctly shown, of the formation of clouds 

 and their configuration. The forms of clouds, 

 those all-enlivening ornaments of the land- 

 scape, are faithful indications of what is going 

 on in the upper regions of the air ; and in 

 calms, and floating in the warm summer's sky, 

 they are also the " projected image" of the 

 heat-radiating surface of the ground. 



Where the influence of the radiation of heat 

 is conditional on the relative position of great 

 continental and oceanic surfaces, as betwixt the 

 East coast of Africa and the West coast of the 

 peninsula of Hindostan, regular periodical chan- 

 ges in the direction of the winds accompany 

 the changes in the declination of the sun, and 

 constitute the Indian monsoons(^^^), the Hippa- 

 los of the Greek navigators. These winds 

 must have been amongst the earliest regular 

 winds recognized and taken advantage of by 

 mankind. In this knowledge of the monsoons, 

 which has certainly been spread over China 

 and Hindostan, the Eastern, Arabian, and 

 Western Malayan Seas, for thousands of years, 

 as well as in the still older and more generally 

 diffused observation of the sea and land breeze, 

 lies the hidden germ of the fast-advancing me- 

 teorological science of the present day. The 

 long series of magnetic stations which have 

 now been established from Moscow to Pekin, 

 through the whole of Northern Asia, as they 

 have it also in charge to observe meteorologi- 

 cal phenomena in general, will soon become of 



great importance in establishing the Law oi" 

 THE Winds. The comparison of observations 

 made simultaneously at places many hundreds 

 of miles apart, will determine whether or not 

 the same east wind blows from the barren ta- 

 ble-lands of Gobi to the interior of Russia, or 

 whether, and at what point in the line of sta- 

 tions, the direction of the current becomes 

 changed through a descent of air from the high- 

 er regions. We shall then, in the true sense 

 of the phrase, learn " whence the wind cometh." 

 If we would base the required result on obser- 

 vations continued for not fewer than twenty 

 years, Mahlman's careful notifications assure 

 us that in the middle latitudes of the temperate 

 zone in both continents the west-south-west is 

 the prevailing wind. 



Our knowledge of the distribution of heat 

 in the atmosphere has gained, in some respects, 

 in clearness, since attempts have been made 

 to connect the points that indicate the mean 

 temperature of the year, of the summer and of 

 the winter, by different orders of lines. The 

 system of Isothermal, Isotheral, and Isochim- 

 enal lines, which I first proposed in 1817, may, 

 perhaps, when it has been gradually perfected 

 by the united efforts of natural philosophers, be 

 found to supply a general and grand basis for 

 a comparative Climatology. Terrestrial mag- 

 netism first acquired a scientific shape when 

 scattered partial results were connected graph- 

 ically with one another by lines of equal varia- 

 tion, of equal dip, and of equal intensity. 



The expression Climate, in its most general 

 acceptation, indicates every change in the at- 

 mosphere which sensibly affects our organs — • 

 temperature, humidity, alteration of barometri- 

 cal pressure ; calms or storms of wind from va- 

 rious quarters ; amount of electrical tension ; 

 purity of atmosphere, or its contamination with 

 gaseous exhalations more or less pernicious ; 

 finally, degree of habitual transparency and se- 

 renity of the sky, which is not merely impor- 

 tant in connection with the amount of radia- 

 tion from the ground, the organic evolution of 

 plants, and the ripening of fruits, but also with 

 the feelings and whole mental estate of man- 

 kind. 



Were the surface of the earth composed of 

 one and the same homogeneous fluid mass, or 

 of rocky strata of like colour, like density, like 

 smoothness, like capacity of absorption for the 

 sun's rays, and like power of radiation into 

 planetary space, then would the Isothermal, 

 Isotheral, and Isochimenal lines run parallel to 

 one another, and to the Equator. In such an 

 hypothetical condition of the earth's surface, 

 the power of absorbing and of emitting light 

 and heat would be the same in the same paral- 

 lel of latitude all round the globe. And it is, 

 in fact, from such a mean, and, as it were, pri- 

 mary condition, which neither excludes the 

 transmission of heat to the interior of the 

 earth, nor towards the atmosphere involving 

 it, nor the communication of heat by currents 

 of air, that the mathematical consideration of 

 climates sets out. All that alters the absorb- 

 ing and radiating powers of the surface in par- 

 ticular parts lying in the same parallels of lati- 

 tude, produces inflections in the Isothermal 

 lines. The nature of these inflections, the 

 angle under which the isothermal, isotheral, 



