1174 



METEOROLOGY. 



.. . . 



" U the region in which the phenomena occur, 

 " not exceeding fifteen miles ; horizontal motion of iU 

 MTeral superposed or intermixed currents (like streams 

 of wat.T) ; their opposition, or, at times, combination ; 

 and th-ir perennial progress, bodily (in mass), toward 

 the east, in the temperate zone." These ho considers as 

 the most useful facts on which inferences may be formed, 

 ami the discovery of law can be ednced, in dealing with 

 questions relating to the forecasts and warnings to 

 we hare already alluded. 



With respect to the movements of the atmosphere, 

 " accumulated facts have induced the conclusion that 

 winds move in parallel currents, or circulate around a 

 centrical area ;" and that a uniform law of circulation or 

 tyr.it in exists, equally on the large scale as in the polar 

 and tropical currents, and descending to the slightest 

 ai rial disturbance of any kind, with but rare and limited 

 exceptions. The distinction between perennial trade- 

 winds and local cyclones, seems to arise simply in the 

 very circular direction of wind in the cyclone storm; 

 whilst in the ordinary trade and similar winds, the 

 n is of a loss curved, and consequently more con- 

 stantly direct, character. The direction and force of the 

 polar current are, of course, affected by the rotation of 

 the earth, it becoming equatorial as that region is arrived 

 at. The current, in its return, moves eastward towards 

 each pole, and loses its momentum gradually, from 

 gravitation and various local causes. It has been proved 

 that storms, ami all the greater circulations of the air, 

 have an eastward motion bodily, while circulating 

 around a centrical area, between the tropics aud polar 

 regions ; whilst, within the tropics, the direction is west- 

 ward until the currents recurve. To the circulation of 

 these polar and tropical currents, often in parallel direc- 

 tions, and the effects of heat, moisture, &c., on the 

 atmosphere generally, are assigned the cause of what are 

 called "atmospheric waves," which are attended with a 

 rise and fall of the mercury in the barometer, under 

 certain limitations. 



"Such currents, prolonged and excessively broad, are 

 always flowing in nearly opposite directions, and always 

 moving eastward, at an average rate of about five miles 

 an hour, in our latitude." If near the earth's surface, 

 they may be side by side or parallel ; but if overlapping, 

 or entirely superposed, they then cross in various di- 

 rections, and more or less impinge on, or intermingle 

 with, each other. Henco arise intermingling cyclones, 

 which, affecting each other, apparently in opposition, 

 but really in accordance with these laws, have, from our 

 previous ignorance, much hindered the progress of our 

 deductions, and frequently thrown doubt on the anti- 

 cipatory predictions which have been made as to their 

 occurrence. 



Without detailing the entire known chain of causes 

 which tend to produce the results, aud which have, more 

 or less, been already considered in the previous chapters 

 of this section it may be remarked, with respect to the 

 polar current, that " the sensible result on the whole 

 system of circulation, must be a continual easterly pro- 

 gression in the temperate zones ; a general motion of the 

 atmosphere towards the cast, over the earth's surface ; 

 while, in the lower latitudes, and (perennial) trade- 

 winds, its motion is different from, that in higher lati- 

 tudes, being towards the west The normal state 



of our atmoaphe.ro appears to b a regular alternation, or 

 circulation of currents, between polar and tropical 

 regions the polar usually, but not always, advancing 

 along the earth's surface ; the return current (i.e., from 

 the equator) "generally above, at higher elevations." 

 The electrical characteristic of the polar current is 

 generally that of positive excitement, with cold, great 

 specific gravity, and dry ness, except over a warm ocean ; 

 whiUt the tropical current is generally negatively 

 electrical, the positive state being rarely, if ever, ob- 



: 



Those remarks are intended to explain what wo may 

 the ii.rmal state of our atmosphere. The pheno- 

 to which wo have drawn attention may be con- 

 ridmd as the natural condition of the gaseous envelope 



of the surface of the earth, undisturbed by local and 

 abnormal influences. When, however, we descend to 

 us of comparatively slight elevation above the earth, 

 then we find a vast number of other causes coming into 

 operation, such as radiation, the physical character of 

 country, as mountains, plains, swamps, seas, \c. ; all of 

 which tend to break-up the main currents of which we 

 have been speaking, and thus to produce that diversity 

 of wind and weather so proverbial in many parts ; but 

 especially in insular, places, like our own country. In 

 such situations, the study of atmospheric currents be- 

 comes a question of great complexity; and our readers 

 may more fully understand its extent, when we remind 

 them, that it is by no means an uncommon occurrence to 

 find the wind blowing in every possible direction of the 

 compass, at the same moment, in different parts, round 

 and within our own coasts. It will be easily seen, after 

 a careful perusal of our Section on Heat, and the earlier 

 chapters of the present one, that an even-flowing current 

 arriving from any direction at our shores, must imme- 

 diately impinge, in some places, against cold mountain 

 heights ; in others, on warm and moist plains, itc. ; and 

 thus be broken into an immense number of smaller 

 currents, which, re-acting on each other, afford about as 

 difficult a problem for study as any within the range of 

 philosophical investigation. The establishment of tho 

 primary currents, however, tends to assist us in ex- 

 plaining the causo of those of a secondary nature; and 

 as we are assured that nothing occurs by chance, but 

 follows certain definite, although possibly obscure laws, 

 we ore constantly encouraged to pursue these investiga- 

 tions, and, by observations and deductions, gradually 

 arrive at the object of our researches. 



In explaining tho causes of barometric disturbance, 

 Admiral Fitzroy relies chiefly 1st, on the effects of a 

 rapid horizontal current, which tends, by producing a 

 partial vacuum at the place of observation, to depress tho 

 mercury : 2ndly, on the presence of aqueous vapour, which 

 diminishes the specific gravity of the air; and, 3rdly, on 

 the expansion of the air by heat, which has precisely 

 the same efl'ect. As regards the influence of atmospheric 

 waves on the rise and fall of the barometer, he is disposed 

 to consider that their action is chiefly of a secondary 

 character. It will hence be seen, that any change of 

 wind and weather, whilst depending on a vast variety of 

 causes, must always bo attended by an increase or 

 decrease of temperature, and moisture ; and, conse- 

 quently, accompanied with a variation in the height of 

 the mercurial column. The barometer, thermometer, 

 and hygrometer, are, therefore, par excellence, the in- 

 struments, taken together, on which the meteorol> 

 essentially depends for unravelling the complex character 

 of the winds, &c. 



A popular illustration of these statements may be 

 readily presented by referring to the experience of 

 weather-changes in our own country. With a north 

 wind, which is always dry, of great specific gravity, and 

 low temperature, a rise of the mercury occurs, both at 

 the moment, and shortly preceding the advent of this 

 wind. On the other hand, the south-west wind, loaded 

 with moisture, of a comparatively high temperature, 

 and consequently low specific gravity, is always usli 

 in, and attended, by a fall of the mercury, lift, 

 this point and tho north-east, as tho wind \ 

 northerly, tho barometric column generally rises; whilst, 

 as tho wind travels southerly, from north-east, a baro- 

 metric fall gradually occurs in most cases. These facts 

 are palpable to the most cursory observer of the 

 barometer ; and those who object to the truth of fore- 

 cast*, admit them tacitly, and condemn themselves prac- 

 tically, whenever they rely on barometric chanj,'. 

 predict, for a few hours, a change of the weather. The 

 philosophic observer differs from such in arguing on a 

 broader basis : he not only notes the barometric varia- 

 tions, but associates with them the antecedent changes of 

 temperature and hygrometrio condition, which, In 

 perienco, he has found invariably to occur. Thus hi* 

 deductions have reason and foot for their basis, whilst 

 the hasty weather-wise simply follow an empiric and 



