BAR 



BAR 



Copland, fiirgeon at Du'mfiies, arc of peculiar importance. 

 This gentleman is of opinion that tlie changes of the baro- 

 meter ird;cate approaching cold and hot weather, with 

 much greater certainty than dry and wet. " Ever)' re- 

 markable elevation of the barometer, he fays, when it is of 

 «nr duration, is followed by very warm or dry weather, and 

 moderate as to wind, or by all of them ; but heat feems to 

 have moft influence and connexion ; and when it is deficient, 

 the continuance of the other two will be longer and more 

 remarkable ; therefore the calculntion mud be in a com- 

 pound ratio of the excefs and deficiency of the heat, and of 

 the drynefs of the weather in comparifon of the medium of 

 the ftafon ; and with regard to the want of ilrong v/ind, it 

 appears to be intimately connefted with the \ai\, as they 

 fhew that no precipitation is going -on in any of the neigh- 

 bouring regions." 



In his 14th and 15th remarks, he had faid, 



• 14th, That the barometer being lower, and continuing 

 (o longer than what can be accounted for by immediate 

 faKs, or ftormy weather, indicates the approach of very cold 

 weather for the feafon ; and alfo, coM weather, though dry, 

 is always accompanied by a low barometer, till near its ter- 

 mination.' 



' 15th, That warm weather is always preceded and 

 luoftly accompanied by a high barometer ; and the rifing of 

 the barometer in the time of broken or cold weather, is a fign 

 of the approach of warmer weather : and alfo if the wind is 

 in any of the cold points, a fudden rife of the barometer in- 

 dicates the approach of a foutherly wind, which in winter 

 generally brings rain with it.' 



In the two following remarks, Mr. Copland had explained 

 certain phenomena from a principle fimilar to that oa which 

 Dr. Darwin has fo much infilled. (Botanic Garden, I. notes 

 p. 79, &c.) / 



• That the falling of the barometer may proceed from a 

 decompofition of the atmofphere occuning around or near 

 that part of the globe where we are placed, which will occa- 

 fion the electricity of the atmofphere to be repelled upwards 

 in fine lambent portions ; or driven downwards or upwards 

 in more compared balls of fire ; or laftly, to be carried along 

 with the rain, S:c. in an imperceptible manner to the furface 

 of the earth ; the precipitation of the water)' parts generally 

 very foon takes place, which diminifhes the real gravity of 

 the atmofphere, and alfo by the decompofition of fome of 

 the more aftive parts, the air lofes part of that elaftic 

 and repulfive power which it fo eminently poflcfred, and will 

 therefore prefs with lefs force on the mercury of the baro- 

 meter than before, by which means a fall enfues. 



• That the caufe of the currents of air or winds, may alfo 

 be this way accounted for : and in very fcvere ftorms, where 

 great decompofitions of the atmofphere take place, this is 

 particularly evident, fuch as generally occur in one or more 

 of the Weft India iflands at one time, a great lofs of real 

 gravity, together with a confiderable diminution of the 

 fpring of the air immediately enfues ; hence a current com- 

 mences, firft in that direction whence the air has moft 

 gravity, or is moft difpofed to undergo fuch a change ; but 

 it being foon relieved of its fuperior weight or fpring on that 

 fide, by the decompofition going on as faft as the wind 

 arrives in the ifland, it immediately veers to another point, 

 which then rufhes in moftly with an increafe of force ; thus 

 it goes on till it has blovs-n more than half way round the 

 points of the compafs during the continuation of the hurri- 

 cane. For in this manner the Weft India phenomena, 

 as well as the alteration of the wind during heavy rains in 

 this country, can only be properly accounted for.' S«e re- 

 mark No. 4. 



Mr. C's 4th aphorifm is, ' That the heavieft rains, nhen 

 of long continuance, generalFy begin with the wind blow- 

 ing eafterly, when it gradually veers round to the fouth ; and 

 that the rain does not then begin to ceafc till the wind 

 has got to the weft, or rather a little to the noithward of 

 it, wiien, it may be added, it commonly blows with fomc 

 violence.' 



Mr. Kirwan, in ?.n elaborate paper on this fubjeft (fee 

 Irifli Tranfiiftions, vol. ii. p. 49, &c.) examines the difil^erent 

 caufes to which the phenomena of the bnroi"ttcr ha\e been 

 af?ribed. He begins with the influence of diflFerent tempe- 

 ratures. It apptars, he fays, by obferiation, that a vari- 

 ation of the miifi of the atmofphere is not a necefiary con- 

 fcquence of an alteration of the temperature ; for the mer- 

 cury is often at the fame height at different feafons, and at 

 different places in the fame feafon ; and even when the 

 height of the mercui-y changes fimultaneoufly with the tem- 

 perature, the change is often direftly contraiy to that 

 which the change of temperature would lead one to expeft. 

 Befides, great changes of temperature take place only ia 

 the lower atmofphere ; but in the higher regions they are 

 inconfiderable. Any increment or decrement of the mafi 

 of the lower atmofphere that can be afcribed to a change of 

 temperature, is too ftnall to produce any confiderable altera- 

 tion in the height of the barometer ; for in winter the height 

 to -.vhich any confiderable variation of temperature may be 

 fuppofed to extend, fcarcely exceeds 5000 feet, as we learn 

 from the teftimony of aeronauts and the height of the 

 clouds ; and indeed the vrinds that prevail on the furface of 

 the earth, and v\ hich are the primary agents in producing 

 a change of temperature, feldom reach higher, and in the 

 more northern regions not fo high. This caufe, the effedt 

 of which is eftimated by calculation, and compared with the 

 aftual variation, though not abfolutely inefficient, on the 

 fuppoGtion that the whole mafs of the fuperincumbent co- 

 lumn is increafed by the acccflion of new air in proportion 

 to the condenfation, is neverthelefs inadequate to the effe£t 

 produced. 



Mr. Kirwan next examines the efficacy of winds in pro- 

 ducing the variations of the barometer ; and thefe are fuch. 

 as reign in the lower regions of the atmofphere. If, ac- 

 cording to Dr. Halley's theory, the rife of the barometer 

 above its mean altitude were owing to the accumulation of air 

 o'.er the place of obfervation, occafioned by two contrary 

 winds blowing towards that place, we (hould always have a 

 cahn when the mercury ftands higheft ; but it is notorious, 

 that the greateft mercurial heights are accompanied by an 

 eafterly or northerly n-ind, as Halley himfelf has obferved. 

 Nor can that equality of barometrical heights, which take* 

 place in very diftant countries, where very different winds pre- 

 vail, be explained by this hypotheCs. This bypothefis af- 

 cribes the defcent of the mercury below its mean altitude to 

 the rarefatlion of the atmofphere over the place of obferv*. 

 tion, which rarefaftion is owing to its exhauftion by two 

 contrar)- currents ; for inftance, over England, if it fhould 

 blow a wcfterly wind on the German, and an eafterly wind 

 on the Irifh fea. But Mr. Kirwan thinks, that a rarefackioa 

 in fuch circumftances from fuch a caufe is impoffible ; for 

 if fuch currents took place, the northern or fouthem air 

 would flow in to maintain the equilibrium in the fame pro- 

 portion ; or if this did not happen, and that four contrary 

 currents took place, the higher air flionld defcend, and 

 caufe a fenfible cold, which yet is feldom obfer\cd in Eng- 

 land, when the mercur)' is low ; on the contrary, a wana 

 fouth wind commonly prevaib, to whofe teniperature never- 

 thelefs the rarefaftion cannot be afcribed. Dr. Halley's 

 •xplicatioB of tie Ucfce«t of the mercwiy oa bigU winds in 

 4 & a &aiTa» 



