UM 



METEOROLOGY. 



[OOWDUCTIOS, ETC., OT HIAT. 



sphere itself the medium, the temperature of which 

 wo desire to investigate ; but it becomes of practical 

 importance in all -othor cases; and its cot 

 teaches the thornioiiiftric observer the necessity for em- 

 ploying instrument*, the bulbe of which nro as small as 

 compatible with well-marked amounts of columnar ex- 

 paiuion. This remark especially applies to cases in which 

 the bulk of liquid or solid operated upon is small. As 

 concerns the graduation of thermometers, the mos: 



instruments are those of which the graduations are 

 effected on the tube of the glass itself. If the gr;i'l na- 

 tion be made on a scale of brass, the resulting indica- 

 tions will be very incorrect, except the relative expansi- 

 bility <>f brass and glass be taken into consideration, and 

 duly allowed for ; this, however, is so troublesome, that 

 it will be too frequently evaded, and errors will creep in. 

 Far betu r than brass is box-wood ; and slate and ivory 

 are better still. 



Corrttpondence bettoern Thermometers. Scarcely any 

 two thermometers exactly correspond, even though the 

 same materials be employed in their construction, so 

 numerous are the points which require attention. Be- 

 tween thermometers constructed with different materials 

 air thermometers and mercurial thermometers, for in- 

 stance, or either of these, aud spirit thermometers the 

 discrepancies are still more considerable. 



The experiments of M. Uegnault relative to the dis- 

 crepancies subsisting between mercurial and air thermo- 

 ; s, are amongst the latest on this important subject. 

 He found, that between 32 and 21*2 Fahrenheit, there is 

 an almost absolute coincidence between the degrees of 

 the air and the mercurial thermometer. From 212 to 

 482 Fahrenheit, the mercurial and air thermometers 

 remain pretty equal ; but after the latter point the mer- 

 curial gains on the air thermometers. 



Hitherto we have treated of the atmosphere, statically, 

 that is, in a condition of repose ; but perfect atmospheric 

 quiescence is unknown in natxire it is always agitated, 

 more or less ; hence we are led to the consideration of 

 atmospheric currents or winds. 



So variable are winds in these northern latitudes, that 

 their incertitude has passed into a proverb; primary 

 atmospheric currents, nevertheless, are constant in their 

 direction, and are referable to variations of temperature 

 simultaneously existing in different parts of the 

 world. 



Heat, in its non-latent condition, or, in other words, 

 that condition of heat which is recognisable by the ther- 

 mometer, has a tendency to equalise itself. Hence, if 

 two bodies a and 6, of which a is hotter than b, be 

 situated in proximity to each other, there is an imme- 

 diate tendency to equalisation of temperature as between 

 the two ; but the thermal conditions which regulate the 

 production of winds, will be most readily appreciated by 

 ing on what takes place when a heated solid is 

 suspended in the atmosphere by a small chain or wire ; 

 we shall find, on investigation, that a heated solid thus 

 circumstanced gradually becomes cool by the operation of 

 three distributive influences conduction, radiation, and 

 convection. It will be unnecessary for us to do more 

 than give a very general illustration of these terms, as 

 the subjects are fully investigated iu the first section of 

 this volume. 



Conduction. If the chain by which the heated cannon- 

 Fig, ss. ball is represented to be suspended in 

 the annexed diagram (1 

 examined from time to time by tlm 

 thermometer, or even by the fingers, 

 it will bo found to inrn-a.se in heat, 

 evidently because it removes a portion 

 of temperature from the heated can- 

 non-bull by conducting it away. 

 This function of heat is so well 

 understood, and so commonly exem- 

 plified, that no further consideration 

 of it will be necessary here. 



Radiation. If a tliermonu " 

 held even at tho distance of some feet from the 1. 

 '!i-ba!l, the mercurial column will be sensibly ai! 

 thus demonstrating the transmission of heat. But how 

 transmitted ? By contact with the atmosphere ? Clearly 

 not, inasmuch as tho result just indicated, still occurs if 

 the cannon-ball be placed in the vacuum of an air- 

 pump. By experiment, it has been detei -mined that tho 

 temperature, in the case under consideration, has been 

 given off in tho condition of rays, precisely as light is 

 evolved, and hence the propriety of the term radiant 



Convection. Independently of the two processes of 

 heat-distribution already described, there is yet a third 

 Referring to the suspended hot cannon-ball, we shall find 

 that, if suspended in a room from the ceiling, the air near 

 the ceiling becomes hotter than the air below : hence a 

 portion of the temperature of the hot cannon-ball must 

 have become accumulated there, by reason of some cause 

 besides those of conduction and radiation, both of which 

 distribute the temperature equally in all directions, 

 through a homogeneous medium such as the atmo- 

 sphere for example in our assumed experiment. Tho 

 process of heat-distribution known as convection, is the 

 necessary result of the expansion of liquids and fluids by 

 heat ; for when expanded they are specifically lighter ; and 

 when specifically lighter, they must necessarily ascend. It 

 is by virtue of the process of heat-distribution, termed 

 convection, that tho child's soap-blown bubbles ascend 

 instead of at once falling to the surface of the earth. 



Temperature and pressure being equal, breath evolved 

 from the lungs is heavier than atmospheric air, because 

 it holds more carbonic acid. Nevertheless, inasmuch as 

 it is evolved from the lungs hotter than tho surrounding 

 atmosphere, soap-bubbles blown with it ascend. Pre- 

 sently, however, they descend, because tho heat acquired 

 from the lungs being evolved, and equalisation of tem- 

 perature with that of the surrounding atmosphere having 

 ensued, the great specific gravity of the gas wherewith 

 the bubble is blown, causes the latter to sink to the sur- 

 face of the earth. 



It is to the process of heat convection that we owe 

 the salutary draughts in our chimneys, and also nusalu- 

 tary draughts in our apartments. No sooner is fuel 

 lighted in a fire-place, than the superincumbent air be- 

 comes higher aud specifically lighter ; it therefore ascends, 

 and cold air rushes in to fill its place. Thus we have, in 

 point of fact, a local wind ; and the causes which deter- 

 mine that wind are exactly comparable to tho causes of 

 winds which take place in the grand economy of nature, 

 as will soon be rendered manifest. 



CHAPTER II. 

 ATMOSPHERIC PHENOMENA. 



THE TRAI>R WIST*. Applying tho facts already de- 

 \ 1- 1 us now regard tho surface of ourglobe'in the 

 aggregate, with reference to the localities of maximum 

 and minimum temperature, and the consequence of such 

 difference of temperature in originating an aerial < 

 It in evident that tho hottest portions of our globe's sur- 

 fsee an udod within tho tropics, and the cold- 



est portions are the arctic regions north and south. 



Those circumstances being promised, wo arc now in a 

 condition to anticipate the direction of the aerial cur- 

 rents or winds which must necessarily ensue,. Firstly, 

 an ordinary current of heated air should rise aloft in the 

 1 1 i] .ic.il regions, then diverge and pass north and south to 

 !<>, thus constituting what may be term--! 

 tho \ipper trade current. This current, as it proceeds 

 north and south, gradually becomes cold, in which condi- 



