

METEOROLOGY. 



[BAltoMl.TT.iC VAUIATIOXJ. 



are influenced by luniiy other circumstances, which we 

 mart not omit to consider. Amongst the most 



thews are winds. Having regard to the- ultiniiito 

 cause of winds, it will he evident that the existence of a 

 win. 1 bespeaks the condition i>f dim-rent mper.iti;- 



..ii . >. Hunoe, every wind naOHWtily 

 raries to some degree the temperature which v 

 liave subsisted at any given place under a perfectly tr.m- 

 quil atnio.-; -ow, as the atmosphere expands by 



. to a corresponding 

 degree, its density or specific gravity, so it follows that 



:<" barium-trie 'column will be intiii' 



by wind*. In I will be generally found that a 



fall of the barometer corresponds with a rise in the 

 thermometer ; this rule also prevails for the tropics: 



: -.In-less it is subject t" many variations. Tin- i 

 meter may rise and fall without any corresponding change 

 in the thermometer, or both may rise and fall together. 

 The application of the barometer as a weather glass is 

 tlu r collateral and secondary ; nevertheless its in- 

 :oiis in this respect are, for the most part, worthy 

 of confidence : generally, the barometric column sinks 

 the day before rain occurs, and rises during i 



The barometric column is much agitated during 

 the existence of a storm, owirtg to the conflict which 

 thru ensues bet worn atmospheric currents tending to- 

 wards opposite directions. 



Other Variation* in the Barometer. Besides the eleva- 

 tion of barometric mercury due to direct atmospheric 

 pressure and to aerial current*, there exist other fluc- 

 tuating causes, both diurnal and annual. The former is 

 scarcely noticeable in tempi-rate, but very conspici; 

 in the torrid zone. Every day the barometric column 

 twice attains a maximum, and as often a minimum. 

 The two periods of maximum elevation occur bet'. 

 8J and 1W.V A.M. (say an average of Oh. Win.) ; and be- 

 . !> and 11 P.M. (say an average of lOli. llm.) The 

 two periods of minimum elevation are between 3 and 

 6 A.M. (average 3J) ; and between 3 and 5 P.M. (average 

 m.) During winter and the rainy tropical season, 

 the diurnal elevations of the barometer are least, ami 

 they assume their maximum in April. The variations 

 are much less on elevated mountains than in the p 

 below. 



Mean Barometric Condition of a Plate. It was formerly 

 assumed that' everywhere, at the level of the sea, the 

 barometric condition for the same time was identical. 

 This opinion is fallacious, latitude having a well-deter- 

 mined influence in tliis respect. It is least of all at the 

 equator, whence it increases north and south, attaining 

 its maximum about 30 or 43 of latitude : it then de- 

 creases to between 60 and 70. Within the polar circle 

 it would appear to reascend ; but further experiments 

 fur this locality are a rfr.tr 



ude also appears to exert some influence over the 

 elevation of the barometric current. It is greater in the 

 | Atlantic than in the Pacific, by a small but readily per- 

 i ceptible quantity. 



Caiaet of Periodical Barometric Variations. Various 

 opinions have been advanced to account for these 

 "lical barometric variations. To say that they are 

 'Mitable. to ditl'uifnce of temperature, is to advance 

 a cause too remote from the result. Many philosophers 

 have attributed the.su variations to the existence of veri- 

 table atmospheric ti<los ; but the most plausible explana- 

 tion of diurnal barometric variations would seem to be 

 that of Dove, who assumes them to depend upon the 

 varying amount of aqu-ons vapour. Aqueous vapour 

 and atmospheric air are possessed of diiferent specific 

 gravities; and the baron -lit of a column of 



mercury for any time, will be the sum of pressure of dry 

 "pheric air and associated moisture ; as the relative 

 .nt of the two varies, so will vary the height of the 

 barometric column. 



ATJK.M-IH KM M..r-it KK AM. ITS UMUVATIVFS. When 

 treating, at page 1 1 _>, of the means to be employed for 

 weighing a gas, the facility wherewith gaseous bodies 

 absorb moisture, was adverted to. Some idea then may 

 be gained of the amount of moisture present in the at- 



monphoro, seeing that tho latter is over in contact with 

 large expanse* of water. Tlio atmosphere, in point of 

 fact, is never dry, or in any v 



: the air > >!uu a-nl 



ring vegetable*, it is easy to .1 

 aid of chemical agents, tho existence of I 

 turo ; without the presence of which, neither the f uno- 

 t'otisof animal or of vegetable life could bu maintai: 

 KVI-II when the air a; ; ie con.li-ion of drynem, 



within very remote limits, breathm,- is ditli -iilt, ami 

 symptoms of feverish restlessness speedily set in. Tho 

 natural craving of the lun^< for inoisturn is demonstrated 

 by the presence of a close stove in a small room. Tho 

 sensations, which are very unpleasant, i-.ui always be 

 alleviated by placing a small dish of water on tile s: 

 so that evaporation may go on continuously. It is of 

 tho utmost importance, 'then, to be enabled, not only 

 to demonstrate tho existence of atmospheric 

 but to determine its quantity. A few experiments for 

 ng this demonstration wo shall now detail. 



mjit 1. The accompanying diagram (Fig. 39) 

 represents a balance, or pair of scales, into oue pair of 



tig. J9. 



which there has been placed a small dish of oil of vitriol, 

 and into the other a counterpoise. If the apparatus bo 

 exposed to the air, even when the earth is hottest and 

 ih vest, nevertheless, the equilibrium of the pair of scales 

 will soon be destroyed. Some ponderable increase will 

 have been acquired by the pan containing the oil of 

 vitriol ; and analogy demonstrates the increase in question 

 to be due to the absorption of water. Founded on this 

 property, oil of vitriol is frequently employed by tho 

 chemist for desiccating substances which could not bo 

 heated without damage. Accordingly, if a pan of oil of 

 vitriol and a moistened sheet of paper be enclosed 

 together, under an inverted glass, tho paper will in 

 course of time become dry. Far more rapid and power- 

 ful is the operation of the oil of vitriol when, instead of 

 being placed together with the substance to lie ; 

 and in a mere bell-glass, the two are placed under the 

 receiver of an air-pump, and the air exhausted. Under 

 these circumstances, an atmosphere of aqueous vapour 

 soon fills the air-pump receiver, and the absorptive 

 operation of the oil of vitriol being continuous, the 

 wafer is speedily evaporated. 



K.?lieriment 2. Instead of oil of vitriol, carbonate of 

 >" l;i, or chloride of calcium, and, in an inferior il 

 impure common salt (chloride of sodium) may be used ; 

 for these bodies are all hygrometric that is to say, they 

 have tho property of absorbing water from the air. Of 

 the three bodies mentioned, chl n.le of calcium is the 

 most hygrometric, and is of constant application by the 

 chemist. Founded on the hygrometric quality of 

 common salt, ami other saline materials contained in 

 sea-water, is the property which certain sea-weeds 

 have of becoming moist in damp weather, and of iiidi- 

 ', by their dry crispuess, an opposite atmospheric 

 condition. 



Although aqueous \.ipmir is always present in the 

 atmosphere, it is not always visible. Frequently it is 

 quite transparent, and only demonstrable by the process 

 of extracting it by such means as we have descri- 

 but at other times it aggregates, becoming vesicular, 

 and forming clouds, fog, dew, rain, snow, bail, or 



Dtw. Although the philosophy of dew is now 



