HKTKOROLOOY. 



(THE BAKOMF.TFR. 



already soon ; then-fore sm-h deviation from the standard 

 of uniformity, and indeed many other circumstance)", 

 mint 1 Uken cognizance of before we are enabled to 

 employ the baromeUr aa an indicator of atmospheric 

 elevations. 



The law of Marriotte teaches that the dilation of a 

 Tolnme of air is proportional to its density, BO long as 

 the temperature to which it is exposed is constant ; 

 whence it follows, that the density of the atmosphere 

 diminishes from below upwards, in geometrical pmgres- 

 sion. Reasoning on this basis, it appears that, a/winning 

 any particular elevation above the point of observation, 

 if/ immoral exponent may be regarded as the logarithm 

 of the density of the lowest atmospheric layer, or, in 

 other words, of the barometric column. A consideration 

 of the mutual relation subsisting between numbers and 

 their logarithms will render this evident, for logarithms 

 are nothing more than numbers increasing by arithmetical 

 progression, corresponding to other numbers, the in- 

 crease of which is also in geometrical progression. Being 

 possessed of a table of logarithms, expressing the densi- 

 ties of atmospheric layers, one might calculate the 

 height of a mountain by two observations made at two 

 stations ; but the same result may be arrived at by using 

 a common table of logarithms, and multiplying them 

 by a constant factor. According to Deluc, the factor is 

 10,000. 



Many other considerations have to be allowed for 

 before the barometer can bo accurately applied as a 

 measure of mountain elevations : they are temperature, 

 latitude, relation subsisting between the specific gravities 

 of air, and mercury, and dilation of mercury for each 

 degree of the thermometer = 0-OOOt for each degree of 

 Fahrenheit's scale. All these general considerations have 

 been embraced in tables. 



Although the circumstances necessary to be taken 

 cognizance of when employing the barometer as an indi- 

 cator of mountain elevation, are numerous and compli- 

 cated, nevertheless, the results obtained are susceptible 

 of a considerable degree of accuracy. Subjoined is a 

 table of comparative results between trigonometric and 

 barometric observations. The difference, it will be seen, 

 is only trifling. 



COMPARISON OF TRIGONOMETRIC AND BAROMETRIC 

 MEASUREMENTS OP MOUNTAIN HEIGHTS. 



When an approximate result is alone required, and 

 the height is inconsiderable, a fall of } l ,,th of an inch may 

 be allowed for every ninety feet of elevation, or -nAroth 

 of an inch for every foot. This rule suffices for the 

 small differences of elevation at which barometers are 

 hung, and enables the observer to institute a comparison 

 between them. Correction for temperature must, how- 

 ever, not be omitted. The ratio of expansion for HT- 

 cury, glass, and brass the materials employed in tho 

 manufacture of barometers will be pointed" out here- 

 after ; meantime we may as well indicate, that perhaps, 

 after all, it is well in practice to ignore these complex 

 element*, and to consider -pHioths of an inch as the 

 allowance for mercurial expansion for every degree 

 above 32, and viee vtriA. Applying the above correc- 

 tions for temperature and pressure to practice, let it be 

 required to know what height tho mercury would stand 

 at the level of the sea and at 32 Fah., if at an altitude 

 of ISO feet, and temperature 65, it indicated 29 '565 



i:, 1,.-, I 



Actual height of mercurial column . 

 Deduct for 23 of temperature above 32 



3 23 69 



.,)' 



KM) X 1 



Altitude of mercury . 

 Add for elevation -001 x 



150 



Inrhet. 



20505 



- OC3 



- -150 



Altitude of level at sea, at temperature 32 Fah. = " 



The reader will remember tha* the previous remarks 

 have reference to the barometer as affected by air a' 

 this being the simplest a'mospheric condition which theory 

 can assume. Hereafter we shall discover that the baro- 

 meter, when influenced by the atmosphere in mo: ion 

 by winds, in other words is subject to great varia- 

 tions. 



Further Improrrmeiits of the Barometer. The baro- 

 meter in its simplest form, as already described, is a more 

 perfect instrument than many in which that simplicity of 

 form is departed from, in deference to portability; Fig. 6. 

 nevertheless, it is not quite correct. To be abso- 

 lutely correct, it is indispensable that the mercurial 

 level in the basin should bear a constant ratio to 

 the mercury remaining in the tube, a condition 

 which evidently cannot be obtained in the instru- 

 ment just described. In proportion as mercury 

 descends out of the inverted tube, the level of the 

 mercury in the basin will bo elevated, and to the 

 extent of such elevation tho indications of the in- 

 strument will be prejudiced. Various means are 

 had recourse to for lessening, or absolutely remov- 

 ing, this evil. It may be lessened by inert -aMii^ 

 the width of tho basin to such extent, that tho 

 ratio of elevation of the mercurial surface may be 

 so greatly diminished that it will practically cease 

 to impart errors. It maybe absolutely removal 

 by one of two devices. One consists in mounting 

 the receiving basin on a screw, which, by eleva- 

 tion and depression, regulates the quicksilver to any de- 

 sired level. Such is the contrivance of M. Fortin. \\hoso 

 construction of a barometer is hero annexed (Fig. 6) ; 

 Fig. 7. Tig. 8. but it is more usual to attach to the 

 barometer, a long scale having a slide 

 motion, so that the lower end, or com- 

 mencement of the scale, may be made 

 to coincide witli the level of (lie mer- 

 cury in tho basin. Practically, how- 

 ever, the basin is usually dis| . 

 with, the reservoir for mercury being 

 a mere extension of the barometric 

 tube, in some cases bulbed, and in 

 others quite plain. Both these forms 

 of construction are shown in figures 

 7 and 8. 



The Wfather-g'nss. The primary 

 and only direct function of the baro- 

 meter is, that its 

 mercurial column in- 

 dicates, by its rising 

 and fulling, the vary- 

 ing weight of the superincumbent at- 

 nioi] .hero. Very frequently this direct 

 function is taken no account of ; 

 variation in the state of tho weather 

 being all which the observer desires 

 to make himself acquainted with. 

 Subserviently to this intention, all direct 

 rise and fall of the barometric column 

 is lost sight of, and the indications 

 of a dial-plate with movable hand are 

 substituted. Such an instrument is 

 termed the dial weather-glass, the con- 

 struction of which is an follows : T 

 (Fig. 9) is a barometer tube ; W is a small 

 float attached to one end of a cord, the othor extremity 



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