BAR 385 BAR

burning amt of giving light, but becomes incapable of affectingthe most sensible thermometer, in which case it is said to be in alatent state: and, 2d. 'J hat it the atmosphere be affected by anyunusual degree of heat, it thence becomes incapable of supportingso long a column of mercury as before; for which reason it is thatthe barometer sinks. From these axioms it would follow, that asvapours are formed by an union of lire and water, whether by at-traction or a solution of the water in the fire, tlie vapour cannotbe condensed till tiiis union, attraction, or solution, is at an end.Hence the beginning of the condensation of the vapour, or thefirst signs of approaching rain, must be the separation of the lirewhich is latent in the vapour. In the beginning, this maybeeon-'-slow and partial, or it may be sudden and violent; in thefirst case the ram will come on slowly, and after a considerabletime but in the other, it will come very quickly, and in greatquantitv. Yet Dr. Black has proved, that when iire quits its la-tent state, how ever long it may have lain dormant and insensible,it always reassumes its proper qualities, and affects the thermome-ter just the same as if it had never been absorbed. The consequenceof this is, that in proportion as the latent heat is discharged fromthe vapour, those parts of the atmosphere into which it is dischargedmust be sensibly affected by it; and, in pioportion to the heat,communicated to those parts, they become specifically lighter, andthe mercury sinks of course.

In the construction and use of the barometer, the following ob-servations are to be attended to: 1. The length of the tubeshould be 33 or 34 inches; the diameter one-third or one-fouithof an inch to prevent the effects of cohesion. 2.‘The diameterof the cistern must be large in proportion to that of the tube, forthe divisions, denoting the altitude of the mercury, are markedfrom a fixed point; and therefore the surface of the mercury inthe cistern should remain as nearly as possible stationary at thatpoint. 3. The mercury should be purified of air and other ad-mixture by boiling it in a glazed earthen vessel, and pouredthrough a small glass lube into the lube of (he barometer, washedwith rectified spirit, and cleaned with a pi-ton of shammy leather,if both ends were not hermetically sealed when it was made, andheated and rendered electrical by rubbing. 4. Unless the tem-perature of the air remain the same, the dimensions of a givenquantity of mercury will be variable, and the altitude of the mer-cury is an uncertain measure of the weight of the atmosphere,because it is dilated by heat, and contracted by cold, when per-haps the weight of the atmosphere is unaltered. If very great ex-actness be therefore required, the difference of temperature, at thedifferent times of observation, and the depression or elevation ofthe mercury produced by it, must be ascertained, before! the heightof the column, raised by the weight of the atmosphere, can be dis-covered.

The barometer applied to the measuring of altitudes.—-The se-condary character of the barometer, namely, as an instrument formeasuring accessible heights or depths, was first proposed by Pas-cal, and Descartes and succeeding philosophers have been at greatpains to ascertain the proportion between the fall of (he barometer,and the height to which it is carried ; as Ilalle, Mariotte, Shuck-burgh, Roy, and more especially by De Luc, who has given acritical and historical detail of most of the attempts that have atdifferent times been made for applying the motion of the mercuryin the barometer to the measurement of accessible heights. Andfor this purpose serves the portable barometer already describedwhich should be made with all the accuracy possible. V ariousrules have been given by the writers on this subject, for comput-ing the height ascended from the given fall of the mercury in thetube of the barometer, the most accurate of which was that of Dr.Halley, (ill it was rendered much more accurate by the indefati-gable'researches of De Luc, by introducing into it the correctionsof the columns of mercury and air, on account of heat. This rule

is as follows: viz. 10000 x log. of ^ is the altitude in fathoms,

in the mean temperature of 31°; and for every degree of thethermometer above that, the result must be increased by so manytimes its 435th part, rnd diminished when below it: in whichtheorem M denotes the length of the column of mercury in thebarometer tube at the bottom, and m that at the top of the hill, orother eminence; which lengths may be expressed in any one andVOL. no. 25.

the same sort ot measures, whether feet, or inches, or tenths, Ac. andthe result is always fathoms of six English feet each, i ’or furtherinformation concerning the above theorem, see Atmosphere.

The requisite precepts are the following: 1. Observe theheight of the barometer at the bottom of any height or depth,proposed to be measured ; together with the temperature ot themercury, by means of the thermometer attached to the barometer,and also the temperature of the air in the shade by another ther-mometer which is detached from the barometer. 2. Let the samething be done also at the top of the said height or depth, and asnear to the saint time with the former as maybe. And let thosealtitudes of mercury lie reduced to the same temperature, if it bethought necessary, by correcting either the one or the other, viz.augmenting the height of the mercury in the colder temperature,or diminishing that in the warmer, by its 96()Otli part for every de-gree of difference between the two : and the altitudes of mercuryso corrected, are what are denoted by M and m, in the algebraicformula above. 3. Take out the common logarithms of the twoheights of mercury, so corrected, and subtract the less from tiregreater, culling off from the right-hand side of the remainderthree places for decimals; so shall those on the left be fathoms in, whole numbers, the tables of logarithms being understood to heJj such as have seven places of decimals. 4.- Correct the numberlast found, for the difference of the temperature of the air, as fol-low;,; viz. take halt the sum of the two temperatures of the air,j shewn by the detached thermometers, for the mean one; and for1 every degree which this differs from the standard temperature of31°, take so many times the 435th part of the fathoms above found,and add them iftiie mean temperature be more than 31°, but sub-tract them if it be below 31°; so shall the sum or difference bethe true altitude in fathoms; or, being multiplied by 0, it will givethe true altitude in English feet.

Example. Let the slate of the barometers and thermometers bi-as follows, to find the altitude : viz.

Thermometers.

detached.

57

42

mean 4<ft

attached.

57

43

dif. 14

Barometers.

29,68 lower.25,28 upper.

As 9600 : 14 : : 29,68 : ,04cor. ,04 logs.

mean 494 M = 29,64 - 471S7B2stand. 3l” m = 25,28 - 4007771

dif. 18 j As 435: 18*:: 691,011 : 29,38*

the altitude (

29,388720,399 fath.

sought is \ or 4322,394 feet.

The mean height of the barometer in London , upon an averageof two observations in every day of the year, kept at the house ofthe Royal Society , for many years past, is 29,88; the mediumtemperature, or height of the thermometer, according to the same,being 5S°. But the medium height at the surface of the sea, ac-cording to Sir George Slutckburgh, (Phil. Trans. 1777, p. 586,)is 30,04 inches, the heat of the barometer being 55°, and oi theair 62°

BARON , in British customs, a degree of nobililv next to aviscount, but the highest in point of antiquity. In the House ofBeers dukes, marquisses, earls, viscounts, and barons, are all equalmembers, whence they are collectively < ailed a House of Beers ,or equals; but, in other respects, they claim and eniov certainhonours and distinctions, peculiar to their respective ranks andthe date of their creations. 'I he Latin baro was used in the pureage of that language for nr, a stout or valiant man; whence,according to Menage, those placed next the King in battles werecalled barons, as being (he bravest men in the army ; and asprinces frequently rewarded the bravery and fidelity ot tho-eabout them with Ices, the word came to be used lor" any noble

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