BAROMETER. 



IJAKO'METKH. 



below; UMO any de*cnt of DM mercury in the cylinder it shown by 

 tan tune* M grit donant of the point < in the upper tube, becauw 

 portion of UM cylinder mut I* filled out of the tub*. When Torn 

 rath barometer bill an inch, the mercury her* will fall | of an inch 

 and the oil Y, or q of an inch ; thi. f of an inch of oil being equiva- 

 lent only to I of an inch of mercury ; and a fall of uho of an inch 

 .1 Tomcelli* barometer would be marked by a fall of ^ of an inch 

 in that of Descartes. Buygbww tried to oonctruct thin barometer 

 (DMcarUM baring died before be completed it), but found that the air 

 contained in the upper fluid alwayi escaped into the vacuum He 

 therefor* suggested the next jOan. 



5. J/tyyknu. To the siphon barometer he added a cistern at the 

 vacuum end of the tulw, equal in diameter to the cistern in which the 

 mercury oommunioate* with the air. The Utter cutern communicates 

 with a arrow tube, *ay one-tenth of tho horizontal section of the cis- 

 tern, and the barometer was completed with such a quantity of mercury 

 as would always leave *ome in both cisterns. The remainder of tin 

 lower cistern, sod a portion of the tube above it, were filled with water. 

 Now it U evident, that the water is merely to be considered as a very 

 email addition to the weight of the atmosphere. A depression of an 

 inch hi Torricelli's barometer would cause a depression of half an inch 

 in the higher cistern, and a rise of half an inch in the lower. Neglect- 

 ing the effect of the weight of the column of water, it appears that a 

 rise of half an inch in the lower cistern would be accompanied by ten 

 times as great a rise of the water in the tube, on account of the pro- 

 portion of the horizontal sections. Hence the water multiplies the 

 indications of Torricelli's barometer five times. The objections to this 

 construction are : that the portion of the tube abandoned by the sinking 

 of the water remains wet, or a part of the water is left behind, so that 

 it appears to have descended somewhat lower than it ought to do ; and 

 also that the evaporation of the water produces a similar effect. We 

 need hardly observe that, except for extreme exactness of observation, 

 no improvement upon Torricelli's barometer U here pretended to have 

 been made ; so that, if this end be not answered, the whole peculiarities 

 of the construction are useless. Dr. Hooke slightly varied this baro- 

 meter, by adding another fluid above the first, and making the tube 

 terminate in a third cistern. 



6. Hoott't wheel barometer is a well-known plaything, for as to 

 accuracy it scarcely deserves a better name. On the mercury in the 

 siphon barometer a weight is placed, which is very nearly counter- 

 poised over a pulley by another weight. The ascent of the mercury 

 rains the weight, and the Hiring which connect* the weights makes 

 the pulley revolve more or less. A hand attached to the pulley shows 

 the quantity of revolution, and the plate U divided BO as to show how 

 much revolution of the pulley corresponds to a hundredth of an inch 

 (usually) of rue or full in the barometer. In the common instruments 

 it is usual to mark fair, rhanyea/Je, Ac., at certain places ; an innocent 

 practice, because those who use the instrument .are generally aware 

 that it is not the tlale of the barometer which furnishes any probable 

 tost of the weather, but the rkamjt which is taking place for the time 

 being. For observing the mere fact of a change, and roughly whether 

 it is much or little, this instrument is sufficiently well adapted for 

 thaw who wish to " give an air of philosophy to their parlours," but 

 for mAinfl the exact quantity of the change, or the absolute height of 

 the column, it U worth very little. 



7. Sir Samuel Mordand. By inclining the tube of the barometer 

 iu indications were supposed to be rendered more sensible. The mer- 

 cury standing at ' in the upright barometer (dotted) would stand at < 

 on the same level in the Miqtu or diagonal barometer, by a well-known 

 law of hydrostatics ; and since any difference of levels cuts off a longer 

 apace from an inclined, than from a vertical line, the indications of 

 change on the former must be more marked than on the latter ; but 

 the friction of the mercury is increased, and the place of the head of 

 the column of mercury is difficult to be read by a vertical sqale, because 

 of iu inclination. 



8. Jvh* Bernoulli. The mercurial column is here made to end in a 

 nailer horizontal tube of considerable length, the pressure of the air 

 acting horizontally against the column of mercury. The vacuum is 

 made in a cistern, M in No. 5. Thin, and the smallnenH of tli.- liri- 

 lonul tube, render the indications very great, in the manner already 

 explained. There is no change in the lower level of the mercury ; but 

 other circumstance* render this construction not more worthy of 

 confidence than the preceding. An improved form of thin instrument 

 ha* lately been introduced by M. de CeUes. 



9. Amamimu. A conical tube of glass U closed at the upper end. It 

 i* partly filled with mercury, and the tube is inverted. The pressure 

 of the air from underneath prevents the total deaoent of the mercury, 

 but allow* it to descend in the cone until it forms a column of a height 

 sufficient to counterpowe the weight of the atmosphere. It is evident, 

 that in a conical tube the column, as it descends, will spread hri/.on- 

 tally and decrease in height But by the laws of hydrostatics it i the 

 vertical height of the column only on which depends the pressure per 

 square inch on the base. If the weight of the air decrease, the mer- 

 cury must fall until, by increase of its horizontal dimensions, the height 

 has been decreased a* much as is nece**ary. But a tube of the requisite 

 degree of accuracy is almost an ideal supposition. The principle iteelf 

 is the moat simple and elegant of all those which have been applied to 

 UM instrument 



10. AmoHtotu. This U a barometer in which the column of external 

 air is balanced by several different columns of mercury, an follows : 

 From a to t is a vacuum as usual ; from to p mercury ; from p to ** 

 air ; from * to p' mercury. When a ip has been filled in the usual 

 manner, the mercury t p' i* admitted at g, which is then closed. Neg- 

 lecting entirely the weight of the tir pi, and considering }t merely as 

 a medium for communicating pressure, the difference of levels of p and 

 t, and of p' and t", will each be half of the column for the time being 

 in the Torricellian barometer. Kur the pressure of the external air at 

 p' if counterbalanced by the pressure of the two columns, that of the 

 column beginning at * being communicated to that beginning at i' by 

 the intermediate air pi'. By a repetition <>f the some principle, each 

 column might be made one-third, one-fourth, Ac., of the Torricellian 

 column. But the exactness required in the several parts U impossible 

 to be attained. 



11. Mairan. This is a simple siphon barometer, so short in the 

 tube that the mercury does not descend until the density of the super- 

 incumbent air u considerably lean than that of natural air. It is used 

 under the receiver of an air-pump [AlR-PoMp] to indicate the degree of 

 exhaustion which has taken place. 



12. llvokt. This barometer was intended to be used at sea. It is 

 not a mercurial barometer, but a portion of air confined in the bulb 



i lie liquid which mounts in the tube. Any increase of weight 

 in the exterior air compresses the air in the bulb by pressing on the 

 liquid. This is not at all to be depended upon, as the effect of changes 

 of temperature makes it rather more of the thermometer than the 

 barometer. 



13. Print. This is a Torricellian barometer, with a contrivance for 

 keeping the lower level always the same. The cistern is closed at the 

 top, excepting an orifice very little larger than is necessary to admit 

 the tube. The mercury escaping through this orifice, and ita cohesion, 

 with the repulsion which it exercises towards glass, preventing any part 

 separating from the rest, it forms a bulb round the tube, which bulb, 

 when the mercury falls into the tube, instead of rising, spreads itself 

 out upon the glass, in the manner shown in the dotted section. 

 (Fig. 13'.) 



14. Gay Ltutot. This barometer is very convenient for carriage. It 

 ia a siphon barometer, differing from others of that species only in form, 

 and communicating with the external air through a note y, pierced too 

 small to allow mercury to pass through it 



15. Fort in. Thin barometer is distinguished from the rest by a 

 method of adjusting the lower level of the mercury exactly to the zero 

 point of the scale before commencing the observation. It is a Torri- 

 cellian barometer, in which the bottom of the cistern can be raised or 

 lowered by a screw. An ivory needle points downwards, the point of 

 which is on a level with the zero of the scale. The bottom of the 

 cistern is raised or lowered by the screw, until the point of the needle 

 and its image in the mercury precisely coincide. The observation is 

 then made. 



Several other forms of barometer will be described hereafter. 



In order to construct good barometers, such that two or more may 

 always stand (if possible) at the some height when in the same place, 

 or may be correct indexes of the differences of height in different 

 places, the following points must be attended to : 



1. The mercury must be chemically pure, and the interior of the 

 tube must be freed from that coating of air which adheres to all bodies 

 in their natural state, which, if allowed to enter with the mercury, 

 would in time expand and render the vacuum above the mercury 

 imperfect It was formerly thought sufficient to expel the air by 

 simply boiling the mercury in the tube previously to inverting it, and 

 allowing the vacuum to form. De Luc found that all his barometers 

 pive different heights until he boiled the mercury, after which the 

 jreater port of the difference disappeared ; but he was not aware that, 

 liowi-ver carefully a barometer is prepared on this plan, it is liable to 

 slow but certain deterioration by the infiltration of the air, as will be 

 explained at greater length presently. The old plan was to l>il a 

 portion of the mercury first in the tube, and then to odd the rest in a 

 lot state, after which the boiling was repeated. One of the best tests 

 of a good vacuum ia when, by gently inclining the tube backwards and 

 forwards, the mercury striken the glass at the closed end of the tube with 

 a hard, well-defined, and instantaneous tap. The vacuum can never be 

 quite perfect ; for, generally speaking, a small quantity of air will 

 remain ; and besides this, mercury itself will rise in vapour into the 

 iresumed vacuum [MEBCCKT], though not to so great a degree as to 

 cause any perceptible pressure [ATMOSPHERE], and not more than it 

 would rise in the air. The presence of mercurial vapour is a fact 

 Ktablished as well by chemical tests a* by the effect upon human 

 lealth of breathing an atmosphere to which much of the metal has 

 >een exposed. If there bo moisture in the supposed vacuum, the 

 mercury will sink on applying the hand or any other warm substance. 



Many barometers likewise, when the mercury is shaken in the dark, 

 inhibit a luminous appearance in ill.- v.irmun over the mercury, and 

 Lnown as the barometrical ';;/'' ; tint light l>eing sometimes apparently 

 iniform throughout the vacuum, sometimes appearing almost entirely 

 >n the surface of the mercury. This appearance was first noticed by 

 'icard, and afterwards by Cassini, Lahira, Ac. Though it appears to 

 w an electrical phenomenon, we are not aware that any satisfactory 

 xpLination has been given of it, and particularly of the reason why it 



