BAROMETER, an instrument for mea suring the weight or pressure of the at mosphere ; and by that means the varia tions in the state of the air, foretelling the changes in the weather, and measuring heights or depths, &c. About the begin ning of the 17th century, when the doc trine of a plenum was in vogue, it was a common opinion among philosophers, that the ascent of water in pumps was owing to what they called nature's abhorrence of a vacuum ; and that thus fluids might be raised by suction to any height what ever. But an accident having discovered that water could not be raised in a pump, unless the sucker reached to within 33feet of the water in the well, it was conjec tured by Gallileo, who flourished about that time, that there might be some other cause of the ascent of water in pumps, or at least that this abhorrence was limit ed to the finite height of 33 feet. Being unable to satisfy himself on this head, he recommended the consideration of the difficulty to Torricelli, who had been his disciple. After some time Torricelli fell upon the suspicion, that the pressure of the atmosphere was the cause of the ascent of water in pumps ; that a column of water 33 feet high was a just counter poise to a column of air of the same base, and which extended up to the top of the atmosphere ; and that this was the true reason why the water did not follow the sucker any farther. And this suspicion was soon after confirmed by various ex periments. See krattosruzus.
It was some time, however, before it was known, that the pressure of the air was various at different times in the same place. This could not, however, remain long unknown, as the frequent measuring of the column of mercury must soon shew its variations in altitude ; expe rience and observation would presently shew that those variations in the mercu rial column were always succeeded by certain changes in the weather, as to rain, wind, frosts, &c.: hence this instrument soon came into use as the means of fore telling the changes of the weather, and on this account it obtained the name of the weather-glass, as it did that of barome ter, from its being the measurer of the Weight or pressure of the air. We may now proceed to take a view of its various forms and uses.
The common mercurial barometer, (plate Miscel. fig. 9.) or weather-glass, is a cylindrical glass tube, whose diameter is generally about one-third or one-fourth of an inch in diameter, and length 34 inches, filled with prepared mercury ; one end of the tube, A, is hermetically sealed, and the open end,I3, inserted into a basin of mercury. The tube and ba sin are fixed to a frame of wood, and suspended in a vertical situation. The height of the mercury in the tube above the surface of the mercury in the basin is called the standard altitude, and the dif ference between the greatest and least altitudes is called the limit or scale of va riation.
The mercury in the barometer tube will subside, till the column be equivalent to the weight of the external air upon the surface of the mercury in the basin, and it is therefore a criterion to measure that weight, and chiefly directed to that purpose. In this kingdom the standard altitude fluctuates between 28 and 31 inches ; and from hence it is justly infer red, that the greatest, least, and interme diate weights of the atmosphere, upon a given base, are respectively equal to the weights oecolumns of mercury upon the same base, whose vertical altitudes are 28, 31 inches, and some altitude contain ed between them.
The standard altitude ought to be the same, whatever be the diameter of the barometer tube : but when this diameter is very small, the attraction of cohesion between the mercury and glass prevents a variation of altitude, which ought to be, and in larger tubes is, sensible from small differences in the weight of the at mosphere.
Writers on this subject have given the following lemma :—If a given line, L, be into nto n equal parts, and L X be also divided into n equal parts, each divisison of L will be less than that of m-Fm 7/1 L X by L X — • n' When L is divided into n equal parts, 77 each part is equal to L X 1 —, or L X —; and each part of L , thus divid ed, is equal to L X which -is n' greater -than the former by L x If each inch of the scale of variation, A D. (fig. 10.) of a barometer tube be di vided into ten equal parts, marked with 1, 2, 3, &c. increasing upwards, and a vernier or nonius, L M, whose length is :laths of an inch, be divided into ten equal parts, marked with 1, 2, 3, &c. in creasing downwards, and so placed as to slide along the graduated scale of the ba rometer, the altitude of the mercury in the tube above the surface of that in the basin may be found in inches and hun dredth parts of an inch in this process. If the surface of the mercury in the tube do not coincide with a division in the scale of variation, place the index of the ver nier, M, even with the surface, and ob serving where a division of the vernier coincides with one in the scale, the figure in the vernier will shew what hundredth parts of an inch are to be added to the tenths immediately below the index. Let, for instance, the surface of the mercury be between 7 and 8 tenths above 3:.) inch es, and the index of the vernier being placed even with it, and the figure .5 upon the vernier being observed to coincide with a division upon the scale, the alti tude of the barometer will be 30 inches and five hundredths of an inch : for each division of the vernier being greater than that of the scale by one hundredth of an inch (lemma), and there being five divi sions, the whole must be five hundredths of an inch above the number 7 in the scale, and the height of the mercury is there fore 30.75 inches.