A very isimple.mode of enlarging the divisions ofthe barometer is commonly ascribed to Sir Samuel More land, the same person who had invented, or perhaps only revived, the Speaking Trumpet. (Seefig. 8.) consisted in merely bending the upper part of the tube into a very oblique position. By this plan, however, the scale, which depends onthe perpen dicular altitude, cannot be augmented beyond three or four times, without incurring evident risk of' in accuracy. 'Air instrument is called the Inclined or Diagonal Barometer. The form has been some times varied by the fancy of artists, who, repeating the inclination of the tube, have occasionally given the upper part a zigzag appearance.
The most ingenious barometer, filled with mercury only, and yet admitting a scale of any extent, was in vented by Cassini and by John Bernoulli, who first garb the description of it in 1710. fig. 9.) A wide cylinder is annexed to the top of the main tube, at the bottom of which there is joined at the right angles another long and =TOW tube. The mercury, in as cending or descending within the wide cylinder must, therefore, run along this horizontal tube. If that cylinder have a diameter only four times greater than the bore of the tube, the scale of variation will be augmented sixteen times. This instrument is," from its shape, called the Square Barometer. It is not found in practice to answer so well as the theory might lead us to suppose. The mercury creeps along the horizontal tube with difficulty, and by de. sultory advances ; and these irregularities increase, as it becomes, from its motion and exposure, cover ed with dust and partial oxidation.
The simplest of all the barometers, with an enlarg. ed scale, and, at the same time, one Of the most Inge. nious, is the Conical or Pendant Barometer, invented and described in 1695, by Amontons, a French philo. sopher, who being afflicted with total deafness, in con• eequence of a fever in his infancy, had devoted.him. 'self to mechanical contrivances. (See fig. 3, Plate XXXII.) This instrument consists merely of a tube, four feet or more in length, with a bore narrower than ordinary, and tapering regularly to the top. The width at the bottom must hardly exceed three-twentieth parts of an inch, while near the top it may be contracted to about one tenth. A column of thirty-one inches of mercury being introduced, the tube is gently inverted and held perpendicular ; the cohesion of such a narrow column is sufficient to prevent it from dividing and admitting the air, unless it be shaken : but over powering the atmospheric pressure, it descends till it has contracted into altitude, by passing into a wide part of the tube. To obtain
equal divisions on the scale, it is necessary that the tube should have an uniform taper. The most ac curate construction of a barometer of this kind is, therefore, attained by forming together two tubes that have even but unequal bores, the longer and narrower one being uppermost. If the width of the upper tube were supposed to be to that of the under one as two to three, the scale would be enlarged three times, since, hy descending three inches from the top, and consequently two at the bottom, the column would suffer a contraction of one inch in height.
This species of barometer is thus recommended by its simplicity and its ample range. But the bore of the tube being indispensably narrow, the mercury moves with difficulty, and resists the impression of minute changes of external action. When the co nical shaped tube is retained, the instrument is lia ble to some inaccuracy from the Influence of the cohesion of the mercury, which varies with the diameter of the column in different parts of the tube.
Amontons likewise proposed another form of ba rometer, in which the mercurial column is subdivided among several short connected branches. (Seefig. 10.) Suppose the instrument were to have only the third part of the usual height ; the first, third, and fifth branches enlarged above and below into very short cylinders, are filled with mercury; and the second, fourth, and sixth branches, which may have their bores narrower, are occupied with some light fluid, or simply with air. If the external pressure should suffer any di rainution, the three niCileuvia columns which Fa. duce the counterpoise, will each descend and push up the last fluid of the series by their combined effects. It is evident, that, by multiplying. those branches, the barometer will have its altitude pro• portionally reduced. But this construction, though Specious in theory, is found to have no practical ad vantages. The instrument is, from its complication, very difficult to construct ; its motions are sluggish, owing to the multiplicity of tubes, and the conjunc tion of fluids, and they are subject to derangements from the variable influence of temperature. It has, therefore, been generally abandoned.