428 



BAROMETER. 



in an experiment of Torricelli, an Italian, who flour- 

 ished about the middle of Uie seventeenth century ; 

 and we conceive that a short account of this experi- 

 ment will be the best means of teachklg the principle, 

 while, at the same time, it unfolds the early history 

 of the barometer. Torricelli took a glass tube, about 

 three feet in length, l>eing open :ii one end only, 

 and, having filled it with mercury, he placed the 

 open end in a cup containing the same fluid metal, 

 taking care the while that none of the mercury 

 flowed out of the tube, or that any air was admitted. 

 When the tube was placed in a perpendicular direc- 

 tion, with the open end in the cup, he found that a 

 portion of the mercury descended into the cup, the 

 height of the column of the metal in the tube being 

 only about thirty inches. It was inferred, by the 

 experimenter, tliat the atmosphere, by reason of its 

 weight, pressed on the surface of the mercury in the 

 cup, and forced it up the tube to the height of thirty 

 inches ; because a column of air from the cup to the 

 top of the atmosphere was only equal to the pressure 

 arising from the weight of a column of mercury of 

 the same base, and thirty inches high. The cele- 

 brated Pascal repeated and varied this experiment, 

 and confirmed the truth of the conclusion which Tor- 

 ricelli hail drawn. These experiments were made in 

 1645, and six years afterwards, from observation, it 

 was found, by Perrier, Uiat the height of the mercury 

 in the Torricellian tube varied with the weather; 

 and the same tube was proposed to be employed for 

 the measurement of the height of mountains by 

 Boyle in 1665. The common barometer is a modifi- 

 cation of the Torricellian tube. It consists of a glass 

 tube thirty-three inches in length, the diameter of 

 the bore being about one-third of an inch. This 

 tube is hermetically sealed at the top, and the bottom 

 is curved up, and terminates in a glass bulb, whose 

 top opens into the atmosphere. The tube is filled 

 with purified mercury, and there is affixed to it a 

 scale which marks the height of the mercurial 

 column. In this country the changes of the weather 

 seldom produce a variation in the height of the mer- 

 cury which passes the limits of twenty-eight and 

 thirty -one inches ; and this will, therefore, be a suf- 

 ficient length for the scale of the instrument when it 

 is only to be used as a weather-glass, and, indeed, in 

 the scales of the barometers of the common construc- 

 tion, these weather-points are marked in the follow- 

 ing manner: At 28 inches, stormy weather; 285, 

 much rain or snow ; 29, rain or snow ; 29^, change- 

 able ; 30, fair or frost ; 30^, settled fair or frost ; 31, 

 very dry weather or hard frost. But to use the ba- 

 rometer as a weather-glass, several particulars must 

 be attended to, which have been given by different 

 authors in the fonn of rules, as those of HaJley, 

 Walker, Patrick, c. The rules of the last-mentioned 

 individual are, without doubt, the best, and we there- 

 fore present them to the reader. 1st. The rising of 

 the mercury presages, in general, fair weather, and 

 its felling the contrary, as rain, snow, high winds, 

 and storms. 2d. In very hot weather, the falling of 

 the mercury indicates thunder. 3d. In winter, the 

 rising presages frost, and in frosty weather, if the 

 mercury falls three or four divisions (tenths of an 

 inch), there will certainly follow a thaw; but in a 

 continual frost, if the mercury rises, there will be 

 snow. 4th. When foul weather happens soon after 

 the falling of the mercury, expect but little of it ; 

 and, on the other hand, little fair weather may be 

 expected when it becomes quickly fair after the ris- 

 ing of the mercury. 5th. In foul weather, when the 

 mercury rises much and high, and so continues for 

 two or three days before the foul weather has gone 

 away, then a continuance of fair weather may be ex- 

 pected to follow. 6th. In fair weather, when the 



mercury falls much and low, and thus continues for 

 two or three days before the rain comes, then a great 

 deal of wet and high wind* may be expected to fol- 

 low. 7th. The unsettled motion, or frequc'it rising 

 and falling of the mercury, denotes changeable wea- 

 ther, sih. The chords on the plate (scale) are not. 

 M> strictly to be observed as the rising and falling of 

 the mercury ; for, if it stands at much rain and then 

 rist s to changeable, it presages fair weather, though 

 not to continue so long as though the mercury had 

 risen higher; and so, on the contrary, if the mercury 

 stand a I fair and then fall to changeable, it pre 

 foul weather, though not so much as if it had sunk 

 lower. In plate IX., fig. 11, the common barom.at T 

 or weather-glass is represented. The ordinary ob- 

 jection to this form of the instrument is, tlint the 

 divisions on the scale are too small to enable one to 

 determine correctly the amount of the variation in 

 the height of the mercury, and various means have 

 been adopted to remedy this defect. One of th<- 

 simplest of these contrivances is, to bend the upper 

 part of the tube so that the part within the range of 

 variation should lie in a slanting direction, ami the 

 mercury will thus rise or fall through two inches of the 

 tube, while the difference of its levels is not more 

 tlian one inch. This arrangement gave an awkward 

 form to the instrument, and the wheel-barometer 

 was introduced, which presented more symmetry in 

 form, and possessed the same advantage as the bent 

 barometer in enlarging the divisions of the scale. A 

 section of this instrument is seen in plate IX., fig. 5. 

 Here the tube lias a large ball at top, and is bent up 

 at the lower or open end, where an iron or glass 

 ball floats on the surface of the mercury, and to a 

 small balance weight, by a thread, which passes over 

 a pulley, on the axis of which there is fixed an index- 

 hand, which traverses the circular index-plate. The 

 rising or falling of the quicksilver in the tube causes 

 a similar rise or fall of the float, which, by the action 

 of the cord, turns the pulley, and thus the index 

 hand attached to its axis will also move, and indicate 

 the change in the altitude of the mercurial column. 

 It is to be remarked, that the friction of the addi- 

 tional apparatus connected with the pulley detracts 

 from the sensibility of the instrument, and it is, 

 therefore, unfit for purposes where great nicety is 

 required. For very delicate operations, such as the 

 measurement of altitudes, the scale of the barometer 

 having a straight tube, is furnished with a nonius or 

 vernier, which greatly increases the minuteness and 

 accuracy of the scale. But several other additions 

 are necessary to be made to the barometer which is 

 intended for the measurement of heights, the instru- 

 ment being then called the portable barometer. Of 

 portable barometers there are various kinds, but we 

 shall confine our attention to that of Troughton, 

 which experience has proved to be the best. In 

 plate IX., fig. 6, the external form of this instrument 

 is shown, and in this respect the barometer of 

 Troughton is nearly the same with that of Ramsden ; 

 but the great and distinguishing quality of Trough- 

 ton's construction, is the method he has employed to 

 raise the mercury in the cistern to zero in the tube. 

 For this purpose a glass cylinder, about two and a 

 half inches in diameter and length, contains the 

 mercury. An external covering of hollow brass, 

 terminating in a female screw above and another 

 below the glass, which are furnished with male 

 screws, the ends of which being leathered and press- 

 ed against the ends of the glass, prevent the escape 

 of the mercury. Near the upper end of the brass 

 cover are two slits, made horizontally, one before and 

 the other behind, exactly similar, and opposite to 

 each other. At the bottom is a screw (seen in the 

 section, fig. 7), which, acting upon the leather bag 



