Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 1, 1802).djvu/200

176&#93; »7«] EAR as free from decay as the posts or beams of the bam." BAROMETER, an instrument of modern invention, for measur- ing and ascertaining the weight of the atmosphere, as well as the height of mountains, and likewise foretelling, with tolerable accuracy, the probable changes of the wea- ther. That such an instrument must be of extensive utilitv to every person engaged in the active pursuits of life, whether those of gardening and agriculture, or in the various departments of the do- mestic and useful arts, will be uni- versally admitted. Hence we pro- pose to bestow a considerable share of attention on this interesting subject. History and analysis : — When Galileo, in the beginning of the seventeenth century, discovered that water could not ascend in a pump, unless the sucker reached within 33 feet of its surface in the well, he justly concluded that the ascent of water in pumps, was effected by the pressure of the atmosphere, and not by the power of suction ; that a column of water 33 feet high was a counterpoise to one of air of an equal diameter and base, the height of which extended to the top of the atmosphere j and that consequently the water could not be attracted any farmer by the sucker. This important discovery induced his great pupil Torki- celli, to substitute a column of mercury for that of water j because the former fluid being about 14 times heavier than the latter, he wanted, according to that propor- tion, only about 2(ji inches of quicksilver to determine the accu- racy of his experiment. He ac- cordingly found that, after having filled a glass tube with mercury, BAR and inverted it in a bason of the same semi-metal, it descended in the tube till it became stationary at about 2g inches above the sur- face of that contained in the lower vessel. Many years, however, elapsed after this experiment, before any notice was taken of the circum- stance, that this pressure of the air considerably varied at different times, though the tube was uni- formly kept in the same situation. Indeed, these variations in the mer- curial column, were too obvious to remain long unnoticed 5 and philoso- phers began minutely to mark their degrees. As soon, therefore, as this point was properly attended to, they observed that the changes in the rise and fall of the mercury were in general very speedily suc- ceeded by variations in the wea- ther. Hence the instrument ob- tained the name of weather-glass, for which purpose it has, since that period, been generally employed. It is surprizing that the ancients were unacquainted with tire laws by which the ambient air presses on our bodies as well as on all in- animate matter; and that Otto Guericke, a German, to whom the world is indebted for the dis- covery of the air-pump, was the first who excited the attention of philosophers to this important sub- ject. Although Tokricei.li had previously ascertained, by his ex- periment made in the year 1646, that the mercury in a tune of four feet in length did not remain stati- onary at 2f}| inches, but varied ac- cording to the greater or less de- gree of density of the atmosphere ; yet it docs not appear that he ap- plied this great discovery to the purpose of predicting the future, or impending changes of the wea- ther