BAROMETER. 



dicate in the two first propositions. For 

 example : 



Nnllus homo non est bipes : 

 Non omne animal est bipes : 

 JVon omne animal est homo. 



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 33 feet 

 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 ATMOSPHERE. 



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 ; ..and 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, B, 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 of columns 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 

 divided into n equal parts, and L X 



-" m _hg also divided into n equal parts, 



n 

 each divisison of L will be less than that of 



When L is divided into n equal parts, 

 each part is equal to L X -, or L X ; 



and each part of L X , thus divid- 



ed, is equal to L X 



which is 



greater -than the former by L x -y 



