PNEUMATICS. 



;ht of a column of mercury, whose 

 base is a square inch and whose height 

 isFE. 



It might appear, that in this experi- 

 ment the weight of the column of mer- 

 cury F E suspended in the tube must be 

 equal to the total pressure on the surface 

 of the mercury in the cistern, and that, 

 therefore, (supposing, as before, the base 

 of the column in the tube to be equal to 

 a square inch,) this pressure being dis- 

 tributed over as many square inches as 

 are in the surface of the mercury in the 

 cistern, the proportion of pressure by 

 which the ascent of each square inch 

 must be resisted, is as many times less 

 than the weight of the column F E, as 

 the surface of the mercury in the cistern 

 Is greater than the base of the column. 

 This, however, is not the case ; for it is 

 the peculiarity of fluids, not merely to 

 transmit pressure equally in every direc- 

 tion, but to transmit whatever pressure 

 is exerted on any one part of its surface, 

 imdiminished, to every part equal in 

 magnitude with the first. 



( 1 5 .) If a scale be adapted to the tube 

 A B, in the apparatus which we have 

 .just described, suited to indicate the 

 height of the column of mercury F E, 

 it will become a barometer*, which is an 

 instrument constructed to determine 

 the weight of the atmosphere. It should 

 be observed, that the height of the mer- 

 cury in the barometric tube will be the 

 same, whatever be the bore of the tube. 

 Thus it is the height of the column, and 

 not its absolute weight, which measures 

 the weight of the atmosphere. 



That it is the weight of the atmos- 



Shere which, pressing on the surface of 

 le mercury in the cistern, sustains the 

 column of mercury in the tube, will be 

 made manifest by breaking the upper 

 end B of the tube, and admitting the air 

 to press on the mercury E. The conse- 

 quence will be, that the mercury in the 

 tube will fall to the level F of the mer- 

 cury in the cistern. 



There is another very satisfactory 

 proof that the weight of the atmosphere 

 is the cause which sustains the mercury 

 in the tube. If a tube of more than 

 thirty-four feet long be immersed in a 

 cistern of water, and the air be with- 

 drawn from it, by means which shall be 

 hereafter explained, the water will rise 

 according as the air is expelled ; but the 

 ascent of the water will be limited to 



* From two Greek words, fixpog (baros), weight, 

 and ftsrpn (mctron), meas'trc. 



about thirty-two perpendicular feet : at 

 the same time it will be found that the 

 column of mercury suspended in the 

 barometric tube will be about twenty- 

 eight perpendicular inches. If, then, 

 the weight of the atmosphere be the 

 cause which sustains both the water and 

 the mercury, we may expect to find that 

 a column of water thirty-two feet high, 

 and a column of mercury twenty-eight 

 inches high, ought to have the same 

 weight when they have the same base. 

 To determine whether this be the case, 

 let equal measures of mercury and wa- 

 ter be accurately weighed, and it will be 

 found that the mercury is about thirteen 

 and a half times heavier than the water. 

 Hence we perceive, that a column of 

 water, whose base is a square inch and 

 whose height is thirteen inches and a 

 half, will have the same weight as a 

 column of mercury whose base is a 

 square inch and whose height is one 

 inch. Hence it appears that columns of 

 water and mercury, with equal bases, 

 will have equal weights, if the column 

 of water be thirteen and a half times 

 the height cf the mercury. In the pre- 

 sent instance, the height of the water is 

 32 feet, or 384 inches ; and that of the 

 mercury is 28 inches. If 384 be divided 

 by 13i," the quotient will be nearly 28 

 inches. 



If similar experiments be tried upon 

 other fluids of different specific gravi- 

 ties, it will be found that they will be 

 sustained at different altitudes in their 

 respective tubes ; but that if the weights 

 of the several columns be determined as 

 above, they will be found to be equal. 



When the barometric column is thirty 

 inches, the atmosphere presses with 

 about 1 5 Ibs. av. on the square inch ; 

 and, therefore, in general, we may esti- 

 mate the pressure nearly by allowing 

 1 Ib. for every two inches in the column. 



(16.) In the construction of a baro- 

 meter which will give an exact measure 

 of the atmospheric pressure, there are 

 many circumstances to be attended to, 

 the details of which would be unsuitable 

 to the present treatise. It may not, 

 however, be uninteresting, and certainly 

 not uninstructive, in a general way, to 

 state some of the most important pre- 

 cautions to be taken in the construction 

 of this instrument. 



In order that the weight of the column 

 suspended in the tube should be exactly 

 equal to that of a column of the atmos- 

 phere, of an equal base, it is evidently 

 necessaiy that the space in the tube 



