DIAGONAL AND WHEEL BAROMETERS. 



Fig. 5. 



to the pressure of the atmosphere acting on the mercury in the 

 cistern. If the tube be inclined so that the mercury will fill the 

 top of it, and the stop- cock be then closed, the instrument may be 

 transported from place to place without risk of air 

 entering the tube, or other derangement. The 

 diameter of the globular cistern bears so great a 

 proportion to that of the tube, that a rise or 

 fall of the mercury in the tube, within the usual 

 limits of barometric variation, produces a 

 very inconsiderable variation in the level of the 

 mercury in the cistern, or if greater accuracy 

 be desired, the scale attached to the tube may 

 be so divided as to measure, not the actual varia- 

 tion of the level of the column in the tube, 

 but its variations relatively to that of the mer- 

 cury in the cistern. Thus, if the diameter of the 

 cistern be four times the diameter of the tube, 

 the area of its section will be sixteen times that of 

 the tube, and when the level of the mercury in 

 the tube falls through an inch, the level of the 

 mercury in the cistern rises through the sixteenth 

 of an inch, so that the real decrease of the 

 mercurial column is only fifteen sixteenths of an 

 inch. It is evident that, if it be desired, the 

 scale may be so divided that an actual fall of 

 an inch may be marked as one-sixteenth less 

 than an inch, and so of other variations. 



In barometers for domestic use this extreme 

 precision is not necessary, and is so much the 

 less so, as the most important indications of the 

 barometer are those which depend on the varia- 

 tion of the height of the column, and not on its absolute height. 



14. It has been shown, that when a barometer is carried 

 upwards in the atmosphere, the column of mercury in the tube 

 falls, because the force which sustains it is diminished by an 

 amount equal to the weight of the column which it leaves below 

 it. By comparing, therefore, the height of the column in the 

 barometer at any two stations, one of which is above the other, 

 we can ascertain directly the weight of a column of atmosphere 

 extending from the lower to the higher station. Thus, for ex- 

 ample, if the column of mercury in the barometer at the lower 

 station be 30 inches, and at the higher station 20 inches, it 

 follows that a column of air whose base is at the lower station, 

 and whose summit is at the higher station, will have a weight 

 equal to that of a column of mercury 10 inches high, and there- 



183 



