25 



the joints and levers are perfectly free, the hand will jump away and 

 then return quickly with a vibratory movement to its original posi- 

 tion. If the instrument is defective, the hand in some cases will 

 not respond to the slight knocks, or will do so without exhibiting any 

 vibratory movement, or upon being disturbed it may move a little, 

 but will not return to its original position. 



III. Miscellaneous Barometers 



59. Many curious and interesting forms of barometers have been 

 devised for the purpose of showing the changes of air pressure in a 

 much more magnified manner than is possible with ordinary barome- 

 ters, especially of the mercurial pattern. A few of these will be 

 briefly mentioned. Many others equally curious and meritorious are 

 necessarily omitted. 



60. 'Water harometer. — This may be constructed in practically the 

 same manner as the mercurial barometer, except that water is used 

 instead of mercury. Now, as mercury is 13.6 times as heavy as water, 

 it will result that the water column will be 13.6 times higher than the 

 mercurial column, or about 34 feet high; also a change of 1 inch in 

 pressure by the mercurial barometer will appear as a change of 13.6 

 inches in the water barometer. 



The great defect of the water barometer, aside from its incon- 

 venient proportions, is the shortening of the column, due to the 

 pressure of water vapor in the vacuum. This shortening amounts to 

 about 10 inches at a temperature of 70° F., and if the temperature 

 were to rise 10° the pressure would seem to fall nearly 4 inches, 

 when really no change of pressure had occurred. 



61. Glycerin harometer. — Glycerin, sulphuric acid, or nonvolatile 

 oils may also be used in place of mercury, each affording a magnified 

 indication of pressure changes, as in the case of the water barometer, 

 and with the advantage that the errors due to vapor pressures are 

 much less, or are quite inappreciable. 



62. Diagonal 'barometers. — It is easily seen that if the top por- 

 tion of a barometer tube be bent off at a small inclination upward 

 from a horizontal position, the movement of the mercury along this 

 diagonal portion will exhibit, in a magnified manner, the ordinary 

 fluctuations of atmospheric pressure. The same result is secured by 

 forming the top portion of a barometer tube into a helical coil having 

 suitable graduations. 



63. Dial harometers. — Probably the most practical expedient for 

 magnifying the indications of an ordinary siphon mercurial barome- 

 ter is the form known as the " dial barometer," shown in Figure 18. 



The action of the instrument will be readily understood from the 

 diagram without further explanation. 



64. Synipiesometer. — This is another form of sensitive barometer, 

 in which the pressure of the air, acting through a short column of 

 liquid of low specific gravity, is made to compress a portion of air 

 confined within the instrument. In this case it is necessary to make 

 allowance for the expansion and contraction of the confined air with 

 changes of temperature. This is conveniently accomplished by pro- 

 viding the instrument with two scales, one of which, containing the 

 readings of pressure, is set to a certain line on the scale according 



