MECHANICS, ol 
neutralizing in some measure the external pressure, and causing the mer- 
cury to stand at too low a point; this undue depression will be increased, 
also, whenever expansion of the included air is produced by an increase of 
temperature. The empty space above the mercurial column of every 
barometer is called the Torricellian vacuum. The simplest barometers 
have only a straight tube, dipping directly into a separate vessel of 
mercury. 
Since the barometer has been applied to the measurement of heights, the 
older construction for this purpose has been changed, and the syphon 
barometer (fig. 33) employed. This also consists of the tube, b, bent into 
a syphon shape at a, and closed at both ends. The short limb has atc a 
capillary opening which admits the entrance of air, but not the exit of mer- 
cury, so that the tube may be inverted without the contents escaping. To 
prevent the entrance of air into the larger limb during this inversion, Bunten 
has invented the construction represented in fig. 35. Here the mercury 
on inversion enters the space, d, so that the point of the downward pro- 
jecting tube is, during inversion, constantly closed air-tight by the superin- 
cumbent mercury. It will readily be understood that in the figure only the 
lower part of the barometer is represented. In the syphon barometer, the 
quicksilver surface exposed to the pressure of the atmosphere has no fixed 
position, and the zero of the scale must therefore be brought to the place of 
the inferior surface. 
In the barometer of Gay Lussac ( fig. 34), the long limb, 8, is bent in 
such a manner, that its upper part and the short limb, a, lie in the same 
straight line; the stations of the two surfaces can therefore be read off on 
the same scale, and then the zero is in the centre, so that the reading is of 
how much one scale is above, and how much the other is below 0; the 
sum is then the proper height of the barometer. This double obser- 
vation is necessary on account of the influence of temperature upon the 
mereury. 
The barometer of Fortin (figs. 36-88) is a cistern barometer, and has the 
advantage over others, that the mercury in the cistern. a, has an invariable 
level. The bottom of the cistern is formed by a leather pouch, h (fig. 37), 
against which a screw, k, presses, by which the surface of the mercury may 
be elevated or depressed. If then g be screwed fast to 2, the surface of the 
mercury in the cistern must correspond exactly with the zero of the scale, 
_ which is at the extremity of a fine point. When the image of this point in 
the surface of the mercury is made to coincide with the point itself, the 
adjustment is made. The barometer is surrounded by a metallic tube, in 
whose upper part there are two opposite slits for observing the top of the 
mercury. ‘The scale is attached to the metal tube. To assist the eye in 
determining the exact height of the mercury, there is a slider on the metal 
tube, which has also two slits corresponding to those of the tube, only a little 
broader. The slider is so adjusted that the upper edges of its slits coincide 
exactly with the top of the mercurial column. 
Experiments and calculations instituted for the purpose, assign to a 
station of the barometer of 28.6 inches, an atr ospheric pressure of about 14.6 
ICONOGRAPHIC ENCYCLOPZDIA.—VOL. I. 15 225 
