There are two forms of barometers—namely, the mercurial barometer and the aneroid. Of mercurial barometers there are two forms, the "cistern" and the "siphon." The cistern form is the one most generally used for scientific observations, and is the best for measuring heights. One of the most approved forms of cistern barometers known as "Fortin's barometer"—consists of a glass tube closed at one end and filled with mercury, the lower portion of which dips into another tube of larger diameter which contains a reservoir of mercury forming the "cistern." The bottom of the cistern is formed of leather and fitted with an adjusting screw below, for the purpose of adjusting the level of the mercury in the cistern to an ivory index point above, which marks the zero of the graduated scale. By means of this adjusting screw the mercury may also be raised so as to completely fill the cistern and tube, and thus adapt the instrument for traveling.
We need not discuss here the manufacture of barometers and the filling of the tube with mercury, an operation which must be done carefully so as to exclude air from the tube. Suffice it to say that the best method is to fill the tube gradually, and boil the mercury as we proceed by means of a spirit lamp, in order to drive out all bubbles of air which may be contained in the mercury. The tube may be filled without boiling, but the resulting instrument will not be so accurate as one in which the mercury has been boiled.
To determine the difference of elevation between two places with a mercurial barometer, several points must be attended to. In the first place, the temperature of the barometer and the temperature of the air must be noted at each station. As the mercury in a barometer is affected by heat—in the same way that a thermometer is—the temperature at which the barometer is read must be observed. For this purpose a thermometer is usually attached to the barometer. The temperature should be read as accurately as possible, for an error of one degree Fahrenheit would make a difference of about three feet in the resulting altitude. The reading of the attached thermometer should be first noted, and then the height of the barometer. To do this, first bring the surface of the mercury in the cistern accurately to the index point by means of the adjusting screw. Then tap the tube gently near the top of the column in order to get rid of the adhesion between the mercury and the glass. The height of the mercury may then be read by means of the attached scale and vernier. Sometimes the amount of aqueous vapor in the atmosphere is ascertained by another instrument. The above data being known for two stations, we substitute the values found in one of the barometric formulae, and thus obtain the height, or
