BAROMETRIC MEASUREMENT OF HEIGHTS. i6j 



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 bot- 

 tom of the cistern is formed of leather and fitted with an adjust- 

 ing 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 adjust- 

 ing 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. Sufiice 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 mer- 

 cury. 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 tem- 

 perature of the air must be noted at each station. As the mer- 

 cury 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 Fahren- 

 heit 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 atmos- 

 phere 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 



