9-t A MANUAL OF TOPOGEAPHIC METHODS. 



THE CISTERN BAROMETER. 



In work having a large contour interval, 50 feet or more, the cistern 

 barometer is used to some extent, though not as much as formerly. Its use 

 is now confined to the work in the far West, where it is employed in the 

 determinations of heights of points in the valleys not easily reached by 

 vertical angles. 



The barometer is an instrument for measuring the pressure of the 

 atmosphere. At the level of the sea this pressure of about 15 pounds per 

 square inch supports a column of mercury about 30 inches in height. As 

 one rises above sea level and leaves a portion of the atmosphere behind 

 him the pressure diminishes and the column of mercury sustained by it is 

 of less height. 



The cistern barometer, in its most portable form, is made by II. J. 

 Green. It consists of a cistern into which dips the lower open end of a 

 glass tube 31 or 32 inches in length, the whole being inclosed in a brass 

 case. The cistern consists of a number of parts, including a short glass 

 cylinder, below which is fitted the inverted frustum of a hollow cone of 

 boxwood. This is succeeded by a second frustum, placed upright, from 

 the lower end of which depends a bag of buckskin. The bottom of the 

 latter is raised or lowered by means of a screw in the brass case of the 

 cistern. The cistern is closed at the top by a boxwood ring, which is fitted 

 to the top of the glass cylinder. By means of an annular piece of kid, 

 which is securely lashed to the boxwood ring and to the barometer tube, 

 the cistern and the tube are connected. From the under surface of the 

 boxwood ring depends an ivory point about a quarter of an inch in length. 

 Upon the brass casing of the tube is a graduation into inches and twentieths, 

 by which, with the aid of verniers, the scale may be read to 0.002 of an 

 inch. To this brass case is attached a thermometer, for indicating the tem- 

 perature of the instrument. For carriage the barometer is placed in a 



w len case fitted to its shape, and this in turn in a case of heavy leather 



fitted with a shoulder strap. It should always be carried in an inverted 

 position. 



To read the instrument it should be hung where it can swing freely. 

 Then, by lowering the screw at the bottom, drop the mercury in the cistern 



