THE CISTERN BAROMETER. 95 



until its top just touches the ivory point above mentioned. This can be 

 best effected by making- the ivory point and its reflection from the surface 

 of the mercury barely touch one another. Then move the vernier until its 

 bottom is just tangent to the convex top of the mercury in the tube. 



The vernier is read like other verniers and requires no special expla- 

 nation. Besides reading the height of the column of mercury in the 

 barometer, it is necessary to read its temperature by means of the attached 

 thermometer, and also the temperature of the air by means of a thermom- 

 eter hung in the shade. 



The barometer is used differentially — that is, the difference in height 

 between two points is determined by the difference in the indications of two 

 barometers, one at each point. In order to obtain the height above sea level 

 of one of these points, that of the other must be known. The latter is called 

 the base station, and its altitude should be determined either by leveling or 

 by a long series of barometric observations referred to some other point 

 whose altitude has been established. The proper selection of a base station 

 or a system of base stations for reference of work to be done in a certain 

 locality is a matter involving considerable judgment and a knowledge of 

 the peculiar errors to which the barometer is liable, as well as a knowledge 

 of the topography of the country and its probable influence upon the 

 fluctuations of barometric pressure. The base station should be near the 

 middle of the area. If but one base station is employed, it should be near 

 the middle altitude of the region. If two be used, one should be at the 

 altitude of the low or valley country and the other should in altitude be 

 near the high summits. In the Hayden survey of Colorado three base 

 stations were employed at once — one at Denver, at an altitude of 5,300 

 feet; one at Fairplay, 10,000 feet, and one near the summit of Mount 

 Lincoln, 14,200 feet. To these base stations were referred severally those 

 observations taken at points most nearly approaching them in height. 



Comparisons should be made between the readings of the base barome- 

 ter and the readings of those to be used in the field. These comparisons 

 should be made with the barometers hung side by side and should be made 

 in full — i. e., by lowering the mercury from the tubes, its level in the cistern 

 to the ivory point, and resetting the verniers at each reading — and the 



