64 FIELD GEOLOGY. 



reduce our results to " sea-level," even from the com- 

 mencement. 



LEVELLING. 



By Aneroid. The possibility of taking heights by 

 this instrument is, of course, based on the weight, gener- 

 ally termed pressure, of the atmosphere, which varies as 

 the height above the surface of the earth. B ut this pressure 

 is subject to almost constant change, at the same place, 

 and at the same height, owing to varying atmospheric 

 conditions of wind, moisture, and so on. Therefore 

 some means must be adopted, when levelling by the 

 aneroid, to compensate for any change that may occur 

 during the time of making the observations, and by 

 which cause, in addition to the difference of height, the 

 needle would be deflected. 



The circumference of the aneroid barometer is grad- 

 uated to inches and hundredths, and on this scale the 

 needle indicates the varying measure of a column of 

 mercury, that would be sustained or counter-balanced 

 by the changing weight of the atmosphere. Generally 

 it has also a movable circumference, divided into parts, 

 about 950 of which correspond with 1 inch on the 

 pressure scale ; because a column of air, about 950 feet 

 high, is equal in weight to a similar column of mercury, 

 which measures 1 inch in height. Therefore, as the 

 mercury in an ordinary barometer would "fall 1 inch for 

 every 950 feet higher, or rise for 950 feet lower, the 

 needle of an aneroid gives equivalent indications on this 

 outer circumference, which thus saves the labour of cal- 

 culation. 



At starting, the zero point of the circumference must 

 be brought round to a position exactly opposite the point 



