USE OF THE ANE ROM BAROMETER IN 
GEOLOGICAL SURVEYING. 
AN attempt to unravel the geologic structure of any area 
should always be preceded by the establishment of a datum plane, 
and the reference of all points in the area to it, or in other words 
the geologic, should always be preceded by a topographic survey. 
This I take to be axiomatic, yet in practice such a preliminary 
survey is seldom made, except by the larger organizations, which 
are under state or national patronage, and not always even by 
these, because of the great expense involved, and because very 
few geologists are sufficiently familiar with the instruments and 
methods of the engineer to undertake such a work, and are not 
in a position to employ an engineer to do it for them. In reality 
however, results sufficiently accurate for practical purposes may 
be reached with an expenditure of time and money almost nomi- 
nal, and by a method which should be mastered by every field 
geologist, whether he expects to make such surveys or not, on 
account of the use he can make of it in other directions. It is 
the object of this paper to set forth a method which I have used 
successfully for this purpose. 
The aneroid barometer is essentially a metallic box having 
one or both ends formed by a thin elastic plate, which is usually 
corrugated to increase its elasticity, and the ends are often con- 
nected by a delicate spiral spring with the same object in view. 
If the air is partially exhausted from such a box, the elastic 
diaphragms will be forced slightly inward, and in this condition 
will respond to the least change in weight of the air. The small 
motion thus engendered is rendered visible by a system of three 
levers, the long arm of the last being connected by a chain with 
an axis carrying an index, which revolves in front of a graduated 
arc. The graduated circle is usually double, the inner arc indi- 
cating the weight of the air column in inches of mercury, while 
128 
