ANEROID BAROMETER IN GEOLOGICAL SURVEYING. 133 
wishes to use an aneroid for topographic: purposes, to make him- 
self thoroughly familiar with all their characteristics. Perhaps 
there is no better way to do this than to draw across a cyclonic 
area as delineated on the daily weather map, a line which shall 
be considered the trace of the observer’s position as the area 
passes over him, and consider carefully the changes in the direc- 
tion of wind, character of cloud, and barometric pressure that 
would occur. A series of such lines, some straight, others 
crooked, will bring out the cyclonic characters better than any 
amount of description could. For further information on this 
subject, see ‘‘Weather’”’ by Abercrombie, Appleton’s Science 
Series, “Elements of Meteorology” by Davis (Ginn and Com- 
pany), and “A Popular Treatise on the Winds” by Ferrel, 
(Wiley and Sons). The last is not nearly so popular (elemen- 
tary) as its title indicates, but is an excellent work. 
From what has preceded, and an inspection of the barogram 
in Plate I, it is evident that the errors which may arise from 
changes in atmospheric pressure are very great, and that, if no 
measures are taken to correct them, the barometer would be 
entirely useless to the topographer, or even for hypsometry, but 
it is also evident that the errors so far considered may be cor- 
rected in either of two ways. The area to be surveyed may be 
divided into sections, each covering only a few square miles, and 
a self-registering barometer, carefully adjusted to read with that 
which is carried into the field, placed near the center of each 
section while it is being surveyed. The trace of this instrument 
will indicate the changes in pressure that have occurred, and 
furnish data for the correction of the readings taken in the field, 
provided the time at which each reading is taken is made part 
of the record. This method is open to the objection that the 
observer works mechanically and blindly, and is thus unable to 
apply the many little checks which he would otherwise find so 
useful, as well as to detect the larger errors, arising from surges, 
that will be spoken of further on. Another method is that by 
the use of gradients. Here the observer, before beginning work 
in the morning, carefully ascertains the barometric gradient, that 
