MARYLAND GEOLOGICAL SURVEY 509

The purposes of such lines are:

(a) To enable the surveyor to readily determine himself at any
time the declination or the " variation of the compass " at the county-
seat and thus to provide the means for determining the amount of
secular change.

(b) To furnish a common line whose magnetic bearing as well as
its true one has been accurately determined, on which surveyors can
from time to time test and compare their compasses.

If such lines had been established long ago at the various county-
seats the re-running of old lines would have been greatly simplified;
for in addition to the secular change being now accurately determin-
able, the condition of the compass used in the early survey would
have been known. The law provides, namely, that every surveyor
shall duly file with the county clerk the " amount of variation " or
magnetic bearing given by his instrument on the authoritatively
established county meridian. It is well known that compasses may
differ greatly from each other by reason of the fact that the proper
care is not, in general, bestowed on them. A magnetic needle poised
on a fine pivot point is a delicate instrument and must be carefully
handled if it is to settle in the right place when it comes to rest.

Nine counties have thus far availed themselves of the privilege of
having their meridian lines established in connection with the mag-
netic survey. These counties are: Baltimore, Dorchester, Charles,
Frederick, Harford, Kent, Queen Anne's, Talbot and Wicomico.

The method used in the establishment of the lines was that of
altazimuth observations on the sun before and after noon. When
for some reason, which only occurred once, observations could not
be made on both sides of noon, then special observations for a suffi-
ciently accurate determination of the latitude were made. The accu-
racy aimed at was, that the established line should be correct within
one minute of are. The instruments employed were those used in the
magnetic survey.

The line was permanently marked by two granite monuments about
four feet long and seven by seven inches square. The monuments
were generally imbedded in several courses of concrete and were