108 
KIRTLEY F. MATHER 
ment of a portion of a United States Geological Survey topo- 
graphic map or a Land Office map. 
The use of the magnetic needle for orientation of a plane- 
table sheet requires the utmost care. The length of the needle 
from its pivot to either end is only 1| to 3 inches, and lines will 
frequently be drawn on the map which are much longer than 
that. Errors in observing the needle may therefore be multi- 
plied during the progress of the work and thus piay become of 
more than trivial importance. Careful attention should be paid 
to see that the needle is swinging freely; a pocket magnifier 
should be used in observing its position with respect to the indi- 
cators; observation should be made from directly behind the 
instrument so that the observer sights along the needle from 
above rather than from the side. 
In many instruments the needle is not perfectly straight, or 
the two indicators and the needle pivot are not in a straight 
line; it is then best to regard only one end of the needle, being 
careful always to use that end and not the other. Or, again, 
the needle points may be blunt and the indicators of consider- 
able width; in that case select a definite relationship of needle 
point to marker and adjust the table or alidade so that the 
needle always returns to that relationship each time the map is 
oriented. 
The surveyor must, moreover, be always on guard lest steel 
or iron bodies in proximity to the compass box deflect the needle. 
Articles about his own person should receive his attention; he 
should stand so that his pocket knife is at least a foot from the 
needle; a Brunton or other compass must be kept at least 3 feet 
distant and must, therefore, be removed from the belt before 
attempting to orient the table; the margin of safety for a geo- 
logic hammer is a little less than 2 feet, and it may therefore be 
left in the belt if carried in the rear and if the surveyor stands 
well away from the table in observing the needle; a harmless 
appearing, leather-covered metal binocular or monocular case, 
if permitted to come within 2 feet of the needle, may lead to the 
erroneous conclusion that one is endowed with a superabundance 
of personal magnetism. Metallic bodies in the general vicinity 
