MARYLAND GEOLOGICAL SURVEY 



181 



ten minutes. Before each reading of the needle it would' be well to 

 tap' the plate lightly with the finger or a pencil so as to slighth' disturb 

 the needle from the position of rest it may have assumed. The accurate 

 time should be noted opposite each reading and a note entered in the 

 record-book as to the date, the weather aiul the kind of time the observers 

 watch was keeping. It is very essential that the surveyor should have 

 some knowledge as to the error' of his compass. He can determine this 

 by making observations as stated at the South Meridian Stone. He 



TAKLE IL 



.1 anuary 



February +0.B 



March +1.3 



April +3.5 



May +3.0 



June +3.9 



July +3.1 



August +3.9 



September... 1+1.8 



October 1+0.6 



NoA'ember +0.6 



December +0.3 



-1.0 

 -0.5 

 -0.0 I 



should reduce the value of 4° 53' to the date of his tests, by allowing an 

 annual increase since January 1, 1900, as above stated, of 3', and the 

 difference between this value and his own will be his compass error. 



If the surveyor has an instrument which admits of the refinement to 

 take into account the change in the magnetic declination during the day, 

 he may use the above table to correct his readings. 



To reduce an observation of the magnetic declination to the mean 

 value for the day of 2i hours, apply the quantities given in the table 

 above with the sign as affixed. 



"Great care must be taken not to electrify the needle liy rubbing the glass 

 plate in any manner. Remarkable deflectious of the needle can thus be 

 produced. 



'I have found surveyors' compasses to differ at times as much as 1° from 

 the readings with the Coast and Geodetic Survey Standard Magnetometer. 

 The error may be due to a variety of causes, such as an imperfect pivot, non- 

 coincidence of magnetic axis of needle with the geometric axis, loss of mag- 

 netism of the needle, or impurity of brass of which the compass box is con- 

 structed. 



