MARYLAND GEOLOGICAL SURVEY 291 



When the surveyor determines the value of the magnetic declina- 

 tion, it would be well for him to* make the observations on several 

 days, if possible. Probably the best time of day for making the 

 observations would be towards evening, about 5 or 6 o'clock. 1 At 

 this time the declination reaches, approximately, its mean value for 

 the day (see Table II). The observations on any one day should 

 extend over at least one-half of an hour, preferably an hour, and 

 the readings should be taken every ten minutes. Before each 

 reading of the needle it would be well to tap 2 the plate lightly with 

 the finger or a pencil so as to slightly 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 and the kind of time the observer's watch 

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

 some knowledge as to the error 3 of his compass. He can determine 

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

 He should reduce the value of 5° 21' to the date of his tests, 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 following table to correct his readings: 



To reduce an observation of the magnetic declination to the mean 

 value for the day of 24 hours, apply the quantities given in the table 

 below with the sign as affixed: 



1 Or the surveyor may make his observations in the morning and early in the after- 

 noon, at about the time of minimum and maximum values of the magnetic declina- 

 tion. He may regard the mean of the two extreme values as corresponding closely 

 to the mean value for the day (24 hours). 



2 Great care must be taken not to electrify the needle by rubbing the glass plate in 

 any manner. Remarkable deflections of the needle can thus be produced. 



3 T 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, and loss of magnetism of the 

 needle. 



