180 THE JIAGXETIC DECLINATION IN ST. MART's COUNTY 



Mechanicsville. — In the garden of Hotel Mattingly, 18 feet north- 

 east of mulberry tree and 9^ feet west of wooden fence. 



For a description of the methods and instruments used, reference must 

 be made to the " First Report upon Magnetic Work in Maryland," (Md. 

 Geol. Survey, Vol. I, pt. 5, 1897). This report gives likewise an 

 historical account of the phenomena of the compass-needle and discusses 

 fully the difficulties encountered by the surveyor on account of the many 

 fluctuations to which the compass-needle is subject. Surveyors of the 

 county desiring a copy of this report should address the State Geologist. 

 In the Second Eeport (Md. Geol. Survey, Vol. V, pt. 1, 1903), the 

 various values observed in Maryland have been collected and reduced. 

 Surveyors of the county desiring these reports should address the State 

 Geologist. 



ilERIDIAX LINE. 



On June 37, 19nO, Mr. J. B. Baylor, acting under instructions of 

 the Superintendent of the United States Coast and Geodetic Survey as 

 issued to him, in response to a request from the State Geologist, estab- 

 lished a true meridian line at Leonardtown, in the courthouse scjuare. 

 This line is marked by two substantial monuments, suitably lettered and 

 firmly planted in the ground. (See description above.) 



Tile South Stone should be the one to be used by surveyors 

 when making their tests, if still well removed from all disturb- 

 ING influences. 



When the surveyor determines the value of the magnetic declination, 

 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.' At this time the declina- 

 tion 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 



' Or the surveyor may make his observations in the morning and early in 

 the afternoon, at about the time of minimum and maximum values of the 

 magnetic declination. He may regard the mean of the two extreme values as 

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



