MARYLAXD GEOLOGICAL SURVEY 



211 



to tap " 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' of his compass. He can 

 determine this by making observations as stated at the South or North 

 Meridian Stone, whichever is best suited. He should reduce the value 

 of 5° 20' 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 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 : 



TABLE II. 



Month. 



.lanuar^- ... 

 February.. 



March 



April 



May 



June 



July 



August — 

 September. 



October 



November . 

 December . 



6 7 



A.M. I 



-0.1 

 +0.6 



+1.2 

 + 2.5 

 +3.0 

 +2.9 

 +3.1 

 +2.9 

 +1.8 

 +0.5 

 +0.5 

 +0.2 



+0.2 

 +0.7 

 1+2.0 

 +3.1 



+3. a 



!+4.4 



+4.r, 



,+4.9 

 1+2.8 

 + 1.6 

 + 1.2 



+0.3 



+1.0 

 +15 

 +3.0 

 +3.4 

 +3.9 

 +4.4 

 + 4.9 

 + 5.4 

 + 3.4 

 +3.1 

 + 1.7 

 +0.8 



10 



11 



d 

 o 

 o 



+2.1 

 +1.9 



+2.8 

 +2.6 

 +2.6 

 +3.3 

 +3.9 

 +3, 7 

 +2.5 

 +2.8 

 + 1.8 

 +1.8 



+2.4 

 + 1.4 

 +1.6 

 +0.8 

 +0.1 

 +1.1 

 + 1.8 

 +0.4 

 +0.3 

 + 1.4 

 + 1.1 

 +1.8 



+1.2 

 -0.1 

 -0.6 

 -2.1 

 -2.4 

 -2.0 

 -1.2 

 -2.8 

 -2.7 

 -1.0 

 -0.5 

 0.0 



-2.6 

 -2.5 

 -3.7 

 -4.2 



-4.9 

 -4.2 

 -3.4 

 -2.6 

 -2.3 



-2.1 

 -2.0 

 -3.3 

 -3.6 

 -3.6 

 -3.8 

 -4.2 

 -3.7 

 -4.0 

 -2.4 

 -1.8 

 -1.8 



-1.3 

 -1.2 

 -2.3 

 -2.3 

 -2.3 

 -2.6 

 -2.8 

 -1.9 

 -1.4 

 -1.3 

 -1.0 

 -1.1 



6 



P. M. 



-0.2 

 -0.8 

 -1.3 

 -12 

 -0.9 

 -1.2 

 -1.3 

 -0.6 

 -0.3 

 -0.4 

 -0.2 

 -0.3 



+ 0.2 

 -0.4 

 -0.5 

 -0.2 

 +0.1 

 -0.2 

 -0.3 

 +0.3 

 -0.1 

 -0.4 

 1-0.2 

 +0.1 



This table shows that during August, for example, the magnetic 



" 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. 



' 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, and loss 

 of magnetism of the needle. 



