VARIATION OF TIIE COMPASS NEEDLE.
 

37
 

COMPASS.
 

Variation of the Compass Needle, Bucks county, Pennsylvania.
 

Magnetic Declination, or as it is commonly called, the variation of the compass
needle, may be known for any period during the last two hundred years in Bucks
county, Avith as much precision as the instruments used by early observers Avould al-
loAv. Determinations of the Avesterly declination before and about the year 1700 Avere
made at Hatborough, and also at Philadelphia city, and repeated at irregular inter¬
vals doAvn to the present time. These give for the city Avesterly magnetic variation
at fourteen intervening periods, and for at least as many periods at the first-named
place, but the instruments used in early days Avere rough in comparison Avitli such as
are noAv employed for like purposes. But, taking the tAvo series here mentioned, and
api3lying an adjustment of special utility in such calculations, the average deriA'^ed is
knoAvn to be less in error than the single series of observations actually made at either
of the two places. The following average values for Avesterly deviation of the com¬
pass needle, as betAvecn Philadelphia and Hatborough, Avere furnished by vSuperin-
tendent C. P. Patterson, from the office of the United States coast-survey, Avhere
computations from similar records are made by Charles A. Schott, chief of the Comput¬
ing Division:
 

YEAR.
 

variation.
 

YEAR.
 

variation.
 


 

West.
 


 

West.
 

1680
 

8°.9
 

1780
 

2°.55
 

1690
 

8°.7
 

1790
 

2°.l
 

1700
 

8°.3
 

1800
 

1°.95
 

1710
 

7°.9
 

1810
 

2°.06
 

1720
 

7°.4
 

1820
 

2°.40
 

1730
 

6°.65
 

1830
 

2°.94
 

1740
 

5°.9
 

1840
 

3°.59
 

1750
 

5°,0
 

1850
 

4°.34
 

1760
 

4°,1
 

1860
 

5°.14
 

1770
 

3°.25
 

1870
 

5°.99
 


 


 

1880
 

6°.78
 

In accordance with the laAV inferred from computations, the variation of the com¬
pass needle can be predicted for a fcAV years beyond the present time, as appears by
the table.

Supposing a proportionate distribution of magnetism at Philadelphia, Hatborough,
and DoylestOAvn, the variation of the needle at DoylestoAvn would be about 17^ (say a
Quarter of a degree) greater than at Philadelphia, and about 10' (one-sixth of a degree)
Greater than at Hatborough for any period Avithin the last tAvo centuries. If the dis¬
tribution has been, as supposed, the variations for DoylestoAvn are readily found by
adding 0°.22 to each of the values given in the preceding table ; we have therefore
the foUoAving :