484 FIRST REPORT UPON MAGNETIC WORK

All that the surveyor has to do when he wishes to know the amount
of change of the magnetic meridian between any two dates is to turn
to the series at the nearest county-seat to the locality under considera-
tion and subtract from each other the values of the magnetic declina-
tion at that county-seat for those dates as obtained from the table.

If he wishes to be more precise he can make various combinations
of the county-seats surrounding the particular locality. It is believed
that this table will suffice for all practical needs. To facilitate inter-
polations the following dates in decimals of year are appended:

	Date.	Decimal of year.	Date.	Decimal of year.
	January 1	0.00	August 7	0.60
	February 6	0.10	September 13	0.70
	March 14	0.20	October 19	0.80
	April 20	0.30	November 25	0.90
	May 26	0.40	December 31	1.00
	July 2	0.50

EXAMPLE OF THE PRACTICAL APPLICATION OF THE TABLE.

For this purpose I shall select an actual case submitted to the Geo-
logical Survey. We received last March (1897) a letter from Waters-
ville, Carroll County, from which we quote as follows:

" Will you please inform me what variation of the needle should
be used in tracing old lines, say 1750 to 1800, also when the needle
was true to the pole and whether or not the needle was east prior to
1800. What is the correct variation for 1897? "

In reply to this letter, which was received before table No. XII was
constructed, the following table was furnished:

Approximate values of the magnetic declination (variation) at Watersville, Maryland.

	Year.	Declination.	Year.	Declination.
		0 '		0 '
	1750	2 28 W.	1855	2 09 W.
	60	1 50	60	2 29
	70	1 14	65	2 49
	80	0 41	70	3 09
	90	0 17	75	3 29
	1800	0 06	80	3 49
	10	0 05	85	4 09
	20	0 17	90	4 28
	30	0 42	95	4 46
	40	1 14	1900	5 03 W.
	50	1 50 W.