MARYLAND GEOLOGICAL SURVEY 457

The total change between the morning and afternoon extremes, as
will be noticed from the bottom row of figures of the table, is about
1l' in mid-summer and about half this amount in mid-winter. It is
a quantity then that should be taken into consideration for precise
work. In Germany, mine surveying has been brought to such an art
that some of the principal mines maintain small magnetic observatories
where the declination is recorded continuously throughout the day
by photographic means. The mine surveyor then uses the value of
the declination to the nearest minute as prevailing at the very time of
day when he is running his line. The land surveyor generally ignores
the diurnal variation, though it is a quantity that may at times make
itself appreciably felt in his work. In the determination of the de-
clination or in the comparison of compasses the diurnal variation should
be taken into account. Below is a table giving the correction to be
applied to a declination observed at any time between 6 A. M. and
6 P. M., in order to reduce it to the mean value for the day (24 hours).
Apply the tabular quantities to the observed westerly declination with

the sign as affixed.

TABLE II.

Correction of an observed declination for diurnal variation.

Month.

6A. M.

7

8

9

10

11

Noon.

1

2

3

4

5

6 P. M.

Jan.....

— 0'. l

+0'. 2

+1/. 0

+2'. l

+2'. 4

+l'. 2

—V. I

_ 2'. 5

—2'. 6

— 2'. 1

— 1'. 3

— 0'. 2

+0'. 2

Feb....

+0. 6

+0. 7

+1. 5

+1. 9

+1. 4

—0. 1

—1. 5

—2. 1

—2. 5

—2. 0

—1. 2

—0. 8

—0. 4

March..

--1. 2

+2. 0

+3. 0

+2. 8

+1. 6

—0. 6

—2. 5

—3. 4

--3. 7

— 3. 3

—2. 3

—1. 2

—0. 5

April...

--2. 5

+3. 1

+3. 4

+2. 6

+0. 8

—2. 1

—4. 0

— 4. 1

--4. 2

—3. 6

—2. 3

—1. 2

—0. 2

May....

—3. 0

+3. 8

+3. 9

+2. 6

+0. 1

—2. 4

—4. 0

— 5. 0

—4. 5

—3. 6

—2. 3

—0. 9

+0. 1

June....

—2. 9

+4. 4

+4. 4

+8. 8

+1. 1

—2. 0

—3, 6

—4. 5

—4. 5

—3. 8

—2. 6

—1. 2

— 0. 2

July....

-8. 1

+4. 6

--4. 9

+3. 9

+1. 8

—1. 2

—3. 4

—4. 4

—4. 7

—4. 2

—2. 8

—1. 3

--0. 3

August.

+2. 9

+4. 9

--5. 4

+3. 7

+0. 4

—2. 8'

—4. 7

—5. 1

—4. 9

—3. 7

--1. 9

—0. 6

+0. 3

Sept....

+ 1. 8

+2. 8

—3. 4

+2. 5

+0. 3

—2. 7

—4. 4

—4. 6

—4. 2

—4. 0

—1. 4

—0. 3

— 0. 1

Oct.....

+0. 5

+1. 6

--3. 1

+2. 8

+1. 4

— 1. 0

—2. 7

—3. 3

—3. 4

—2. 4

—1. 3

—0. 4

—0. 4

Nov....

+0. 5

+1. 2

—1. 7

+ 1. 8

+1. 1

—0. 5

—2. 0

—2. 7

—2. 6

— 1. 8

—1. 0

-0. 2

+0. 2

Dec....

+0. 2

+0. 3

+0. 8

+1. 8

+1. S

0. 0

—1. 6

--2. 4

—2. 3

—1. 8

--1. 1

-0. 3

+0. 1

These figures represent the mean results of the continuous magnetic
observations made at the old site of the Washington Magnetic Obser-
vatory—the old Naval Observatory Grounds—during the four years
1888-91. The hours are for the seventy-fifth meridian, or Eastern
time, which is 8 minutes and 12. 09 seconds fast of Washington