MARYLAND GEOLOGICAL SURVEY 441

The table below gives an idea of the magnitude of the error at
various hour angles.

Error in the observed solar azimuth when the assumed latitude is 1'
greater than the true latitude.

		APPARENT	TIME.
	Latitude.	A. H.	P. M.
		789 10 11	12345
	85	—0. 32 —0. 70 —1. 22 —2. 12 —4. 55	+4. 55 +2. 12 +1. 22 +0. 70 +0. 32
	40	—0. 35 —0. 75 —1. 30 —2. 26 —4. 87	+4. 87 +2. 26 +1. 30 +0. 75 +0. 35
	45	—0. 38 —0. 82 —1. 42 —2. 45 —5. 28	+5. 28 +2. 45 +1. 42 +0. 82 +0. 38

The formula by means of which the above figures have been
obtained, is—

c_a=c, sec l. cot t.

where c_a is the correction to be applied to the sun's azimuth counted positive in

the direction N, E, S, W.
e, is difference between the assumed latitude and the true latitude, or the

quantity to be applied to the latter to get the former.
l is the approximate latitude.
t is the hour angle or number of hours in angular measure counted from

apparent noon, minus when before noon and plus when after noon.

When the assumed latitude is 1' less than the true latitude, the quan-
tities in the table would, of course, have their signs reversed.

The latitudes of the stations were scaled from the best maps at
present to be had. In the table of geographical positions, given in
another chapter, the sources from which the positions were obtained
are given. From some tests made it would seem that the error in the
adopted latitudes will generally fall below one minute.

Since an error of 1' in latitude causes an error for latitude 39°
varying from 0/7 to 4/8 between the hours from 8 A. M. to 11 A. M.
and from 1 P. M. to 4 P. M., this was an error that should be taken
into account, if thereby the increased refinement was not obtained
at the cost of an additional station. To take this error into account
it was necessary either to make fore- and afternoon observations at