MARYLAND GEOLOGICAL SURVEY 447

that which I had derived from the four years 1887-1891. The varia-
tion now of the hourly reading on any individual day in that month
on the mean hourly reading for that month was regarded as the cor-
rection to be applied for that hour and that day to the normal diurnal
variation as obtained from the four years 1887-91. This correction
might in a certain way be looked upon as that which it was necessary
to apply to an assumed normal diurnal variation in order to obtain
the actual diurnal variation for that particular period. This correc-
tion was generally between 1' and 2'. In a few cases when a heavy
disturbance had been in progress it was necessary to resort to the
actual traces in order to apply a disturbance correction. On Septem-
ber 18, for example, for Massey a correction of —6^r was needed, and
on October 12 at Oakland corrections —16', —10', —2', 0', or
a mean correction of —7' had to be applied. On December 4th at
Salisbury a severe magnetic disturbance was encountered and the
correction on this day will doubtless be about —6' to —7'. All of
these corrections had to be made in the course of a few hours at the
Magnetic Observatory. When the Observatory furnishes the data
called for, more accurate and more careful reductions can be under-
taken. It is not believed, however, that any error due to defective
reduction amounting to over a couple of minutes will be attached to
the figures herewith presented.

And in this connection it should again be recalled that by reason,
of the general plan of the survey, whereby at some of the stations
the observations were made in the morning and at others in the after-
noon, some of the reduction errors, as far as the diurnal variation at
least is concerned, will be plus and others be minus. So that while
it cannot be said that an accuracy of one or two minutes in the decli-
nation has been reached at every one of the individual stations, never-
theless it is believed that the general result will be nearly, if not
entirely, of the same high order as though the utmost refinement had
been reached at each station.. And when it is remembered that the
great desideratum is to increase the number of the stations and that
by practising the utmost refinement, the number of the stations must
necessarily be cut down, if appropriation and time are limited, it is