MARYLAND GEOLOGICAL SURVEY 419

time. This number will be a constant or nearly so for a given place,
except for slight variations, of which we will speak later, but will vary
with change of place. The number of vibrations will decrease or,
what amounts to the same thing, the time of one vibration will increase
with approach towards the " magnetic pole. " The time of one vibra-
tion thus found is a measure of the intensity of the earth's magnetic
force as exerted in the horizontal plane in which the needle swings,
the exact relation being that H, the horizontal component of the earth's
magnetism, is equal to a constant divided by a square of the time, T,
of one vibration. Suppose we obtain the value of T at two different
places with the same magnet. Let these values be TI and T₂', then
will the respective values of H at these two places be inversely to each
other as the squares of the times of vibration, or

H₁H²T₂²T₁₂,

hence, H₂=H₁ T₁²/T₂²

We can thus obtain the horizontal intensity at any place in terms
of some known or unknown value at some initial station. This is
the principle of relative measurement of the earth's magnetic force.

The first one to apply this principle was William Whiston, chiefly
famous as the translator of Josephus. His achievements in the domain
of terrestrial magnetism have only recently been set forth. His
researches were animated by his desire to determine the latitude and
longitude at sea with the aid of magnetic observations, chiefly of dip.
It will be recalled that the problem of determining longitude at sea was
a perplexing one for a long time, and large prizes were offered for the
best solutions. It is largely owing to this endeavor of the early mag-
neticians to determine geographical position magnetically that observa-
tion data were so thoroughly multiplied and new facts brought to
light.

Just so was it with Whiston. In his desire to win the coveted prize,
and having apparently considerable influence at court and being, there-
fore, richly favored with money grants, he made a series of dip obser-
vations in various parts of southern England, and drew the earliest
isoclinics—the lines connecting the places of equal magnetic dip—of