Has been made by the English and French; and much to their
honour, by them in conjunction. The first modern measurement of degrees of
latitude was made by an Englishman of the name of Norwood: he ascertained
the difference of latitude between London and York in 1635, and then
measured their distance: from these premises he calculated, that the length
of a degree was 122,399 English yards. At this time there was no reason to
suppose that the earth was flattened at the Poles. Shortly afterwards, it
having been discovered that the weights of bodies were less at the equator
than at Paris, Huygens and Cassini directed their attention, as we have
already stated, to the subject of the figure of the earth. In 1670 Picard
measured an arc of the meridian in France; and in 1718, the whole area
extending through France was measured by Cassini and other philosophers.
The results of this measurement seemed to disprove Newton's theory, that
the curvature of the earth diminished as we recede from the equator. To
remove all doubts, an arc near the equator was measured in Peru, by some
French and Spanish astronomers; and an arc near the arctic circle by some
French and Swedish astronomers; the result was a confirmation of Newton's
theory, and that the equatorial diameter exceeded the polar by about 1/204
part of the whole.
Since this period, arcs of the meridian have been measured in several
countries. In 1787 it was determined by the British and French governments
to connect the observatories of Greenwich and Paris by a series of
triangles, and to compare the differences of latitudes and longitudes,
ascertained by astronomical observations, with those ascertained by actual
measurement. The measurement in England was extended to a survey of the
whole kingdom; and the accurate maps thus obtained have been since
published. Arcs of the meridian have also been measured lately from Dunkirk
to Barcelona, - in Lapland, by which an error in the former measurement
there was corrected; - and in India.
We have been thus particular in our notice of this subject, because it is
evident that such measurements must lie at the foundation of all real
improvements in the construction of maps.
Let us next turn our attention to the improvements in navigation which have
taken place during the last and present centuries; these seem to consist,
principally, in those which are derived from physical science, and those
which are derived from other sources.
The grand objects of a navigator are the accurate knowledge of where he
exactly is, in any part of his course, and how he ought to steer, in order
to reach his destination in the shortest time. The means of ascertaining
his latitude and longitude, of calculating how far he has sailed, and at
what rate he is sailing, and the direction of his course with reference to
the port to which he is desirous to proceed, are what he principally
requires.
