decreasing, as we recede from the central portions of the oceanic
surfaces we have mentioned towards the land which forms their eastern,
western, or northern boundaries. This influence of the junction line of
land and water, so far as it is yet known, has been kept in view in
framing these instructions, and, as it appears so prominently in Europe,
it is hoped the additional observations between the four daily readings
to which probably many observers may habitually restrict themselves,
making on certain occasions and in particular localities a series of
observations at intervals of three hours, will not be considered too
frequent when the great importance of the problem to be solved is fully
apprehended. It need scarcely be said that the value of these
observations at three-hourly intervals will be greatly increased by the
number of observers co-operating in them. Upon such an extensive system
of co-operation a large space on the earth's surface, possessing
peculiarities which distinguish it from others extremely unlike it in
their general character, or assimilate it to such as possess with it
many features in common, is marked out below for particular observation,
occupying more than two-thirds of a zone in the northern hemisphere,
having a breadth of 40°, and including every possible variety of
terrestrial and aqueous surface, from the burning sands of the great
African desert, situated about the centre, to the narrow strip of land
connecting the two Americas on the one side, and the chain of islands
connecting China and Hindostan with Australia on the other. On each side
of the African continent we have spaces of open sea between 30° and 40°
west longitude north of the equator, and between 60° and 80° east
longitude, in or to the south of the equator, admirably suited for
contrasting the barometric affections, as manifested in these spaces of
open water, with those occurring in situations where the influence of
the terrestrial surface comes into more active operation.