The localities where three-hourly readings are chiefly desirable may be
specified under the heads of _Northern Atlantic, Southern Atlantic,
Indian_ and _Southern Oceans,_ and _Pacific Ocean_.


_Northern Atlantic. Homeward-bound Voyages._--The discussion of
observations made in the United Kingdom and the western border of
central Europe, has indicated that off the north-west of Scotland a
centre of great barometric disturbance exists. This centre of
disturbance appears to be considerably removed from the usual tracks of
vessels crossing the Atlantic; nevertheless some light may be thrown on
the barometric phænomena resulting from this disturbance by observations
during homeward-bound voyages, especially after the vessels have passed
the meridian of 50° west longitude. Voyagers to or from Baffin and
Hudson bays would do well during the whole of the voyage to read off the
barometer every three hours, as their tracks would approach nearest the
centre of disturbance in question. Before crossing the 50th meridian,
the undulations arising from the distribution of land and water in the
neighbourhood of these vast inland seas would receive considerable
elucidation from the shorter intervals of observation, and after passing
the 50th meridian the extent of undulation, as compared with that
observed by the more southerly vessels, would be more distinctly marked
by the three-hourly series. Surveying vessels stationed on the
north-western coasts of Ireland and Scotland may contribute most
important information on this head by a regular and, as far as
circumstances will allow, an uninterrupted series either of six-hourly
or three-hourly observations. The intervals of observation on board
vessels stationed at the Western Isles, the Orkneys, and the Shetland
Isles, ought not to be longer than _three_ hours, principally on account
of the great extent of oscillation observed in those localities. Vessels