barometer, at the level of the sea, does not indicate a mean atmospheric
pressure of equal amount in all parts of the earth; but, on the
contrary, the equatorial pressure is uniformly less in its mean amount
than at and beyond the tropics." Vessels that are outward bound should,
upon passing 40° north latitude, commence the series of three-hourly
observations, with an especial reference to the equatorial depression.
These three-hourly observations should be continued until the latitude
of 40° south has been passed: the whole series will then include the
minimum of the depression and the two maxima or apices forming its
boundaries. (See Daniell's 'Meteorological Essays,' 3rd edition.) In
passages across the equator, should the ships be delayed by calms,
opportunities should be embraced for observing this depression with
greater precision by means of _hourly_ readings; and these readings will
not only be valuable as respects the depression here spoken of, but will
go far to indicate the character of any disturbance that may arise, and
point out, as nearly as such observations will allow, the precise time
when such disturbance produced its effects in the neighbourhood of the
ships. In point of fact they will clearly illustrate the diversion of
the tendency to rise, spoken of in the Report before alluded to, as
resulting in ascending columns and sheets, between which wind flaws,
capricious in their direction and intensity, and often amounting to
sharp squalls, mark out the course of their feeders and the indraft of
cooler air from a distance to supply their void. Hourly observations,
with especial reference to this and the following head of inquiry,
should also be made off the western coast of Africa during the
homeward-bound voyage.

Immediately connected with this part of the outward-bound voyage, hourly
observations, as often as circumstances will permit, while the ships are
sailing from the Madeiras to the equator, will be extremely valuable in