There is another point to which we wish to call attention in immediate
connexion with hourly readings--it is the observation of the instruments
on the days fixed for that purpose: they were originally suggested by
Sir John Herschel, whose directions should be strictly attended to: they
are as follows:--

The days fixed upon for these observations are the 21st of March, the
21st of June, the 21st of September, and the 21st of December, being
those, or immediately adjoining to those of the equinoxes and
solstices, in which the _solar influence_ is either stationary or in a
state of most rapid variation. _But should any one of those 21st days
fall on a Sunday, then it will be understood that the observations are
to be deferred till the next day, the 22nd._ The series of observations
on board each vessel should commence at 6 o'clock A.M. of the appointed
days, and terminate at 6 A.M. of the days following, according to the
usual reckoning of time adopted in the daily observations.

In addition to the twenty-five hourly readings at the solstices and
equinoxes as above recommended, it would be desirable to continue the
observations until a complete elevation and depression of the barometer
had been observed at these seasons. This plan is adopted at the Royal
Observatory, Greenwich, and would be attended with this advantage were
it generally so--the progress of the elevation and depression would be
more readily traced and their velocities more accurately determined than
from the four or eight daily readings.


III.--LOCALITIES FOR ADDITIONAL OBSERVATIONS.

In sketching out a system of barometric observation having especial