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 reference to the acquisition of data from which the _barometric character_ of certain large areas of the surface of the globe may be determined--inasmuch as such areas are distinguished from each other, on the one hand by consisting of extensive spaces of the oceanic surface unbroken, or scarcely broken, by land; on the other by the proximity of such oceanic surface to large masses of land, and these masses presenting two essentially different features, the one consisting of land particularly characterized as continental, the other as insular, regard has been accordingly had to such distribution of land and water.