Seasons For Extra Observations

In reference to certain desiderata that have presented themselves in the

course of my researches on this subject (see Report of the British

Association for the Advancement of Science, 1846, p. 163), the _phases_

of the larger barometric undulations, and the _types_ of the various

seasons of the year, demand particular attention and call for extra

observations at certain seasons: of these, three only have yet been

ascer ained--the type for the middle of November--the annual depression

on or about the 28th of November--and the annual elevation on or about

the 25th of December. The enunciation of the first is as under: "That

during fourteen days in November, more or less equally disposed about

the middle of the month, the oscillations of the barometer exhibit a

remarkably symmetrical character, that is to say, the fall succeeding

the transit of the maximum or the highest reading is to a great extent

similar to the preceding rise. This rise and fall is not continuous or

unbroken; in some cases it consists of _five_, in others of _three_

distinct elevations. The complete rise and fall has been termed the

great symmetrical barometric wave of November. At its setting in the

barometer is generally low, sometimes below twenty-nine inches. This

depression is generally succeeded by _two_ well-marked undulations,

varying from one to two days in duration. The central undulation, which

also forms the apex of the great wave, is of larger extent, occupying

from three to five days; when this has passed, two smaller undulations

corresponding to those at the commencement of the wave make their

appearance, and at the close of the last the wave terminates." With but

slight exceptions, the observations of eight successive years have

confirmed the general correctness of this type. On two occasions the

central apex has not been the highest, and these deviations, with some

of a minor character, form the exceptions alluded to. This type only has

reference to London and the south-eastern parts of England; proceeding

westward, north-westward, and northward, the symmetrical character of

this type is considerably departed from; each locality possessing its

own type of the barometric movements during November. The desiderata in

immediate connexion with the November movements, as observed in the

southern and south-eastern parts of England, that present themselves,

are--the determination of the types for November, especially its middle

portion, as exhibited on the oceanic surface within an area comprised

between the 30th and 60th parallels, and the 1st and 40th meridians

west. Vessels sailing within this area may contribute greatly to the

determination of these types by making observations at intervals of

three hours from the 1st of November to the 7th or 8th of December. The

entire period of the great symmetrical wave of November will most

probably be embraced by such a series of observations, as well as the

annual depression of the 28th. For the elevation of the 25th of December

the three-hourly observations should be commenced on the 21st, and

continued until the 3rd or 4th of the succeeding January.



With respect to the great wave of November, our knowledge of it would be

much increased by such a series of observations as mentioned above,

being made on board surveying and other vessels employed off Scotland

and Ireland; vessels navigating the North Sea; vessels stationed off the

coasts of France, Spain, Portugal, and the northern parts of Africa, and

at all our stations in the Mediterranean. In this way the area of

examination would be greatly enlarged, and the _differences_ of the

curves more fully elucidated; and this extended area of observation is

the more desirable, as there is some reason to believe that the line of

greatest symmetry _revolves_ around a fixed point, most probably the

nodal point of the great European systems.



It is highly probable that movements of a somewhat similar character,

although presenting very different curves, exist in the southern

hemisphere. The November wave is more or less associated with storms. It

has been generally preceded by a high barometer and succeeded by a low

one, and this low state of the barometer has been accompanied by stormy

weather. We are therefore prepared to seek for similar phænomena in the

southern hemisphere, in those localities which present similar states of

weather, and at seasons when such weather predominates. We have already

marked out the two capes in the Southern hemisphere for three-hourly

observations: they must doubtless possess very peculiar barometric

characters, stretching as they do into the vast area of the Southern

Ocean. It is highly probable that the oscillations, especially at some

seasons, are very considerable, and vessels visiting them at such

seasons would do well to record with especial care the indications of

the instruments already alluded to. At present we know but little of the

barometric movements in the Southern hemisphere, and every addition to

our knowledge in this respect will open the way to more important

conclusions.



It has been observed in the south-east of England that the barometer has

generally passed a maximum on or about the 3rd of every month, and this

has been so frequently the case as to form the rule rather than the

exception. The same fact during a more limited period has been observed

at Toronto. With especial reference to this subject the three-hourly

series of observations may be resorted to in all localities, but

especially north of the 40th parallel in the northern hemisphere. They

should be commenced at midnight immediately preceding the 1st and

continued to midnight succeeding the 5th.