The Hurricane Guide Part 2

In other localities besides those hereafter to be mentioned, when opportunities serve, readings at these hours would greatly enhance the value of the four daily readings.

There are, however, portions of the surface of our planet, and probably also phaenomena that occur in its atmosphere, which require still closer attention than the eight daily readings. One such portion would appear to exist off the western coast of Africa, and we recommend the adoption of _hourly_ readings while sailing to the westward of this junction of aqueous and terrestrial surface; more attention will be directed to this point as we proceed. There are also phaenomena the localities of which may be undetermined, and the times of their occurrence unknown, but so important a relation do they bear to the subject of our inquiries, that they demand the closest attention. They will be more particularly described under the head of acc.u.mulations of pressure preceding and succeeding storms, and minute directions given for the hourly observations of the necessary instruments. In the mean time we may here remark that hourly observations under the circ.u.mstances above alluded to are the more important when we consider that the barometer, the instrument employed in observing these moving atmospheric ma.s.ses, is itself in motion. The s.h.i.+p may meet the acc.u.mulation of pressure and sail through it transversely; or she may sail along it, the course of the vessel being parallel to the line marking the highest pressure, the ridge or crest of the wave; or the s.h.i.+p may make any angle with this line: but whatever the circ.u.mstances may be under which she pa.s.ses through or along with such an acc.u.mulation of pressure, it should ever be borne in mind that her position on the earth's surface is scarcely ever the same at any one observation as it was at the preceding, the barometer in the interval has changed _its_ position as well as the line of maximum pressure, the rate of progress of which it is desirable to observe. It will, therefore, be at once apparent that in order to obtain the most accurate data on this head hourly observations are indispensable. To these readings should of course be appended the places of the s.h.i.+p from hour to hour, especially if she alter her course much.

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 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 s.p.a.ces of the oceanic surface unbroken, or scarcely broken, by land; on the other by the proximity of such oceanic surface to large ma.s.ses of land, and these ma.s.ses 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.

As these instructions have especial reference to observations at sea, observations on land have not been alluded to; but in order that the data acc.u.mulated may possess that value which is essential for carrying on the inquiry in reference to atmospheric waves with success, provision is made to mark out more distinctly the barometric effects of the junction of large ma.s.ses of land and water. It is well known that the oceanic surface, and even the smaller surfaces of inland seas, produce decided inflexions of the isothermal lines. They exercise an important influence on temperature. It has also been shown that the neighbourhood of water has a very considerable influence in increasing the oscillations of the mercurial column in the barometer, and in the great systems of European undulations it is well known that these oscillations increase especially towards the north-west. The converse of this, however, has not yet been subjected to observation; there has been no systematic co-operation of observers for the purpose of determining the barometric affections of large ma.s.ses of water, such as the central portion of the basin of the northern Atlantic, the portion of oceanic surface between the Cape of Good Hope and Cape Horn, the Indian and Southern oceans, and the vast basin of the Pacific. Nor are we yet acquainted with the character of the oscillations, whether increasing or decreasing, as we recede from the central portions of the oceanic surfaces we have mentioned towards the land which forms their eastern, western, or northern boundaries. This influence of the junction line of land and water, so far as it is yet known, has been kept in view in framing these instructions, and, as it appears so prominently in Europe, it is hoped the additional observations between the four daily readings to which probably many observers may habitually restrict themselves, making on certain occasions and in particular localities a series of observations at intervals of three hours, will not be considered too frequent when the great importance of the problem to be solved is fully apprehended. It need scarcely be said that the value of these observations at three-hourly intervals will be greatly increased by the number of observers co-operating in them. Upon such an extensive system of co-operation a large s.p.a.ce on the earth's surface, possessing peculiarities which distinguish it from others extremely unlike it in their general character, or a.s.similate it to such as possess with it many features in common, is marked out below for particular observation, occupying more than two-thirds of a zone in the northern hemisphere, having a breadth of 40, and including every possible variety of terrestrial and aqueous surface, from the burning sands of the great African desert, situated about the centre, to the narrow strip of land connecting the two Americas on the one side, and the chain of islands connecting China and Hindostan with Australia on the other. On each side of the African continent we have s.p.a.ces of open sea between 30 and 40 west longitude north of the equator, and between 60 and 80 east longitude, in or to the south of the equator, admirably suited for contrasting the barometric affections, as manifested in these s.p.a.ces of open water, with those occurring in situations where the influence of the terrestrial surface comes into more active operation.

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 phaenomena resulting from this disturbance by observations during homeward-bound voyages, especially after the vessels have pa.s.sed 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 pa.s.sing 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 circ.u.mstances 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, princ.i.p.ally on account of the great extent of oscillation observed in those localities. Vessels arriving from all parts of the world as they approach the United Kingdom should observe at shorter intervals than six hours. As a general instruction on this head the series of three-hourly observations may be commenced on board vessels from America and the Pacific by the way of Cape Horn on their pa.s.sing the 20th meridian, such three-hourly observations to be continued until the arrival of the vessels in port.

s.h.i.+ps by the way of the Cape of Good Hope should commence the three-hourly series either on leaving or pa.s.sing the colony, in order that the phaenomena of the tropical depression hereafter to be noticed may be well observed.

_Northern Atlantic. Outward-bound Voyages_.--Vessels sailing to the United States, Mexico, and the West Indies, should observe at three hours' interval upon pa.s.sing the 60th meridian. Observations at this interval, on board vessels navigating the Gulf of Mexico and the Caribbean Sea, will be particularly valuable in determining the extent of oscillation as influenced by the ma.s.ses of land and water in this portion of the torrid zone, as compared with the oscillation noticed off the western coast of Africa, hereafter to be referred to.

_Southern Atlantic. Outward and homeward bound_.--Without doubt the most interesting phaenomenon, and one that lies at the root of the great atmospheric movements, especially those proceeding northwards in the northern hemisphere and southwards in the southern, is the equatorial depression first noticed by Von Humboldt and confirmed by many observers since. We shall find the general expression of this most important meteorological fact in the Report of the Committee of Physics and Meteorology, appointed by the Royal Society in 1840, as follows: "The 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 pa.s.sing 40 north lat.i.tude, commence the series of three-hourly observations, with an especial reference to the equatorial depression.

These three-hourly observations should be continued until the lat.i.tude of 40 south has been pa.s.sed: 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 pa.s.sages across the equator, should the s.h.i.+ps 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 s.h.i.+ps. In point of fact they will clearly ill.u.s.trate 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 circ.u.mstances will permit, while the s.h.i.+ps are sailing from the Madeiras to the equator, will be extremely valuable in elucidating the origin of the great system of south-westerly atmospheric waves that traverse Europe, and in furnis.h.i.+ng data for comparison with the amount of oscillation and other barometric phaenomena in the Gulf of Mexico and the Caribbean Sea, a portion of the torrid zone essentially different in its configuration and in the relations of its area to land and water, as contra-distinguished to the northern portion of the African continent; and these hourly observations are the more desirable as the vessels may approach the land. They may be discontinued on pa.s.sing the equator, and the three-hourly series resumed.

There are two points in the southern hemisphere, between 80 west longitude and 30 east longitude, that claim particular attention in a barometric point of view, viz., Cape Horn and the Cape of Good Hope; the latter is within the area marked out for the three-hourly observations, and too much attention cannot be paid to the indications of the barometer as vessels are approaching or leaving the Cape. The northern part of the South Atlantic Ocean has been termed the _true Pacific Ocean of the world_; and at St. Helena a gale was scarcely ever known; it is also said to be entirely free from actual storms (Col. Reid's 'Law of Storms,' 1st edition, p. 415). It may therefore be expected that the barometer will present in this locality but a small oscillation, and s.h.i.+ps in sailing from St. Helena to the Cape will do well to ascertain, by means of the three-hourly observations, the increase of oscillation as they approach the Cape. The same thing will hold good with regard to Cape Horn: it appears from previous observation that a permanent barometric depression exists in this locality, most probably in some way connected with the immense depression noticed by Captain Sir James Clark Ross, towards the Antarctic Circle. The general character of the atmosphere off Cape Horn is also extremely different from its character at St. Helena. It would therefore be well for vessels sailing into the Pacific by Cape Horn, to continue the three-hourly observations until the 90th meridian is pa.s.sed.

Before quitting the Atlantic Ocean it may be well to notice the marine stations mentioned in my Third Report on Atmospheric Waves,[5] as being particularly suitable for testing the views advanced in that report and for tracing a wave of the south-westerly system from the most western point of Africa to the extreme north of Europe. A series of hourly observations off the western coast of Africa has already been suggested.

Vessels staying at Cape Verd Islands should not omit to make observations at three hours' interval _during the whole of their stay_, and when circ.u.mstances will allow, hourly readings. At the Canaries, Madeiras, and the Azores, similar observations should be made. Vessels touching at Cape Cantin, Tangier, Gibraltar, Cadiz, Lisbon, Oporto, Corunna, and Brest, should also make these observations while they are in the localities of these ports. At the Scilly Isles we have six-hourly observations, made under the superintendence of the Honourable the Corporation of the Trinity House. s.h.i.+ps in nearing these islands and making the observations already pointed out, will greatly a.s.sist in determining the increase of oscillation proceeding westward from the nodal point of the two great European systems. We have already mentioned the service surveying vessels employed on the coasts of Ireland and Scotland may render, and the remaining portion of the area marked out in the report may be occupied by vessels navigating the North Sea and the coast of Norway, as far as Hammerfest.

In connexion with these observations, having especial reference to the European system of south-westerly atmospheric waves, the Mediterranean presents a surface of considerable interest, both as regards these particular waves, and the influence its waters exert in modifying the two great systems of central Europe. The late Professor Daniell has shown from the Manheim observations, that small undulations, having their origin on the northern borders of the Mediterranean, have propagated themselves northward, and in this manner, but in a smaller degree, the waters of the Mediterranean have contributed to increase the oscillation as well as the larger surface of the northern Atlantic. In most of the localities of this great inland sea six-hourly observations may suffice for this immediate purpose; but in sailing from Lisbon through the Straits of Gibraltar, in the neighbourhood of Sicily and Italy, and in the Grecian Archipelago, we should recommend the three-hourly series, as marking more distinctly the effects resulting from the proximity of land; this remark has especial reference to the pa.s.sage through the Straits of Gibraltar, where, if possible, hourly observations should be made.

_The Indian and Southern Oceans. Outward and homeward bound._--On sailing from the Cape of Good Hope to the East Indies, China, or Australia, observations at intervals of three hours should be made until the 40th meridian east is pa.s.sed (homeward-bound vessels should commence the three-hourly readings on arriving at this meridian). Upon leaving the 40th meridian the six-hourly observations may be resumed on board vessels bound for the Indies and China until they arrive at the equator, when the readings should again be made at intervals of three hours, and continued until the arrival of the vessels in port. With regard to vessels bound for Australia and New Zealand, the six-hourly readings may be continued from the 40th to the 100th meridian, and upon the vessels pa.s.sing the latter, the three-hourly readings should be commenced and continued until the vessels arrive in port. Vessels navigating the Archipelago between China and New Zealand, should make observations every three hours, in order that the undulations arising from the configuration of the terrestrial and oceanic surfaces may be more distinctly marked and more advantageously compared with the Gulf of Mexico, the Caribbean Sea, and the northern portion of the African continent.

_The Pacific Ocean._--As this ocean presents so vast an aqueous surface, generally speaking observations at intervals of six hours will be amply sufficient to ascertain its leading barometric phaenomena. Vessels, however, on approaching the continents of North and South America, or sailing across the equator, should resort to the three-hourly readings, in order to ascertain more distinctly the effect of the neighbourhood of land on the oscillations of the barometer, as generally observed, over so immense a surface of water in the one case, and the phaenomena of the equatorial depression in the other: the same remarks relative to the latter subject, which we offered under the head of South Atlantic, will equally apply in the present instance. The configuration of the western sh.o.r.es of North America renders it difficult to determine the precise boundary where the three-hourly series should commence; the 90th meridian is recommended for the boundary as regards South America, and from this a judgment may be formed as to where the three-hourly observations should commence in reference to North America.

In the previous sketch of the localities for the more important observations, it will be seen that within the tropics there are three which demand the greatest regard.

I. The Archipelago between the two Americas, more particularly comprised within the 40th and 120th meridians west longitude, and the equator and the 40th degree of north lat.i.tude. As a general principle we should say that vessels within this area should observe the barometer every three hours. Its eastern portion includes the lower branches of the storm paths, and on this account is peculiarly interesting, especially in a barometric point of view.

II. _The Northern portion of the African Continent, including the Sahara or Great Desert._--This vast radiating surface must exert considerable influence on the waters on each side northern Africa. Vessels sailing within the area comprised between 40 west and 70 east, and the equator and the 40th parallel, should also make observations at intervals of three hours.

III. _The great Eastern Archipelago._--This presents a somewhat similar character to the western; like that, it is the region of terrific hurricanes, and it becomes a most interesting object to determine its barometric phaenomena; the three-hourly system of observation may therefore be resorted to within an area comprised between the 70th and 140th meridians, and the equator and the 40th degree of north lat.i.tude.

The southern hemisphere also presents three important localities, the prolongations of the three tropical areas. It is unnecessary to enlarge upon these, as ample instructions have been already given. We may, however, remark, with regard to Australia, that three-hourly observations should be made within the area comprised between the 100th and 190th meridians east, and the equator and the 50th parallel south, and hourly ones in the immediate neighbourhood of all its coasts.

IV.--STORMS, HURRICANES, AND TYPHOONS.

The solution of the question--How far and in what manner are storms connected with atmospheric waves?--must be extremely interesting to every one engaged in either the naval or merchant service. As we have in the former chapters directed attention to their connexion, our great object here will be to endeavour to mark out such a line of observation as appears most capable of throwing light, not only on the most important desiderata as connected with storms, but also their connexion or non-connexion with atmospheric waves. We shall accordingly arrange this portion of the instructions under the following heads:--_Desiderata_; _Localities_; _Margins_; _Preceding and Succeeding Acc.u.mulations of Pressure._

_Desiderata._--The most important desiderata appertaining to the subject of storms, are certainly their origin and termination. Of these initial and terminal points in the course of great storms we absolutely know nothing, unless _the white appearance of a round form_ observed by Mr.

Seymour on board the Judith and Esther, in lat. 17 19' north and long.

52 10' west (see Col. Reid's 'Law of Storms,' 1st edit. p. 65), may be regarded as the commencement of the Antigua hurricane of August 2, 1837.

This vessel was the most eastern of those from which observations had been obtained; and it is the absence of contemporaneous observations to the eastward of the 50th meridian that leaves the question as to the origin of the West Indian revolving storms unsolved. Not one of Mr.

Redfield's storm routes extends eastward of the 50th meridian; this at once marks out, so far as storms are concerned, the entire s.p.a.ce included between the 20th and 50th meridians, the equator and the 60th parallel, as a most suitable area for observations, under particular circ.u.mstances hereafter to be noticed, with especial reference either to the commencement or termination of storms, or the prolongation of Mr.

Redfield's storm paths.

_Localities._--The three princ.i.p.al localities of storms are as follows:--I. The western portion of the basin of the North Atlantic; II.

The China Sea and Bay of Bengal; and III. The Indian Ocean, more particularly in the neighbourhood of Mauritius. The first two have already been marked out as areas for the three-hourly observations; to the latter, the remark as to extra observations under the head of Desiderata will apply.

_Margins._--Mr. Redfield has shown that on some occasions storms have been preceded by an unusual pressure of the atmosphere; the barometer has stood remarkably _high_, and it has hence been inferred that there has existed _around_ the gale an acc.u.mulation of air forming a margin; barometers placed under this margin indicating a much greater pressure than the mean of the respective localities. With regard to the West Indian and American hurricanes--any considerable increase of pressure, especially within the s.p.a.ce marked out to the eastward of the 50th meridian, will demand immediate attention. Upon the barometer ranging _very high_ within this s.p.a.ce, three-hourly observations should be immediately resorted to; and if possible, _hourly_ readings taken, and this is the more important the nearer the vessel may be to the 50th meridian. Each observation of the barometer should be accompanied by an observation of the wind--its direction should be most carefully noted, and the force estimated according to the scale in page 21, or by the anemometer. It would be as well _at the time_ to project the barometric readings in a curve even of a rough character, that the extent of fall after the mercury had pa.s.sed its maximum might be readily discernible by the eye. A paper ruled in squares, the vertical lines representing the commencement of hours, and the horizontal tenths of an inch, would be quite sufficient for this purpose. The _force_ of the wind should be noted at, or as near to the time of the pa.s.sage of the maximum as possible. During the fall of the mercury particular attention should be paid to the manner in which the wind changes, should any change be observed; and should the wind continue blowing steadily in _one_ direction, but gradually _increasing_ in force, then such increments of force should be most carefully noted. During the fall of the barometer, should the changes of the wind and its increasing force indicate the neighbourhood of a revolving storm, (independent of the obvious reasons for avoiding the focus of the storm,) it would contribute as much to increase our knowledge of these dangerous vortices to keep as near as possible to their margins as to approach their centres. The recess from the centre towards the margin of the storm, will probably be rendered apparent by the _rising_ of the mercury; and so far as the observations may be considered valuable for elucidating the connexion of atmospheric waves with rotatory storms (other motives being balanced), it might be desirable to keep the s.h.i.+p near the margin--provided she is not carried beyond the influence of the winds which characterize the latter half of the storm--until the barometer has nearly attained its usual elevation.

By this means some notion might be formed of the general direction of the line of barometric pressure preceding or succeeding a storm.

Should a gale be observed commencing without its having been preceded by an unusual elevation of the mercurial column, and consequently no additional observation have been made; when the force of the wind is noted in the usual observations at or above 5, then the three-hourly series should be resorted to, and the same care taken in noting the direction, changes, and force of the wind as pointed out in the preceding paragraph.

The foregoing remarks relate especially to the central and western portions of the North Atlantic; they will however equally apply to the remaining localities of storms. Under any circ.u.mstances, and in any locality, a _high_ barometer not less than a low one should demand particular attention, and if possible, _hourly_ readings taken some time before and after the pa.s.sage of the maximum: this will be referred to more particularly under the next head.

_Preceding and Succeeding Acc.u.mulations of Pressure._--Mr. Redfield has shown in his Memoir of the Cuba Hurricane of October, 1844, that two a.s.sociated storms were immediately preceded by a barometric wave, or acc.u.mulation of pressure, the barometer rising above the usual or annual mean. We have just referred to the importance of _hourly_ observations on occasions of the readings being _high_ as capable of ill.u.s.trating the marginal phaenomena of storms, and in connexion with these acc.u.mulations of pressure in advance of storms we would reiterate the suggestion.

These strips of acc.u.mulated pressure are doubtless crests of atmospheric waves rolling forwards. In some cases a s.h.i.+p in its progress may cut them transversely in a direction at right angles to their _length_, in others very obliquely; but in all cases, whatever section may be given by the curve representing the observations, too much attention cannot be bestowed on the barometer, the wet and dry bulb thermometer, the direction and force of the wind, the state of the sky, and the appearance of the ocean during the s.h.i.+p's pa.s.sage _through_ such an acc.u.mulation of pressure. When the barometer attains its mean alt.i.tude, and is rapidly rising above it in any locality, then _hourly_ observations of the instruments and phaenomena above noticed should be commenced and continued until after the mercury had attained its highest point and had sunk again to its mean state. In such observations particular attention should be paid to the direction and force of the wind preceding the barometric maximum--and the same phaenomena succeeding it, and particular notice should be taken of the time when, and amount of any change either in the direction or force of the wind. It is by such observations as these, carried on with great care and made at every accessible portion of the oceanic surface, that we may be able to ascertain the continuity of these atmospheric waves, to determine somewhat respecting their length, to show the character of their connexion with the rotatory storm, and to deduce the direction and rate of their progress.

V.--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 a.s.sociation 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 ascertained--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 pa.s.sed, 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 a.s.sociated 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 phaenomena 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 pa.s.sed 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.

CHAPTER IV.

PRACTICAL DIRECTIONS FOR AVOIDING THE CENTRES OF ROTATING STORMS.