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 o eanic 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.



As these instructions have especial reference to observations at sea,

observations on land have not been alluded to; but in order that the

data accumulated 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 masses 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 masses 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 space on the earth's surface, possessing

peculiarities which distinguish it from others extremely unlike it in

their general character, or assimilate 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 spaces 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 spaces 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 phænomena resulting from this disturbance by observations

during homeward-bound voyages, especially after the vessels have passed

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 passing

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

circumstances 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, principally 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 passing the 20th meridian, such three-hourly

observations to be continued until the arrival of the vessels in port.

Ships by the way of the Cape of Good Hope should commence the

three-hourly series either on leaving or passing the colony, in order

that the phænomena 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 passing 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 masses 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 phænomenon, 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 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

elucidating the origin of the great system of south-westerly atmospheric

waves that traverse Europe, and in furnishing data for comparison with

the amount of oscillation and other barometric phænomena 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

passing 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

ships 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 passed.



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 circumstances 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. Ships in nearing these islands and

making the observations already pointed out, will greatly assist 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

passage 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 passed (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

passing 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 phænomena. 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 phænomena 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

shores 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 latitude. 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 phænomena; 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 latitude.



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.