OCEANICCIRCULATION.
OCEANICCIRCULATION.
The existence of currents in certain localities was known at a very early date, and navigators in their voyages to the new world soon discovered the Gulf Stream and other currents of the Atlantic. The first current charts were published more than two hundred years ago. Theories were soon advanced to explain the causes, one group of scientific men attributing the origin of currents to differences of level produced by an unequal distribution of atmospheric pressure over the oceans, another set connecting the tidal phenomena with the cause of ocean currents, and still another finding in the rotation of the earth a sufficient reason for their existence. The polar origin of the cold deep water found in low latitudes has long been considered probable, and has given rise to a theory of a general oceanic circulation in a vertical and horizontal direction, produced by differences of temperature and density. Recent theoretical investigations, however, seem to indicate that these causes alone are incapable of producing currents, and, to-day, the theory that the winds are mainly responsible for all current movements very largely predominates. Benjamin Franklin was probably the first who recognized in the trade winds the cause of the westerly set in the tropics, and Rennel soon after made the division of drift and stream currents. The objections which have appeared against the wind theory have been met with the reply that the present state of oceanic movements is the result of the work done by the winds in countless thousands of years.
Current phenomena is briefly summarized as follows by one of the latest authorities on the subject:
1. The greater portion of the current movement of the ocean must be regarded as a drift, produced by the prevailing winds, whose mean direction and force are the measures for the mean set and velocity of the current.
2. Another group of currents, and in fact a fraction of all currents, consists of compensating or supply streams, created by the necessity of replacing the drifted water in the windward portion of the drift region.
3. A third group results from drifts deflected by the configuration of the coasts; these which are denominated free currents, quickly pass into compensating streams.
4. The deflecting force of the rotation of the earth is considered as of subordinate importance, but may have some influence on currents that are wholly or in part compensating or free.
Late investigations of the Gulf Stream by the U. S. Coast Survey give interesting facts in regard to that notable current.
A satisfactory explanation of the cause of the stream has not yet been found, but many believe, with Franklin, that the powerful trade drift entering the Gulf of Mexico through the broad channel between Yucatan and Cuba presses the water as a strong current through Florida Strait, where the stream is turned to the northward along the coast. Since 1850 American naval officers have added greatly to our knowledge of the characteristics of this stream, particularly within the last decade, during which notable investigations have been carried on by Commanders Bartlett and Sigsbee and Lieut. Pillsbury, U. S. N., under the direction of the U. S. Coast Survey, and by Lieutenant Commander Tanner, U. S. N., in the Fish Commission steamer Albatross.
Of special importance are the valuable and interesting results in regard to tidal action in the stream obtained by Lieut. Pillsbury, U. S. N., in the Coast Survey steamer Blake, from observations begun by him in 1885 at the narrowest part of Florida Strait, between Fowey Rocks and Gun Cay (Bah.), and continued since between Rebecca Shoal and Cuba, and between Yucatan and Cape San Antonio (Cuba), and off Cape Hatteras.
During the past year Lieut. Pillsbury extended the field of operations to the passages between the islands encircling the Caribbean Sea, and in order to study the Atlantic flow outside the limits of the trade drift a station was to have been occupied about 700 miles to the north-east of Barbados; this, however, was unfortunately prevented by bad weather.
The deductions from the observations in Florida Strait showed very clearly adailyand amonthlyvariation in the velocity of the stream, the former having a range of 2½ knots, and reaching a maximum on the average about 9h9mbefore and 3h37mafter the moon's upper transit, and the monthly variation reaching its maximum about two days after the maximum declination of the moon. The variations in this section were found greater on the western than on the eastern side of the strait, and the axis of the stream, or position of strongest surface flow, was located by Lieutenant Pillsbury 11½ miles east of Fowey Rocks, and, farther north, about 17 miles east of Jupiter Light. The average surface current at this section was 33/5knots, the maximum 5¼ knots, and the minimum 1¾ knots per hour. The results also indicate that when the current is at its maximum the surface flow is faster than at any depth below it, but when at its minimum the velocity at a depth of 15 fathoms or even down to 65 fathoms is greater than at the surface, and that there is at times a current running south along the bottom in all parts of the stream except on the extreme eastern side.
The results of the investigations in 1887 and 1888 have not yet been published, but from information kindly furnished by the authorities of the Coast Survey, I am able to give a brief outline of the more prominent facts ascertained.
In the section between Rebecca Shoal and Cuba the daily variation in velocity was found as prominent as in Florida Strait, the mean time of eight maxima corresponding to 9h18mbefore, and that of three maxima to 3h25mafter the moon's transit. The axis of the stream in this section was found near the center of the current prism, and the flow was easterly and inclined on either side toward the axis. The axis seemed to occupy a higher level than other parts of the stream, and this appears to be borne out by the fact that about half the number of the current bottles thrown out in Florida Strait on the west side of the axis were recovered along the east coast of Florida, while of those thrown out east of the axis not a single one was heard from. As a rule it was found that the stronger the current the more constant the direction and the deeper the stratum. Remarkable fluctuations in the flow near the axis were noted, the velocity increasing sometimes one knot in ten or fifteen minutes, and then as suddenly decreasing again. Lieutenant Pillsbury attributes this, however, to a serpentine movement of the maximum flow, which would sometimes strike the station occupied by the Blake. The edge of the stream was found at about 30 miles south of Rebecca Shoal light-house.
Between Yucatan and Cape San Antonio the stream was found flowing about north, and the line of maximum velocity corresponds on the average to 10hbefore and to 2h20mafter the moon's transit. The excessive variations were like those in Florida Strait, on the west side of the stream, and the maximum velocity of 6¼ knots was found about 5 miles off the 100-fathom line of Yucatan Bank. The eastern edge of the stream lies about 20 miles west of Cape San Antonio, and between this edge and the island, eddy currents exist. At the time the easternmost station in this section was first occupied, the declination of the moon was low and the set of the surface current north-easterly. At a high south declination of the moon the surface current was found south-easterly in direction, and east or south-east below the surface. The normal flow below the surface was in each case from the Gulf into the Caribbean Sea, and this makes it probable that the station was situated inshore of the average limit of the stream. On Cape San Antonio Bank the currents are tidal, flood running northward and ebb southward. On the Yucatan Bank the currents were also tidal, but as the edge of the bank is approached the stronger flow of the Gulf Stream predominates. The monthly variation in velocity, which was found clearly defined at the first two sections occupied, appeared at this section to be obliterated by anomalies not existing at the former.
Off Cape Hatteras the Blake accomplished the remarkable feat of remaining at anchor in 1,852 fathoms, and this with a surface current of over 4 knots. Two stations were occupied, and similar variations in velocity were observed as at the other stations. The notable feature at this station was the discovery of tidal action beneath the Gulf Stream, the currents at 200 fathoms depth changing their direction very regularly, the average current flowing about S. S. E. ½ E. for 7 hours and N. N. W. ½ W. for a little over 5 hours.
The first section investigated in 1888 was in the equatorial drift between Tobago and Barbados, where seven stations were occupied. The axis of the stream was found west of the middle, or nearer the South American shore, and the average direction was towards the north. At none of the stations did the current set in the direction of the wind, although the trades were blowing at all times with a force of from 2 to 7. The daily variation was also here very pronounced, the average time of maximum flow occurring about 5h56mafter the moon's transit. At 65 and 130 fathoms depth the current, at three of the stations occupied, was north-westerly; at one south-easterly. The velocity at 130 fathoms was greater than at 65 fathoms, and greater at the surface than at 15 and 30 fathoms.
At all of the three stations between Grenada and Trinidad tidal action was observed, with deflections due to local influences.
The passage between Santa Lucia and St. Vincent appears to be in the line of the equatorial stream. At each of the five stations in this passage tidal action was pronounced, the currents setting in and out of the Caribbean Sea at some depth. The daily variation in this passage reaches a maximum at about 6h3mafter the moon's transit, and a minimum when the moon is on the meridian. The currents entering the Caribbean Sea through this passage are but 100 fathoms in depth, but there is probably an almost equal volume flowing out below that depth.
Between the Windward Islands the currents flow generally westward, but tidal action is everywhere apparent.
To the east of Desirade the currents at all observed depths have a northerly direction, fluctuating between about N. E. by E. to N. W. by N.
In the eastern part of the Anegada Passage the surface current flows into the Caribbean Sea in directions varying between S. S. W. and S. E., but the submarine current down to 130 fathoms flows in a direction lying between north and east.
In the more western part of the passage the currents are more complex, apparently on account of the greater variations in depth in the vicinity of the station occupied.
In the Mona Passage no regular currents were perceptible. Between Mona and Puerto Rico the currents observed set out of the Caribbean Sea, varying in direction from about W. by N. to E. N. E., except at 65 fathoms depth, where there appeared to be an inward flow. On the western side of the passage, near Santo Domingo, the direction of the currents was between S. S. E. and S. W. by W. But few observations could be taken on account of unfavorable weather.
In the Windward Passage, on the western side the currents from the surface down to 130 fathoms set in the directions lying in the S. E. quadrant, and at 200 fathoms the direction changed to W. by S. On the eastern side the surface current varied between E. N. E. and E. S. E., with about ½ knot velocity. Variations in the direction similar in extent characterized also the subsurface currents in the middle and on the eastern side of the passage.
The average of the observations at these three stations gives but a small volume of water passing in either direction.
In the old Bahama Channel, at the station north of Cayo Romano (island off the north coast of Cuba) the currents at and near the surface set south of east; at 65 fathoms, however, the direction varies from about N. W. to E. The deeper current of great volume flowed continually to the north of west with a velocity of over 1½ knots at depths of 130 and 200 fathoms.
Outside the Bahamas, to the north of Great Abaco, a slight current flows about N. W. on the surface and down to 30 fathoms; at 65 fathoms depth the direction changes to a point more westerly, and at 130 fathoms to a point more easterly than the set of the surface current. The maximum in the daily variation at this station occurs about 12hafter the moon's transit.
The observations so far as completed by Lieutenant Pillsbury furnish the most valuable data we have at present concerning the Gulf Stream, and it is hoped that further investigation and the analytical treatment of these observations will clearly develop the dynamic laws involved and lead us to a correct theory of current phenomena in general.
TIDALPHENOMENA.
TIDALPHENOMENA.
The causes for many of the inequalities in the tidal elements observed at different places have not yet been satisfactorily explained. The phenomena are dependent on many purely terrestrial conditions. While we are able to ascertain with tolerable accuracy from certain constants, derived from observation, the times and heights of the tides, the problem to compute theoretically the tides of an ideal ocean of known depth and configuration remains still unsolved. According to Ferrel our present knowledge of tidal phenomena is comparable to that possessed 2,000 years ago of the science of astronomy.
TEMPERATURE OF THESEA.
TEMPERATURE OF THESEA.
The temperature of sea water had already been observed by Ellis, in 1749, in the Atlantic, and subsequent expeditions have furnished a great number of temperature observations in various seas and for various depths. The diversity of instruments and of methods employed by the earlier observers, and the faulty methods of recording, have made the uniform reduction of many of these observations difficult or impossible. The most complete and valuable collection of these older observations up to 1868, with an account of the instruments and methods used by each observer, was published by Prestwich, in 1876, in the Philosophical Transactions, Vol. 165.
With the advent of the great scientific expeditions, which were supplied with modern and refined instruments, our knowledge of the thermal conditions of the sea has progressed immensely, and we are now able to construct charts of all the oceans, showing the distribution of the isotherms with considerable accuracy.
The annual average surface temperature has been found higher in the Indian Ocean than in either the Atlantic or Pacific; the North Atlantic is slightly warmer than the North Pacific, but the South Pacific is warmer than the South Atlantic; this holds generally good also for the temperatures between surface and bottom.
The temperature generally decreases more or less rapidly from the surface down to about 500 fathoms, at which depth it is quite uniformly between 39° and 40° F. From that depth it decreases slowly towards the bottom: in the Polar seas to between 27° and 28° F.; in the middle and higher latitudes of the northern hemisphere and at depths of 2,000 to 3,000 fathoms, to between 34° and 36° F.; at the equator and in southern latitudes it remains in the neighborhood of 32° F.
The low temperatures at the bottom are thought to be due to a steady but slow circulation of water from the Polar seas towards the equator, and, where the circulation is most free and unobstructed, as in the South Atlantic, South Pacific and Indian Ocean, the bottom temperature is slightly lower than in the North Atlantic and North Pacific, both of which are connected with the Polar Sea by comparatively narrow and shallow straits.
The theory of this circulation from the Polar seas is greatly strengthened by the facts appearing from the investigation of the bathymetric isotherms in inclosed seas, i.e., seas which are separated from the deep oceans by submarine barriers. In such seas the temperature decreases slowly from the surface down to the depth of the barrier, and from there on remains constant to the bottom.
The influence of currents on the surface temperature is very marked, cold currents bending the isothermal lines towards the equator, and warm currents bending them towards the poles. The seasonal changes in surface temperatures are considerable, being the least in the tropical zones.
In theAtlantic Oceanthe maximum surface temperature lies near the coast of South America, between Para and Cayenne, and another maximum occurs near the west coast of Africa, between Freetown and Cape Coast Castle.
ThePacific Oceanshows the peculiarity that the surface temperatures on the western side are lower than those on the eastern side. Between 45° N. and 45° S. the temperature does not fall below 50°, but between those parallels and the poles it remains most always below that figure.
The warmest water is found in theRed Seawhere the surface temperature has been recorded as high as 90°. North of the equator the mean annual temperature is considerably above 80°, but south of it, to about the parallel of 25°, it varies from 80° to 70°.
CHEMICALCOMPOSITION, SALINITY ANDDENSITY OFSEAWATER.
CHEMICALCOMPOSITION, SALINITY ANDDENSITY OFSEAWATER.
In this branch of inquiry great progress has been made, and sea water is now known to contain at least 32 elementary bodies. Its chief constituents are found to consist of the chlorides and sulphates of sodium, magnesium, potassium and calcium. It also contains air and carbonic acid.
The salinity and density of sea water have been investigated very thoroughly, particularly in the Atlantic. As the salinity of the sea water is an index of its density, changes in the former naturally affect the latter. The salinity has been found generally to decrease in the neighborhood of coasts, where rivers discharge their water into the sea, and it is a maximum in the trade zones, and a minimum in the equatorial rain belt. The salinity is affected by the degree of evaporation and by the frequency of rainfall, and is now recognized as an important factor in the biologic conditions of the sea.
Of the three great oceans, the Atlantic, with a salinity of 3.69 per cent., shows a slight preponderance over that of the Pacific and Indian Ocean, whose average salinity is 3.68 and 3.67, respectively.
In the trade belts the great evaporation augments the salinity, and hence, also, the density, and in the polar zones the formation of ice brings about the same result, though in a lesser degree. In the equatorial calm region the frequent rainfall diminishes salinity and density through the dilution of the salt water. Density and salinity are thus in a certain degree subject to seasonal changes.
In theAtlanticthe density increases in general from the higher latitudes towards the equator, but the maxima are separated by a zone of lesser density. The maximum in the North Atlantic ocean is found between the Azores, the Canaries and the Cape Verde Islands, and the minimum between the equator and 15° N.
In the South Atlantic two maxima occur, one to the north of Trinidad, and the other near St. Helena and between that island and Ascension.
Taking pure water at 4° C. for unity, the maximum density in the Atlantic is 1.0275 and in the Pacific, 1.0270.
In theNorth Pacificthe maximum density occurs between 30° and 31° N., and the minimum in about 7½° N., in the equatorial counter current, where it was found as low as 1.02485.
In theSouth Pacific, which has a slightly greater density than the North Pacific, the maximum has been found in the vicinity of the Society Islands.
The density of the waters of theIndian Oceanis not yet as well known as that of the Atlantic and Pacific, but the results ascertained indicate a lesser density in its northern part, with a maximum in the region between 20° and 36° S. and long. 60° to 80° E.
In the vicinity of Java and Sumatra, probably on account of the extreme humidity of the atmosphere and of frequent rainfall, the density has been found as low as 1.0250.
In regard to the density of the water at various depths, it has been ascertained that as a general rule it decreases from the surface down to about 1,000 fathoms, after which it increases again slowly to the bottom. In the equatorial calm regions, however, where the heavy rains dilute the surface water, the density decreases from the surface down to between 50 and 100 fathoms, after which it follows the law found for other parts of the ocean. The bottom densities of the South Atlantic and Pacific have been found about alike, varying only from 1.02570 to 1.02590; those of the North Atlantic, however, show a greater value, varying from 1.02616 to 1.02632.
GREATESTDEPTHS OF THEOCEANS.
GREATESTDEPTHS OF THEOCEANS.
ATLANTIC.—Rejecting some of the earliest soundings as untrustworthy, the greatest known depth in the North Atlantic is to the north of the island of Puerto Rico, in about latitude 19° 39' N., longitude 66° 26' W., found by the C. S. S. Blake, Lieut. Commander Brownson, U. S. N., in 1882–83, 4,561 fathoms.
The deepest known spot in the South Atlantic is 3,284 fathoms, in about latitude 19° 55' S., longitude 24° 50' W., sounded by the U. S. S. Essex, Commander Schley, in 1878.
The general run of the soundings indicates that greater depressions exist nearer the western than in the eastern or middle part of the Atlantic, North and South.
PACIFIC.—In the North Pacific the greatest depression has been found by the U. S. S. Tuscarora, Commander Geo. E. Belknap, U. S. N., in 1874, 4,655 fathoms, in latitude 44° 55' N., longitude 152° 26' E. The next deepest sounding in the North Pacific was located by the Challenger in 1875, 4,475 fathoms, in latitude 11° 24' N., longitude 143° 16' E. As in the Atlantic, the greater depths appear to exist in the western part and particularly off the coasts of Japan.
In the South Pacific the greatest depths were supposed, up to a recent period, to be in the eastern part. Within the last two years, however, the British surveying vessel Egeria has discovered greater depressions in the western part of the South Pacific, one spot sounding 4,430 fathoms in latitude 24° 37' S., longitude 175° 08' W., and another, 12 miles farther south, 4,298 fathoms.
INDIANOCEAN.—In this ocean the greatest depths appear to exist to the north and west of the Australian continent, where there are more than 3,000 fathoms in a number of widely separated spots, indicating a depressed area of considerable extent.
In the most southerly part of the Indian Ocean, or rather in theAntarctic region, the Challenger obtained, in 1874, a maximum depth of 1,673 fathoms, in latitude 65° 42' S., longitude 79° 49' E.
ARCTICOCEAN.—The greatest depth was sounded by the Sofia in 1868, 2,650 fathoms, in latitude 78° 05' N., longitude 2° 30' W.
In the minor seas the maximum depths so far as ascertained are:
January, 1889.
BYA. W. GREELY.
BYA. W. GREELY.
In presenting to the National Geographic Society a summary of geographic advance as regards the domain of the air, the Vice-president finds a task somewhat difficult. The traveler passes from the east to the west coast of Africa, and his very efforts to struggle across that great continent, impress in his memory an abiding picture of the physical features of the country over which he has passed, and of the distribution of plants and animal life. So, too, a vessel sails from one coast to another, casting here and there a sounding lead, from which measurements it is possible to give quite a definite idea of the relief features of the bottom of the sea.
Small as are the traces which serve to indicate the character of the sea bottom, yet they are infinitely greater than those which enable us to give a description of the air. Atmospheric disturbances are so vast, and their action is so rapid, that it requires the attentive care of thousands of observers before one can well hope to draw the roughest figure of a passing storm. To note changes in the force and direction of the wind, to note the depth of the rain, the increase and decrease of temperature and the varying changes of aqueous vapor, either in visible or invisible form, requires millions of careful, systematic observations, and then when these are made, the task of collating, elaborating and discussing them seems almost too great for any man. Fortunately the value of meteorological work has impressed itself not only upon governments, which have assisted liberally by appropriations and organization, but yet more upon the isolated observer, thousands of whom over the face of the earth give of their time and labor, and add their mite to the wealth of universal knowledge.
In connection with all great physical questions, there is at times a tendency to application to special phases somewhat to the exclusion of others. While it can hardly be said that scientific and theoretical discussion of meteorology has been unduly neglected during the past year, yet it is evident that the greatest activity of meteorologists has been devoted to climatological investigation, and compilations of this character have been particularly numerous during the past year—not in the United States and Europe alone, but throughout the whole world.
The growing practical importance of meteorological researches has been lately evidenced perhaps in no more striking way than in the establishment in Brazil of a most extensive meteorological service, created by a decree of the Imperial government on April 4, 1888. A central meteorological institute, under the Minister of Marine, is to be the centre for meteorological, magnetic and other physical researches, and observations are to be made at all marine and military establishments in the various provinces, on the upper Amazon, in Uruguay, and on all subsidized government steamers. This service should soon be fruitful in results, as the meteorology of the interior of Brazil is almost absolutely unknown.
Another vast scheme has originated in Brazil in the Imperial Observatory of Rio Janeiro. Señor Cruls, its director, contemplates a dictionary of the climatology of the earth, giving monthly means and extremes of pressure, temperature, rainfall, wind, etc. This scheme, of course, can be successful only by international co-operation. The United States Signal Service has pledged its aid as regards this country.
The former tendency among Russian meteorologists to devote their greatest energies to climatological compilations has gradually given way to other practical work in connection with weather and storm predictions, as shown by the institution by the Russian government of a system of storm-warnings for the benefit of vessels navigating the Black Sea.
Blanford has put forth an important paper, which partially elucidates the very intricate question of diurnal barometric changes, particularly bearing on the relation of the maximum pressure to critical conditions of temperature, cloudiness and rainfall. The question viewed in a negative light by Lamont, as to whether the maximum barometric pressure could be attributed to the greatest rate of increase in the temperature of the air, due, it is supposed, to the reactionary effect of the heated and expanding air, has been re-examined by Blanford, whose conclusions are somewhat in favor of this theory.
S. A. Hill has treated of the annual oscillation of pressure, so noticeable in India, and in so doing has investigated the changes of pressure for three levels, up to a height of 4500 meters. The reduction of monthly barometric means at high levels, having regard to the vertical distribution of temperature, shows a double oscillation in the annual curve at the level of Leh, which becomes a single one at the height of 4500 meters, while this is substantially the reverse of the oscillation observed below.
The subject is also treated in another way by Mr. Hill, through analysis of normal monthly means for all India, whereby he succeeds in presenting a formula, the first periodic terms of which represent the two principal factors of the oscillation.
Mr. Hill has also discussed elaborately the anomalies in the winds of northern India in their relation to the distribution of barometric pressure. The anomalies are:—(1) in the hot season the wind direction frequently shows no relation to the barometric gradient; (2) the winds over the plains show little or no relation to pressure gradients, but an obvious one to temperature, being greatest where the temperature is highest.
It is pointed out as highly probable that the copious snowfalls of the late winter in the northwest Himalayas not only produce low temperatures on the Himalayan ranges, but subsequently cause dry northwesterly winds over northern and western India, and on this supposition, reliable forecasts of the character of the coming rainy monsoons have been made for a number of years. Convection currents between upper and lower air strata, it is suggested by Koppen, explain diurnal variations in wind velocity and direction. At low stations the maximum velocity occurs at the time of the highest temperature, while at high stations the reverse obtains. Hill has examined into an important point connected with this subject, that is, the great local differences in the vertical variation of temperature. Hill concludes by saying that high pressures at low levels are the result of low temperatures, and in connection with the fact that wind directions are largely influenced by the irregular distribution of pressure at high levels, it is more important to know the abnormal variations of pressure at the highest hill stations in India than those in the plains.
Overbeck has lately published a paper on the apparent motions of the atmosphere, in which he clearly and admirably outlines the treatment of the dynamics of the air by his predecessors. He comments on the mode of treatment of Ferrel, as well as those of Guldberg and Mohn. Overbeck then sets forth his own method, and elaborately discusses the influence of the earth's rotation with reference to the resistances which oppose the motion of the atmosphere. He touches on the effect produced by rapidly moving fluid entering fluid at rest, the development of discontinuous (so called by Helmholtz) currents, the tendency of parallel currents of unequal velocities towards similar velocities, the effect of friction arising from contiguous currents of different velocities, upon the coefficient of friction, of the temperature distribution over the surface of the earth, etc. He derives three very simple expressions for the motions of the air; the first giving the velocity in a vertical direction at any point, in terms of latitude, and a constant and factor depending on the distance of the point above the surface of the earth. The other expressions give the velocities in a north or south direction, and in an east or west direction, also in terms of constants and latitude. The velocity when charted from Overbeck's equations indicate an ascending vertical current from the equator to 35° north, and thence a descending current to the pole. The meridional current at the equator and pole are zero, and have a maximum value at latitude 45°.
Ciro Ferari, from long and important investigations of thunder storms, shows that these phenomena invariably attend motionless areas of low pressure, and believes the surest elements for predicting such storms will be found to be the peculiarities in distribution of temperature and absolute humidity. He observes that the storm front invariably tends to project itself into the regions where the humidity is greatest, and that hail accompanies rapidly moving storms of deep barometric depression. Ferari considers the chief causes of thunder storms to lie in the connection of high temperature and high humidity. Grossman believes that ascending moist-laden currents are the cause of thunder storms, and hence they are most frequent when the temperature diminution with altitude is very great, so that the over-heating of the lower air strata in the warmest part of the day is the cause of the primary maximum of thunder-storm frequency.
Abercromby and Hildebrandsson have renewed their recommendations for a re-classification of clouds in ten fundamental types, in which the first part of the compound name, such as cirro-stratus, cirro-cumulus, etc., is to be in a measure indicative of the height of a cloud.
Hildebrandsson has charted the differences of monthly means of air pressure for January, 1874 to 1884. In January, 1874, the values at nearly all the stations in the Northern Hemisphere, were plus, and those in the Southern, minus. It is to be hoped that such general discussions of this important meteorological element may be continued.
General A. Von Tillo has determined, by means of the planimeter, the distribution of temperature and pressure from Teisserenc de Bort's charts. The mean pressure over the Northern Hemisphere for January, he finds to be 29.99 inches (761.7 millimeters), and the temperature 46°.9 (8°.3 C.); in July, 29.806 (758.5 mm.) and 72°.7 (22°.6 C.). In Russia he finds an increase of one millimeter of pressure to correspond with a decrease of 1°.6 C. in temperature.
Doberck, after investigation of September typhoons at Hong Kong, attributes their appearance to the relatively low pressure then existing between Formosa and Lyon.
The valuable and elaborate investigation of American Storms, by Professor Elias Loomis has been completed. Loomis has thoroughly discussed barometric maxima and minima areas as presented by the maps of the Signal Service, from which it appears that these areas are in general elliptical, with the longest axis nearly twice that of the shortest in the high areas, while the difference is less in low areas. He has also investigated the winds relative to baric gradients, thus affording valuable data for proving various meteorological theories. Loomis' researches regarding the movement of maximum areas verify those which have been set forth from time to time in Signal Service publications; wherefrom it appears that high areas have a more southerly movement than low areas.
Van Bezold has put forth a memoir on thermodynamics, while Helmholtz, Oberbeck, and Diro-Kitso have contributed valuable memoirs on motions caused by gravitation and the varying density of the air. These furnish meteorologists with important results as to the laws of fluid or gaseous motions. It is gratifying to Americans to note that the valuable results obtained by Ferrel in his many memoirs are confirmed by these later investigations.
Undoubtedly the most important meteorological event within the past year was the discontinuance, on January 1, 1888, of the system of International Simultaneous Meteorological reports inaugurated in accordance with the agreement of the conference at Vienna in September, 1873. As the charts of storm tracks, based on these observations, have been published by the United States Signal Service one year behind the date of the observations, the completion of this work in printed form for the general public should occur about December 31, 1888.
A few remarks in connection with this unparalleled set of observations may not be out of place. The congress which agreed upon this work, met in accordance with invitations issued by the Austrian Government in September, 1873. The co-operation decided upon at this congress took practical shape January 1, 1874, at which date one daily simultaneous report was commenced from the Russian and Turkish Empires, the British Islands, and the United States: the energetic co-operation of these nations being assured through Professor H. Wild for Russia; Professor A. Coumbary for Turkey; Mr. Robert H. Scott for Great Britain; and Bvt. Brig. General A. J. Meyer, for the United States. Concurrent action followed shortly after on the part of Austria, through Professor Carl Jelinek; Belgium through Professor E. Quetelet; Denmark through Capt. Hoffmeyer; France through Monsieurs U. J. Leverrier, Marie Davy, and St. Claire Deville; Algiers by General Farre; Italy by Professor Giovanni Cantoni; the Netherlands by Professor Buys Ballot; Norway by Professor H. Mohn; Spain by Professor A. Aquilar; Portugal by Professor F. de Silveira; Switzerland by Professor E. Plantamour; and the dominion of Canada by Professor G. T. Kingston. Within a year the average number of daily simultaneous observations made outside the limits of the United States increased to 214. Later, the co-operation of the Governments of India, Mexico, Australia, Japan, Brazil, Cape Colony, Germany, and Greece, was obtained, and also of many private observatories at widely separated points throughout the Northern Hemisphere.
In the sixteen years during which simultaneous meteorological observations were continued, reports were received from nearly fifteen hundred different stations, about one-half being from land stations, and the others from vessels of the navies and the merchant marine of the various countries.
The total number of storm centers, counting one for each 5-degree square over which the centre has been traced from the International Simultaneous observations of 1878 to 1887, inclusive, aggregates over forty-two thousand, an annual average of over four thousand two hundred. Less than1/25of 1 per cent. of these storms occurred south of the parallel of 10°, and only ¼ of 1 per cent. south of the parallel of 15°. In marked contradistinction to this freedom of the equatorial regions from storms, there is to be noted the excessive prevalence of these phenomena between the parallels of 40° and 60°, north; in which regions substantially two-thirds of the storms of the Northern Hemisphere occurred; while between the parallels of 45° and 55°, north, 36 per cent. of the entire disturbances are recorded. The most remarkable belt of storm frequency on the Northern Hemisphere is that extending from the Gulf of Saint Lawrence westward to the extreme end of Lake Superior, as nearly 8 per cent. of all the storms of the Northern Hemisphere passed over this limited region; the maximum frequency (1.2 per centum) occurring over the 5-degree square northeastward of Lake Huron.
As regards longitudinal distribution, an unusually large proportion of storms prevailed between the 50th meridian and 105th meridian, west; 37 per cent. or one-third of all the storms of the Northern Hemisphere occurring within this region. A second belt of comparative storm frequency obtains from the meridian of Greenwich eastward to the 30th meridian; over which region 15 per cent. of the entire number of storms occurred.
Only four hundred, or less than 9 per cent. of the entire number of storms, entered the American continent from the Pacific ocean, while about thirteen hundred storms, excluding the West India hurricanes, passed eastward off of the American continent. Over nine hundred storms entered Europe from the Atlantic ocean, of which probably four hundred and fifty, or ten per cent. of the whole number recorded, were developed over the Atlantic ocean. Probably not thirty storms, or less than three per cent. of those which entered Europe from the Atlantic, crossed over the continents of Europe and Asia to the Pacific ocean. Fully two-thirds of the storms which enter Europe from the Atlantic are dissipated as active storm-centres before they reach the Asiatic frontier.
The tendency of great bodies of water, when surrounded wholly or largely by land, to generate storms or facilitate their development, is evident from the unusual prevalence of storms over the great lakes, the St. Lawrence bay and the Gulf of Mexico in North America; over the North and Baltic seas, Bay of Biscay and the Mediterranean in Europe; the Bay of Bengal, and over the China and Okhotsk seas.
Undoubtedly a considerable proportion of these storms are drawn towards these regions owing to the effect of evaporation upon the humidity and temperature of the superincumbent atmosphere, so that a very considerable proportion of the storms credited to these squares have not originated therein, but have been drawn up from neighboring quarters. This tendency is marked in North America, as storms pass over the lake region and St. Lawrence valley, whether they have originated in the Gulf of Mexico, along the central slope of the Rocky mountains in the United States, or further north in the Saskatchewan country. In like manner storms pass southeastward to the Mediterranean from the Bay of Biscay, and northeastward from the Atlantic ocean to the same sea, and then later show a very marked tendency to pass over the Black and Caspian seas.
This tendency of storms originating in diverse sections to move toward the lake regions in the United States, is very evident from the normal storm-track charts for April, May, June, August, November and December.
The opinion that gales rarely, if ever, occur upon the equator is confirmed by these storm-tracks. The most southern storm in the North Pacific ocean, developed in July, 1880, between the Island of Borneo and Mindanao, an excellent account of which is given by Père Mark Dechevrens, S. J., in the Bulletin Mensuelle of Zi-Ka-Wei Observatory. The most southern storm over the North Atlantic ocean, in November, 1878, was remarkable for its origin, duration, length of its path, and its enormous destruction of life and property. It was central on the 1st, as a violent tropical hurricane near Trinidad, the barometer being 29.05, the lowest ever recorded there, and, from its intensity and velocity, it is more than probable that it originated considerably to the eastward, and possibly somewhat to the southward of that island. The storm was described in the U. S. Monthly Weather Review for September, 1878.
The writer looks with considerable interest to the results which may follow from a discussion of the annual fluctuation of the atmospheric pressure as shown by the mean monthly pressures deduced from the ten years' International observations. As far as these means have been examined they show that the periodicity of atmospheric pressure is largely in accord with the results set forth in 1885 in The Report of the Lady Franklin Bay Expedition. The conviction expressed in that year is still adhered to—that, at no distant day, the general laws of atmospheric changes will be formulated, and that later, from abnormalbarometric departuresin remote regions may be predicted the general character of seasons in countries favorably located.
The success of long-time predictions of this class for India, has been set forth in a previous part of this report. It is believed that a further discussion of meteorological phenomena on a broad basis, by means of International Weather Charts, both in daily and monthly form, must eventually result in important and fundamental discoveries. It is gratifying to American pride to know that in this international task of outlining the geography of the air, the United States has liberally provided the labor and means for presenting these ten years' meteorological data in such tabular and geographical forms as to render them available for study by all.
Acknowledgment is due to Professor Thomas Russell, for valuable translations, especially from the German; which translations have been of material value in preparing this report.
December, 1888.