Ocean currents exercise a very important influence not only upon climate but also on commerce. The practical sea¬man today cannot be satisfied with merely general knowledge about an influence which so materially affects him. But there has been so much controversy about the causes of currents, very little of which has had to do with how to take advantage of them, that the study of currents has been at a standstill for sev¬eral years waiting for facts. Yet until the science of aeronau-tics has advanced so as to cause ocean travel to be frowned upon, the circulation of waters will be a subject for study. So much has been written about the existence, kinds, and causes of ocean currents that it would be a waste of time to try to cover ground that has been so carefully gone over. A few facts have been arrived at.
Practically every five-degree square in the North Atlantic Ocean has been crossed in any one month of recent years in some direction, and a great number of observations of the con¬ditions in the majority of squares is available to the Hydrographic Office from which to compute an average current for every portion that ships have navigated. Reports have been received for several years from United States naval officers and from mariners of all nations dealing especially with currents and in most instances on forms provided.
An ordinary current report from a mariner gives only an average current found by him for a certain run and under the circumstances surrounding him at the time, or under special circumstances. But when hundreds of reports of currents flowing over one position are taken, an average of all those currents is a current made up of hundreds of different sets of circum¬stances. Such a current is an “average current under average conditions.” The baffling questions raised on account of wind, declinations of the sun and moon, sizes and shapes of reporting vessels, inaccuracies of navigation, barometric pressures of the atmosphere, temperatures of the water, salinity, etc., are summed up and included in the average. But by the laws of least squares the great compendium of special inaccuracies grows smaller as new averages are added and finally such a current results as will be of material value.
Currents of this kind are to be presented on a chart which the Hydrographic Office is now preparing, and which will indicate to the mariners a definite result from their recorded ob¬servations. Such currents will be shown that shipmasters will be able to choose routes that will have favorable currents and shorten their voyages and thus save millions of dollars every year for the owners of the vessels.
By a current rose the chart will picture the conditions in each five-degree square. This has been decided upon as the best way to present the essential usable features of a heterogeneous mass of information upon the subject. The plan has grown out of and is built upon the developments of a study of these mat¬ters by oceanographers and nautical assistants conducted since the time of Maury. The current rose, in blue color, in each five-degree square will show the character of the currents in that square as computed from a certain number of reports from mariners. The figure on page 401 is an illustration.
The currents are concentrated upon sixteen points. Reports of no currents found are indicated by the figure within the circle. The figure in the lower right hand corner of the square is to represent the total number of reports selected from which the current rose is made up. It includes only reports of no current and of currents flowing in the various directions during the month, that were considered reliable. The arrows give the direction of set, and, in length as applied to the scale of per¬centages below, show what per cent to expect in a given direc¬tion. The figures near the ends of the arrows are averaged from the reported drifts and show about how many knots per day the water flows.
For example: The attached current rose should be read thus: Out of ninety-seven reports of conditions in January, five were reports of no current; two per cent of the remaining ninety- two reports gave a N’ly set with an average drift of thirteen miles in twenty-four hours; six per cent gave a NE’ly set with an average drift of twenty miles in twenty-four hours; fifteen per cent gave an ENE’ly set with fourteen miles average drift; twenty-eight per cent, an E’ly set with a drift of sixteen miles, etc. The number (in red) in the lower left hand corner of the square is the number of the square; by the illustration, 4050 is the square at forty degrees latitude, fifty degrees longitude.
The method of computing the figures is complicated but interesting in that it indicates the significance of the credit given to the reports. The high points can be briefly stated.
Reports are received in batches and after being circulated through the Office are deposited in the Section of Pilot Charts and Ocean Currents. They are separated into piles correspond¬ing to the months in which the first observations were taken. The pile for one month, say January, is then charged in a Jan¬uary ledger which has sections devoted to the various five- degree squares. The noon positions or the positions up to which the currents are determined are charged to the squares in which the positions fall. The dates set opposite the currents and the Hydrographic Office numbers of the documents (placed thereon in the order of receipt) are thus recorded for every current for every square. The documents are then filed according to their numbers.
When a current for any square is to be computed, the docu¬ment numbers charged to that square are transposed from the ledger in chronological order by the dates on which the currents were observed. The documents are then extracted from the files. When the documents are in order one is ready to concentrate the information. By referring to the dates on the list the positions and currents on any document can be found readily without reading over the whole report. But at this stage it becomes necessary to determine how much credence should be allowed to the currents set down on the document. The amount of discretion necessary and the experience of the worker cannot here be portrayed, but assuming that the current report is ac¬cepted, notice must be taken of the weight of the currents in relation to the five-degree squares. For in some instances the ship has only crossed the corner of the square, in other instances the ship has crossed the square diagonally, or possibly the ship has reported a current for a run which ended upon its arrival near the outer edge of the square and while in the square was prevented from making further determination. Obviously all the currents should not have the same weight. So the positions are plotted on a chart or chartlet which is marked into one-degree squares and the current is given weight corresponding to the number of one-degree squares over which the run was made in the five-degree square.
By the use of forms the currents found in every one-degree square are tabulated with their drift and, if unusual, their sur¬rounding conditions, and preserved for future manipulation. When all the documents referring to a single five-degree square have been gone over, another form is taken, the currents are col¬lected by directions, the drifts are averaged, and the percentages are determined. Then the current rose is drawn and its evolu¬tion is complete.
Truly there has been much monotony to the task of compiling an elemental order out of the chaotic flow of mariner’s reports and of plotting and transposing individual currents. But with the gist of the old proverb, “Inspiration stands tiptoe on the back of pure drudgery,” the task has developed a plan that ought to, and probably will, renew and instill interest in this special branch of oceanography about which the world is only gener¬ally acquainted but which with every increase in knowledge will become more valuable to shipmasters.
Developments in the Study of Ocean Currents, or How Ocean Currents Are Depicted
By Theron D. Wilson