56 Lift (iitti Litfrrtt of Fiuinris (UUton 



not affurd the rosulu that seamen principally require; they are only data from which thosu 

 r<»siiiu might b«' cnU-ulattyl hy some liitherto unexplained procewH, which, we can wisily foresee, 

 must Ik- ail excee«linj;ly teilious one." 



It is this process which Cialton proceeds to unfold for moderate winds in 

 the case of a "merchantman of the class that usually navij^ates the Atlantic." 

 To carry it out we rec|uire to know: (1) the proportionate time (or the 

 relative fi-etpiency) that the wind blows in a given area from each of say 

 eight points of the compass, (2) the numl)er of miles that the particular ship 

 will make in an hour at each angle to the wind. Combining these two 

 results we can measure for the average of the winds in that area the average 

 progress of the ship towards each point of the compsiss in an hour. If the 

 distance reached' in an hour be plotted from a centre in the arc, we obtain a 

 closed curve whose radius vector measures the efficiency of the ship in that 

 particular area tor a particular coui-se. If now the chart be divided up into 

 areas and in each area be placed the corresponding polar diagram, we have 

 convertetl a wind chart diagram into a passage chart diagram. A navigator 

 now plots his proposed course acro&s these areas, and sets off' with his 

 comjMisses the distance run jier liour in the direction of the course from the 

 nearest polar diagram. In this way he is able to calculate the average time 

 on the proposed course and cjin compare it with the time on other courses. 



"He will thus be able to si'lect the quickest out of any Mumlx»r of routes that may l>e sug- 

 gested to him, and to d(>terniine, on the most trustworthy of existing data, what is the best 

 course to adopt in sailing from one part of the ocean to another." 



Galton suggests the modiffciition of the polar diagram when (a) force of 

 wind and {h) current are taken into account. 



The next paper on this subject wiis published in the Miimtes of the 

 Meteorological Council for December 2, 1 872". In this communication Galton 

 advances a considerable stage further. The Meteorological Office had sorted 

 out the whole of the data for direction and force of wind and for current into 

 "single degree scjuares." Thus the resultant direction and strength of cur- 

 rent, the average force of the wind and its proportional directions were more 

 or less accurately known for each area, for each month of the year. Galton 

 now terms the polar diagrams of his earlier paper "isixiic curves" or brieffy 

 "isods." He calculates tliem for the month of January for "2 squares" from 

 Longitude 0'' to 10° N. and from Latitude 20" to 30' W., allowing for current, 

 and force of wind as well as direction, and taking as his standard type the 

 " Beaufort ship." The rays now represent the average space run in 8 hours, 

 and Galton enters into details of how to construct ' isodic' charts and passages 

 He seems, however, to have been in some doubt as to whether nis name 

 ' is<Ml ' wjis appropriate. In his own copies of this paper, he (piestions in pencil 

 whether the word shoidd not be 'ishodic' But another doubt must have 

 arisen in his mind; his isods did not represent equal paths, but the paths 



' Not the distance traversed, IxM'ause to reach a given point the ship will generally have 

 to tack. 



' Preaented to Sir Edward Sabine, the Chairman, Mr Galton, Major-General Hmytlio and 

 Sir Gharlea Wheat«tone. 



