348 



Mr. F. Galton on the Conversion of 



The bottom line of the Table gives the results that we seek. 

 In the case we have taken, the diagram in the Wind-chart and 

 that in the Passage-chart would be of the following shapes re- 

 spectively : — 



Wind-chart Diagram. 



Passage- chart Diagram, calculated 

 from that of the Wind-chart. 



The proportion of winds from the 

 neighbourhood of each of sixteen 

 points of the compass is shown by 

 the length of the corresponding 

 lines drawn to the leeward of the 

 centre. The radius of the circle re- 

 presents the proportion of calms. 



The force of the winds is not given 

 in this diagram. It must therefore 

 be reckoned as "moderate" through- 

 out. 



The probable length of an hour's 

 sail in any direction from c, the 

 centre of the diagram, is shown by 

 the length of its radius in that di- 

 rection. This gives a scale to be 

 used throughout the ocean area to 

 which the diagram refers. 



Example. — Since A B is 4^ times 

 the length of the parallel radius c d, 

 therefore the passage from A to B 

 will occupy on an average 4^ hours. 



We should not be justified in usually adopting an "average 

 force" for the winds, though, for simplicity of explanation, we 

 selected the foregoing example, in which we were obliged to do 

 so. If we confined our computation to the effect of simple ave- 

 rages, then an alternation of squalls and calms would be impro- 

 perly reckoned as moderate weather. We must therefore group 

 the winds, not necessarily to each degree of force, but, it may be, 

 in two or perhaps three groups, The Tables would therefore 

 consist not of sixteen lines, but of twice or thrice that number. 

 For the rapid performance of these calculations we should tabu- 

 late the passages of various classes of ships to each of the sixteen 

 points of the compass, under the influence of winds of, say, thirty 

 different degrees of duration, and six of force, making a total of 180 

 lines for each class of ships. In each line the figures should be 

 repeated, so as to sweep not only once but twice round the com- 

 pass. If these are printed on separate slips of paper, the labour 

 of copying them would be wholly avoided ; for the same slips 



