18 



BEPOET 1866. 



■wards each point of the compass, imder the several influences of each of these 

 ■sviuds. In the example taken, the N. wind will he reckoned as lasting G per cent, 

 of an hour, and therefore ships would he ahle to sail during its prevalence, -014 

 mile to the K, -OlS to the N.N.E., and so on. The N.N.E. wind lasting 14 per 

 cent, of an hoiu- will enahle ships to sail '042 mile to the N., -033 mile to N.N.E., 

 and so on. The N.E., E.N.E., and all the other winds would have their influence 

 similarlv calculated. We thus obtain a Table of sixteen lines (not reckoning the 

 line of zeros that correspond to " calms ") and of sixteen columns, whose addition 

 gives the total progress of one particular- class of ships, in one hour, to all points of 

 the compass, under the influence of the winds that blow in the ocean-district under 

 consideration. 



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 respectively : — ^ 



Wind-chart Diagram. 



Passaffc-cJiart Diaf/ram, calculated froyn 

 that of the Wind-chart. 



Tlie proportion of winds from the neigh- 

 bourhood of each of sixteen points of the 

 compass is shown by the length of the cor- 

 responding lines drawn to the leeward of 

 the centre. The radius of the circle repre- 

 sents the proportion of calms. 



The force of the winds is not given in 

 this diagram. It must therefore be reck- 

 oned as " moderate " throughout. 



The probable length of an hour's sail in 

 any direction from c, the centre of the dia- 

 gram, is sliown by the length of its radius 

 in that direction. This gives a scale to be 

 used throughout the ocean area to which the 

 diagram refers. 



Example. — Since A E is 4:J 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 

 averages, then an alternation of squalls and calms would be improperly reckoned as 

 moderate weather. AVe 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 nimiber. For the 

 rapid performance of these calcidations we should tabulate the passages of various 

 classes of ships to each of the sixteen points of the compass, under the influence of 

 winds of, say, thirty difierent degrees of duration, and six of force, making a total 

 of 180 lines for each class of ships. In each line the figm-es should be repeated, so 

 as to sweep not only once but twice roimd the compass. If these are printed on 

 separate slips of paper, the laboiu- of copying them would be wholly avoided ; for 

 the same slips could be used over again. An extract from the foregoing Table will 

 suflice for an example of what is meant ; where, in order to save space, the figures 

 that refer to the eight principal points of the compass are alone inserted. 



