1845.] Thirteenth Memoir on the Law of Storms in India. 733 



It may be possible to estimate this ; approximately at least. 



Let us take our circle of 320 miles, and consider the chords of the 

 wind arcs in a true circle as forming a polygon of 32 sides, or points. 



Now in our Fig. III. the amount of incurving at the two points is 

 about seven miles for an average circle, say, of forty five miles. 



The diameter of our circle of 320 miles is (in round numbers always) 

 102 miles, so that, at this rate of incurving, we may say that the total 

 would be in the same proportion, sixteen ; i. e. 45 : 7 : 102 : 16. 



Now sixteen for 32 points is exactly half a mile for each point, and 

 the chord of each arc of one point is 10-5. An incurving of half a 

 mile in such an arc would give about 5°, or say half a point. 



In a circle of 200 miles in diameter, on which a ship would only be 

 at 100 miles from the centre (at which time in our Bay of Bengal and 

 China Sea Hurricanes a storm is usually fully and unequivocally mani- 

 fest) the whole incurving would be thirty miles ; let us say thirty-two, 

 or a mile for each point. 



Now the incurving of a mile to each point would make a difference 

 on each arc of about 3° only in the direction of the chord, or say a 

 quarter of a point : so that here it would not make much difference. 

 But we may suppose that the incurving is double what we have 

 here assumed, or even more ;* and then the difference as to the bearing 

 of the centre might be a point, i. e. a vessel in the Northern hemisphere 

 with a hurricane commencing at East, would have the centre bearing, 

 not South but SbE. from her ; and if we suppose this on a circle of 

 320 miles circumference as before, this would for our purposes, in 

 protracting the winds and ship's place for the centre, make it rather 

 more than 10 miles to the Eastward of its situation if there was 

 no incurving; and if we again estimated this centre by the cross 

 bearing from the winds of another ship on the Eastern edge of the 

 same circle having the wind at South and the centre supposed to 

 bear (without allowance for incurving) West, it would really bear with 

 this allowance of the incurving WbS. and the position found by these 

 allowances for the incurving winds would be 14 miles to the SE. of that 

 shown without it ! 



I think this may often account for many of the discrepancies we have 



found in reconciling the ship's positions, winds, and bearings of the centre. 



* Is the rate of veering of the winds (in this case, see p. 724, If point per hour) 

 any index to the amount of incurving ? 



