1845.] Thirteenth Memoir on the Law of Storms in India. 723 



at all different, the force of the current must have been greater, as the 

 distance taken is the straight line between the points, and any deviation 

 from that must make a greater distance. In as far then as rate is 

 concerned, we have (supposing the run to be on the whole correctly 

 estimated) taken the lowest. 



3. The construction of the Diagrams in Plate II from the log. 



The seaman will easily understand these (and I hope appreciate the 

 tedious labour they cost), but writing for the meteorologist, and general 

 reader also, I must explain that Fig. 1, is simply the courses and dis- 

 tances of the log corrected for variation, and laid down on a plane 

 chart. 



For Fig. II. every separate course and distance was first worked as 

 for a traverse, and then to it was applied the average current of S. 55° 

 W. four miles per hour, for the number of hours of run on that course, 

 and this corrected course and distance, taken as being the true one, was 

 then laid down ; and the result of all these produces from point to point 

 of the five day's scudding, the singular set of spirals shewn in the Dia- 

 gram !* And these are in all probability not far from the average truth, 

 as we shall now shew. 



The size and probable form of the vortices round which the Charles. 

 Heddle scudded. 



There are three kinds of calculations to be made as to the size of the 

 vortices. The first is to take the number of turns made in the whole 

 five days against the whole distance run by log, and taking this as re- 

 presenting the sum of the peripheries of so many circles as there are 

 turns, the result divided by the number of turns will give the average 

 size of the circles, and consequently from their diameters the average 

 distance from the centre at which the brig scudded. 



The second is to consider each separate turn or circle made accord- 

 ing to the log, with the number of hours employed, and distance run in 

 making it, and to use this to determine the probable Diameter of the circle 

 sailed round ; and the last, which will perhaps assist us in forming a no- 

 tion of the shape of the vortices, to take each half circle only to calcu- 

 late from in the same way. I shall shew the result of each of these 

 calculations, premising that I take the circle or half circle to be com- 



* The points marked with dates on the diagram are the positions of the vessel at 

 noon each day ; and are those taken for the same days on the general chart also. 



