1849.] Eighteenth Memoir on the Law of Storms. 903 



of the 12th. For the extent of the Cyclone on the 14th, as it had 

 then reached the land we have not any data. 



We may then take it that we have here tolerably distinct evidence 

 that though the form of the Cyclone was circular, the continuations of 

 these circles were wanting on the following side of the storm disk, and 

 hence we arrive at a solution of the undoubted fact that the latter half 

 of a Cyclone, as calculated from the passage of the centre, seems 

 always to be of shorter duration than the advancing or front portion 

 of it. I have already (Sailor's Horn Book,) suggested that this 

 may arise from the disk's not being truly tangential to the earth's 

 surface, but slightly lifted up behind, from the friction of the advancing 

 part against the surface of the ocean retarding its progress.* Now 

 assuming that a disk is one of 150 miles in diameter and of 5 miles 

 in thickness, we can easily calculate that while the whole 75 miles of 

 the van or advancing portion, if it was moving at the rate of 6 miles 

 an hour, would take 12^ hours to pass over the ship, a slight inclina- 

 tion of the disk might elevate the following or rear half of it suffi- 

 ciently to leave say 20 or 30 miles of that part free from its surface 

 action ; for an elevation of one mile will, if the whole is projected to 

 scale, be seen to give but a very slight curving upwards to the last 20 

 or 30 miles of the following side of the disk, so that in such a case 

 the centre of the Cyclone would have 7o miles of the storm disk and its 

 tolerably regular circular winds before it, and only 45 miles behind it ; 

 or it would appear to be situated eccentrically on the chart if it was pro- 

 jected according to the winds reported ; and if a strong monsoon or trade 

 was blowing behind it this would soon follow and occupy the vacant 

 space between the disk and the earth's surface with its own wind. It 

 is evident that if we multiply the supposed size of our disk, say by 5 

 or 10, making it 750 or 1500 miles in diameter, and double or treble 

 its velocity, or suppose it to move at the rate of fifteen miles per hour, 

 we shall have a storm disk, which, truly projected on a chart by the 

 logs of the ships, will extend in the same proportions to 375 or 750 

 miles on its van or advancing portion, and to only 225 or 450 on its 

 following or rear part, and be consequently of so many hours less 

 duration to the ships it may meet with. 



* I am not sure that this suggestion was originally my own, and have some remi- 

 niscence that it is Mr. Redfield's, or Sir J. Herschell's, but cannot find the reference. 



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