23C OBSEEVATIONS Oh THE WINDS. 



centre, in the way explained in (168), so that to find the direction ot the 

 wind at any point follow the dotted line out to the margin and read it 

 there. The circles are Isobars, see (24), and the barometer falls -20 inch 

 from one of these circles to the next. This illustrates very clearly the rate 

 at which the barometer falls as you approach the centre — at first slowly, 

 as the broad outer ring is traversed, then more and more rapidly. Near 

 the centre, where the Isobars are very close together, it has been known to 

 fall an inch in 50 miles. Of course, as you recede from the centre, the 

 barometer rises as you pass from one Isobar to the next. 



Lieutenant E. Hayden, U.S.N., gives the following example to show the 

 use of this diagram. 



Suppose that on a certain day at 4 p.m., for instance, the wind is E.S.E., 

 and the barometer -20 inch below the normal : Find at the margin of the 

 diagram the wind-arrow marked E.S.E., and follow the dotted line in 

 toward the centre as far as the Isobar marked " '20 inch below normal ;" 

 this intersection (marked a) is your position on the diagram ; for, by the 

 method of construction just explained, this is the place, and the only place, 

 where, at the same time, the wind is E.S.E., and the barometer -20 mch 

 below the normal. Eeferring to the compass and scale which accompany 

 the diagram, you will find that the centre (low) bears S.W. by S., distant 

 260 miles. Plot this 4 p.m. position of the centre on your track chart, from 

 the 4 p.m. position of your vessel. 



Later in the day, say 8 p.m., suppose that the wind is S.E. by E., and 

 the barometer is -30 inch below the normal (having fallen -10 during the 

 interval) : With this wind your position must be half-way between the 

 dotted lines leading in towards the centre from the arrows marked S.E. 

 and E.S.E., and with this barometer reading it must be half-way between 

 the Isobars marked " -20 inch below normal " and " -40 below ;" it is 

 therefore at the point marked b, and the centre bears S.W., distant 200 

 miles. Plot this 8 p.m. position of the Cyclone centre on your track chart 

 in the same way as before. 



You thus have the position of the Cyclone centre at 4 p.m. and at 8 p.m. 

 plotted on your chart, and the line joining the two positions is the track 

 of the centre and the distance it has moved in 4 hours. 



Suppose, again, that at 10 p.m. the wind is still from S.E. by E., but 

 the barometer stands -40 inch below normal, having fallen -10 in 2 hours. 

 Your position is now at the point marked c on the diagram, found by 

 exactly the same course of reasoning as before, and the centre now bears 

 S.W., distant about 176 miles. Plot this 10 p.m. position in the same way 

 on your track chart. If you have been lying-to, this will evidently indicate 

 ohat the storm's track has recurved, and that you are directly in front of 

 the centre. But no matter whether you have been lying-to or not, your 

 vessel's track and position at any time, and the track and position of the 

 cyclone centre, are both plotted on your chart, and you can closely watch 

 every change of relative position in order to avoid the centre and dangerous 

 Bemicircle of the Hurricane. 



With regard to the use of this and similar diagrams, the Hon. E. Aber- 

 oromby remarks :* — Sometimes good results are thus obtained, but I am 



• " Journal of the Scottish Meteorological Society," 1889, page 328. 



