80 Geometrical Measurement of Barometric Waves. [No. 1. 



roller-like sea waves which I have noticed at p. 360 of my Horn 

 Book as the resultant wave of an advancing or passing Cyclone.* 



6. The centre of the Cyclone at Noon on the 4th May bore S. 

 70° East, 102 miles from Madras, and its course and distance to 3 

 a. m. of the 5th, or for 15 hours when it bore North of Madras was 

 No. 55° West 114 miles, and the various distances given in the 

 Table No. II. are those measured on the Plane Chart on which the 

 positions of the centre at the times of the passage of the Crests of 

 the waves are laid down. The distances from crest to crest of the 

 undulations, and those from the flag-staif of Madras are the actual 

 ones as measured at each successive interval on the Plane Chart, 

 and for each of these again the correction above indicated has been 

 calculated and applied. 



7. The Cyclone was coming up from the S. E. b. E. or from the 

 right to the left of the page, but the hours are naturally read from 

 left to right. It will not then be forgotten that, according to the 

 direction of the arrows, a is the advancing front of the Cyclone, and 

 that the whole passage here shown is that of 19 hours 50 minutes 

 or from 4.11 p. M. of the 4th to 24h. 1' of the 4th— 5th May. 



8. But of this it should be borne in mind that we have really but 

 lOh. 10' of actual measurement of the Cyclone's track at sea, and 

 that we have assumed its inland course and rate to have been the 

 same, as above stated. 



9. The Diagram D represents a Bird's-eye view of the whole of 

 the undulations of the Cyclone, as we may conceive them, or waves 

 analogous to them, to have existed at the time when the first of them 

 a reached Madras, or when the centre passed the Meridian of that 

 place. The regularity on the van or advancing portion of the 

 Cyclone with the exception of g and h, and the grouping of the 

 waves by twos and threes, as they felt the effect of the land (the 

 Pulicat hills) are very remarkable. 



10. The mean result of the whole of the measurements is as 

 follows, i. e. that — 



The mean Time of Transition of each wave is lh. 7'. 



* Our analogies must necessarily be drawn from what we know of the motions 

 of waves in fluids, since we have assumed that the Barometric variations are waves 

 of some kind. Of what medium we know not. 



