1853.] Geometrical Measurement of Barometric Waves. 79 



only the ship's tracks, the Cyclone has been accurately tracked 

 for the day between Noon of the 3rd and Noon of the 4th May, 

 and to 3 a. m. of the 5th May, when its centre must have 

 reached the shore about 30 miles to the Northward of Madras. "We 

 have unfortunately no inland reports beyond that point, so as to 

 enable us to trace accurately the exact passage of the centre, and we 

 have thus to assume that its course and rate of travelling were the 

 same inland as they had been at sea, which I have done in this case 

 — though there is usually some diminution of the rate of travelling 

 on shore — from comparing the veering of the wind, and the rise of 

 the Barometer with its previous fall. 



2. That from the known extent of the Cyclone, as well as by the 

 indications of the Barometer and Anemometer, the true wind-circles 

 of the Cyclone do not appear to have reached Madras before 4h. 11' 

 p. m. of the 4th, from which time the readings are projected on the 

 Diagram C. The greatest depression of the Barometer was 29.316 

 occurring at 5h. 36' A. m. of the 5th May, when the centre of the 

 Cyclone bore about No. 21° 30' West, distant 47 miles from the 

 Madras Observatory. 



3. The course of the Cyclone at sea being accurately known, and 

 consequently the distances of its centre from Madras at any given 

 time, those distances are noted in Diagram C. at the different 

 hours at which the various waves passed the Observatory. These 

 distances are also marked on Diagram B. which is a Plane Chart 

 enlarged from A. A is a copy of the Mercator's Chart to the 

 Memoir on this Cyclone in which the ships tracks are omitted. 



4. As the Cyclone did not advance directly towards Madras but 

 passed to the N. East of it, each of the successive undulations shewn 

 by the projection must have passed the Zenith of the Observatory 

 at a different angle ; so that the simple distance from Crest to Crest 

 as shown by the advancing centre of the Cyclone would not be a 

 direct transverse measurement at right angles to their course, but 

 one more or less diagonal. The correction for this, which varies, as 

 the sines of the angles, has been duly calculated, and the distances 

 in the column of corrected distances are the true transverse ones. 



5. A very steep wave will be remarked at e—f (or 8 p. m. of the 

 4th May). This seems exactly analogous to the steep, and sudden 



