430 MR. T. HEELIS ON THfi HEIGHT ETC. OF WAVES 



XXXII. On the Height and Order of Succession of Waves, 

 as observed off the Cape of Good Hope, By Thomas 

 Heelis, Esq., F.R.A.S. 



Read before the Physical and Mathematical Section, April 2nd, 1863, 



The following observations^, almost entirely of the height 

 of waves, were made in the ship ^ City of Pekin/ in the year 

 1862, on a passage from Calcutta to London, The position 

 of the ship is given for the noon of each day of observation. 

 The estimations v^^ere made by noting the apparent altitude 

 of each wave above the trough when close to the ship, the 

 eye being eighteen feet above the level of the water. All 

 measurements or estimations of height are from the trough 

 of the sea. The above height shows what. observations are 

 most to be depended upon ; but I do not think that in any 

 case the height is overstated. No broken wave-tops were 

 estimated, except when it is expressly stated that such was 

 the case ; but the heights noted were those of the highest 

 waves observed. 



It might be thought that a ship was the best place for 

 this kind of observation ; but such, if the ship be on a 

 wind, is far from being the case. Suppose a ship to be 

 brought to the wind under small sail ; when hove to, or what 

 is technically called head-reaching, she will, if there be no 

 swell running from a different direction to that from which 

 the wind is blowing, cross the ridges of water at an angle 

 of about 75°, mth a speed of two or three knots per hour. 

 In this situation, if a gale be blowing hard, the complica- 

 tions of the waves are so great that it is hopeless to attempt 

 to observe anything beyond the 200 feet or so of the ship's 

 length. This induces an error in the observations, arising 

 from the fact that waves have an origin from which they 

 gradually increase in size (longitudinally) to the crest, and 



