Length, and Velocity of Ocean Waves. 265 



The calculations were made with sufficient accuracy by- 

 means of a slide-rule. 



It is manifest that it is only possible to obtain an approxi- 

 mation to any of the desired data. Standing near the stern 

 it is not easy to observe the precise moment when the crest 

 reaches the bow ; two successive waves rarely run in exactly 

 the same direction ; and, with a heavy following sea, the ship 

 yaws about so much that the angle between her course and 

 that of the waves can only be estimated approximatively. 



I found great advantage in using cards for the original 

 records, as observations must often be made in rain, sleet, 

 spindrift, and frequently at temperatures near the freezing- 

 point, when the fingers are apt to get benumbed. 



The following are some of my most interesting results : — 



June 8, 1885, lat. 47° S., long. 175° W.— Sea too irregular 

 to measure individual wave-heights or lengths, but the baro- 

 meter indicated about 12*5 feet for the vertical motion of the 

 point of observation below decks. Then, pretty constantly, 

 the surface of the sea was about 7 feet below the port-hole in 

 the troughs, but only 1 foot at the crest. This would give the 

 waves an average height of about 18*5 feet. 



The velocity of the waves could be got much better. Five 

 observations gave the accordant speeds of 29, 28, 31, 33, and 

 30 miles an hour, or an average of 30*2 miles. 



The following data were used in the calculation : — Ship, 

 length 380 feet ; course S.E. ; Speed 14 knots. Sea running 

 to S.S.E., and therefore following. 



June 10, 1885. Lat. 51° S., long. 160° W. Height.— I 

 found it impossible single-handed to estimate the height of 

 the water at the moment the aneroid was read, so I took a 

 constant difference of 6 feet for the difference of the height of 

 the eye at trough and crest. The following is an example of 

 the readings : — 



Trough. 



Crest. 



Difference. 



Diff. height 



eye, trough 



and crest. 



Estimated 



height, in 



feet. 



28-955 



28-975 



•020 



6 



26 



•97 



•985 



■015 



6 



21 



•9675 



•985 



•0175 



6 



23-5 



•960 



•980 



•020 



6 



26 



These waves were not consecutive, and the difference be- 

 tween the extreme heights is a good deal larger than the 



