24 WAVES OF THE SEA 



to within i foot in 10 when made by a practised 

 observer. This was the estimate of the late 

 Lieutenant Paris, of the French Navy, for his obser- 

 vations, and the late Lord Kelvin informed me that 

 he relied upon his own measurements to the same 

 extent. When, however, an unpractised observer, 

 judging merely by the look of things from the deck 

 of a ship, guesses the height and length of waves, it 

 is possible for him to err much more widely than 

 he would on the land, where he stands on a firm 

 platform, with objects of known size in the neigh- 

 bourhood to afford a scale. The rolling of the 

 ship, in particular, alters the apparent direction of 

 the vertical so as to mislead the judgment as to 

 height. It is difficult to say how widely these 

 guesses may depart from fact, but I do not think 

 it unlikely that waves 20 feet high may, according 

 to the circumstances, be guessed by unpractised 

 or careless observers at anything from 10 to 

 30 feet. This is a range of error of 100 per cent, 

 as against the i o per cent, of the practised observer. 

 While, therefore, in dealing with the data before 

 me, I have been anxious to obtain numerous 

 records, I have been still more anxious not to 

 include any which might belong to the category 

 of misleading guesses. As far as possible, I have 

 relied upon figures in which the observer has ex- 



