MEASURING 61 



distance divided by the time, minus the speed of the ship. To obtain 

 the wave velocity in feet per second, it is necessary to state the speed 

 of the ship in the same units, a value obtained by multiplying her 

 speed in knots by the factor 1.69. It is also necessary, when attempt- 

 ing to make allowance for the speed of the ship to remember that this 

 may be greatest, temporarily, if she is sliding down the front of a 

 wave crest that is advancing in the same direction, and least if she is 

 meeting a crest advancing from a direction opposite to her own. 

 Proper allowance must also be made for the ship's course if this is at 

 an angle with the line of advance of the waves. 



A single observer can calculate the velocities of waves if these are 

 running with the ship, by paying a chip log out astern for a known 

 distance and then recording the time interval between the instants 

 when the chip is at the top of a crest, and when the latter reaches the 

 bhip, again with proper allowance for the speed of the ship through 

 the water, and for her course relative to that of the waves. 20 The 

 velocities of waves can also be calculated from their lengths, as 

 measured above, or from their periods. 



If the ship is at anchor, the time occupied in the passage of two suc- 

 cessive crests past any fixed point on board gives the period of the 

 wave direct. If she is under way, the simplest method of measuring 

 the periods of the waves is to record on a stop watch the interval, in 

 seconds, between the time when a patch of foam or other flotsam is at 

 the top of one crest, and the time when it is at the top of the next 

 crest — always with due regard to the likelihood that the wind may be 

 blowing the foam ahead over the surface of the water (first proposed 

 by Cornish, 1934, p. 38). 



It is not easy to make accurate measurements of the heights of waves 

 at sea, and rough estimates are notoriously unreliable in this respect. 

 In most cases, the only practical method is to find some place on board 

 from which the crests of the waves appear to be level with the horizon 

 when the ship is in the trough of a wave and on an even keel, i. e., when 

 she is neither pitching nor rolling at the moment. The heights of the 

 waves are then equal to the height of the observer's eye above the 

 water line. 21 But the heights measured in this way are only approxi- 

 mate at best, because it is difficult to pick a moment when the ship is 

 actually on an even keel, and neither rising nor falling fast, and also 

 because successive waves vary so greatly in height that it is often 



20 This method was described by the famous physicist and astronomer, D. F. J. Arago, 

 1857, Oeuvres completes, Paris, vol. 9, p. 550. 



31 This simple method seems first to have been described by Arago, in the Instructions 

 for Scientific Observations, prepared by the French Academy of Sciences for the Command- 

 ing Officer of the Corvette La Bnnite for her voyage of exploration in 1836 and 1837 

 (Arago, D. F. J., 1835, Hauteur vagues, C. R. Acad, sci., Paris, vol. 1, pp. 403-404). 



