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The period of the waves is the time between the passage of two successive wave crests past a fixed point. The average value of the wave period is reported, as obtained from the larger well-formed waves of the wave system being observed.
This is the only element which can actually be measured on board moving merchant ships. If a stop-watch is available, only one observer is necessary; otherwise two observers and a watch with a second hand are required. The observer notes some small objects floating on the water at some distance from the ship: if nothing better is available, a distinctive patch of foam can usually be found which remains identifiable for the few minutes required for the observations. He starts his watch when the object appears at the crest of the wave. As the crest passes on, the object disappears into the trough, then reappears on the next crest, etc. The time at which the object appears to be at the top each crest is noted. The observations are continued for as long as possible; they will usually terminate when the object becomes too distant to identify, on account of the ship's motion. Obviously the longest period of observation will be obtained by choosing an object initially on the bow as far off as it can be clearly seen.
Another method is to observe two or more distinct consecutive periods from an individual group while the watch is running continuously: with the passage of the last distinct crest of a group or the anticipated disappearance of the object, the watch is stopped, then restarted with the passage of the first distinct crest of a new group. The observer keeps count of the total number of periods until he reaches 15 or 20 at least. With observations of a period less than five seconds and low wind velocity, the above observation may not be easily made, but such waves are less interesting than those with longer periods.