Waves of the Sea

If the viewpoint during these three days had been the cock pit of an aeroplane at a considerable height, with good conditions of visibility, the ribbed pattern of the sea would have been marked by stripes, owing to the recurrence of groups of higher and more regular ridges. If the speed of waves so formed be greater than the average they will outrun the others when the wind dies down and herald the approach of the main body by a slow-booming surf. Observations by the writer on the Dorset coast in the winter of 1898-99 when a succession of great storms occurred in the North Atlantic, point to the conclusion that a group of waves of greater speed as well as greater height is associated with each squall. On the afternoon of Dec. 29, 1898, in fine weather dur ing off-shore wind, large breakers succeeded one another for three quarters of an hour without interruption, one-hundred-and-thirty nine in all. The average interval was 19 seconds, showing that their speed when in deep water was 664 miles per hour.

The wave-length calculated from the period is 1,85o feet, so that while in deep water the length of the group from front to rear was 49 miles. This main body had been heralded a few hours earlier by the arrival of five groups of large breakers con taining from four to seven members with an average interval of 20 seconds, corresponding to a speed in deep water of 694 miles per hour. The interval elapsing between the first breaker of the first group and the last of the last group was 52 minutes, which is comparable with the 45-minute duration of the main body that followed in the afternoon. The time occupied by each group in discharging its breakers ranged from one minute to a little more than two minutes. The data indicate therefore that squalls of one to two minutes' duration occurring at about ten minute intervals had engendered groups of waves possessing greater speed which had outrun the main body and reached the shore some hours earlier.

The longest period of swell recorded during this stormy winter was on Feb. 1, 1899 when a group of twelve breakers arrived at intervals of 224 seconds, corresponding to a speed in deep water of 784 miles an hour. Anemometers at shore stations in Great Britain did not record a sustained velocity so great as this, 7o miles an hour maintained for two hours being the highest. On the other hand velocities much greater than 784 miles per hour lasting for some seconds are occasionally recorded by anem ometers on our shore stations, as much as 1064 miles per hour having been registered. Waves with a speed of loo miles an hour would have a period of 28.6 seconds which is far greater than that of any North Atlantic swell recorded by the writer. A storm

which lasts for hours is punctuated by squalls which last for min utes, so the squalls are punctuated by gusts which last for seconds at most, and it appears that the speed of the swiftest groups of waves approximates to the average speed of wind during the squall but does not approach that of its momentary gusts. Our habit of thinking of a squall as short-lived is due to the cir cumstance that we are not able to keep company with its progress. Individual squalls travelling from twenty to forty miles an hour have been traced by the recording instruments of meteorological stations during an unbroken march of a thousand miles.

Squall-action on Waves.

Let us now consider how many waves are simultaneously subject to the action of a squall. A squall which advances at 4o miles an hour (no matter what be the velocity of wind developed therein) and which passes over in three minutes, is two miles, 10,560 feet, from front to back, and this may be called its length of fetch upon the sea. The breakers which came in groups of four to seven on the morning of Dec. 29, 1898 had a period of twenty seconds and the length of the waves while in deep water was therefore 2,050 feet. A three-minute squall travelling at forty miles an hour would act upon five such waves simultaneously, and the waves, reacting on the air, would throw it into conformable undulations superposed upon eddies between the crests of the water waves. A three second gust during this squall would be only 176 feet from front to back and could therefore have no comparable effect upon the group of waves.

It has been pointed out that a single observer upon a vessel under way can readily and quickly determine the period of the waves by noting the time taken by a patch of foam in falling and rising. If a swift running, slow-heaving swell be present, its period of oscillation can also be determined from the foam spots, for the slower heave is easily watched, not, as might be expected, camouflaged by the shorter waves. The time of subsidence and upheaval in seconds multiplied by 31 gives the velocity of the wave or swell in miles per hour. When nearing the northern limit of the trade-wind belt on a voyage from Barbados to South ampton, waves travelling 16 miles an hour were recorded by the writer in an easterly breeze of 19 miles an hour, together with a swell from a northerly quarter with a period of 13-1- seconds and consequently a speed of 461 miles an hour, which must have been produced by a strong gale, the term for a sustained average wind velocity of 5o miles an hour.