and a comparison can be made with the curve in 
figure 12. One obtains: 
D/Lr (D in miles, 
Lr in feet) __---- 1 2 3 4: 5 
Dg) Draws ees 975 1,950 2,920 3,900 4,900 
Hp)/Hr Britishrule- 0.67 0.44 0.30 0.20 0.13 
HAp/Hy Fig. 12... .55 . 37 APE . 21 17 
The values of H,/H, agree for D/Lz;=4. Assum- 
ing 7; equal to 8 seconds, the distance of decay 
equals 1,300 nautical miles. For smaller distances 
of decay, the British rule gives larger wave heights, 
for longer distances, smaller wave heights, but 
the general agreement is quite satisfactory. 
Travel time and distance from the generating 
area.—In additon to the Morocco and Berkeley 
observations, two observations-at Pendeen, Eng- 
land, have been included. The character of the 
observation at Pendeen will be discussed further 
when dealing. with wave periods. In determining 
the arrival time, attention was focused on an 
abrupt change in observed height of swell, usually 
a peak. At Pendeen records of the swell were 
obtained at intervals of 2 hours and the arrival 
time of the peak could be accurately established. 
In the other examples, for which observations 
were widely spaced, the possible error in arrival 
time is indicated by the length of the line drawn 
through the point of observation. The agreement 
between theory and observations is remarkably 
good. 
Certain observations by Berkeley, where the 
winds decreased gradually, have been omitted 
and only observations where the duration was 
fairly definite are included in figure 12. Among 
the omitted observations there was some evidence 
that the theoretical travel time was too long. 
The problem of travel time warrants further 
theoretical study and examination of reliable, 
continuous observations. 
Wave period and distance from the generating 
area.—The observed period increase confirms the 
order of magnitude of the theoretical period in- 
crease, but individual points are too scattered to 
draw any further conclusions. Since the petiod 
increase is implicitly contained in the relationship 
dealing with travel time and, to a lesser extent, in 
the relationship dealing with wave height, the fair 
agreement between theory and observations for 
these quantities may be regarded as indirect evi- 
dence in favor of the theoretical relationship for 
wave period. 
The scatter of the observations is partly due to 
the fact that in the ratio, T>/T», both numerator 
and denominator are subject to considerable un- 
certainty. This uncertainty arises because ob- 
servations of period are difficult and because in 
given localities the periods are not constant but 
vary over wide limits. 
Observations at Pendeen are well suited to study 
the increase in wave, period. They were made by 
means of a pressure recorder placed at a depth of 
about 100 feet and were subjected to a harmonic 
analysis by means of specially designed equipment. 
Twenty-minute records at two-hourly intervals 
were examined for two periods, March 14 to 18 
and Apri] 17 to 19, 1945. For these two periods 
the weather situations were similar, each being 
characterized by one well defined low-pressure 
system traveling toward NE across the North 
Atlantic. Both storms could be expected to 
produce heavy swell that would reach Pendeen 
_ from nearly the same direction. 
32 
In the upper parts of figures 14 and 15 the ranges 
of the periods recorded at Pendeen are shown by 
the bands enclosed between the full-drawn and 
the dashed lines. The full-drawn lines include all 
recorded periods, the dashed lines include the more 
frequent ones. The reason for the great varia- 
bility of the observed periods is probably that 
waves arriving at Pendeen did not come from a 
“line source” but from different portions of a large 
generating area and had used different travel times 
over different distances of decay. 
The computed periods at the end of the fetch 
are indicated by the lower circles and the forecast 
wave periods at Pendeen by the circles directly 
above them. The numbers indicate the travel 
time from the end of the fetch to Pendeen. The 
computed fetch at the ends of the periods are not 
arranged in chronological order because some- 
times waves which left the fetch earlier than other 
waves arrived at Pendeen at a later time. The 
forecast periods generally lie inside the band of 
observed periods except on March 14 and early 
on March 15 and on April 18, when the first 
“forerunner” of the swell appeared at Pendeen. 
These-initial waves were low and may represent 
swell arriving ahead of the energy front. 
The forecast height in feet and the observed 
height on an arbitrary scale are also shown in the 
lower parts of figures 14 and 15. The changes in 
computed and observed heights are in good agree- 
ment. The peaks in the figures have been used 
to check the theoretical travel time and are shown 
by the points marked “‘PENDEEN” in figure 12. 
The concept of an increasing wave period in the 
